# Calendar Help
Source: https://userguide.skysafariastronomy.com/app-specific/calendar-help
Documentation for Calendar Help
## Overview
Calendar Help
The Calendar provides a curated list of stargazing events for each day of the month. To access the full Calendar, tap on the "Tonight" icon in the Toolbar and then select the "View All" button in the Calendar section. Android users can also tap the 'Calendar' icon in the main toolbar.
The Calendar describes all major sky events: eclipses, conjunctions, good meteor showers - miss nothing! Whether you're a newbie skywatcher or an experienced amateur astronomer, the Calendar will become your handy, everyday guide to what's up.
Tap the VIEW icon next to a particular event to view a custom sky map which illustrates the event.
# Compass And Ar Help
Source: https://userguide.skysafariastronomy.com/app-specific/compass-and-ar-help
Documentation for Compass And Ar Help
## Compass & AR Help
If you have an iOS or Android device with a compass, SkySafari can show you the sky in same the direction that you're holding your phone. As you move the phone around, the view on the sky chart follows your motion. You can identify stars and planets by holding your phone up next to them, and you can find any object in the sky by following an arrow that points in its direction.
On iOS devices, SkySafari can show you an Augmented Reality (AR) view that blends the star chart with the real sky, captured from your device's camera.
Please note: some devices, like the iPod Touch and Kindle Fire, have a gyroscope but no compass. The icon for Compass will switch to a Gyro instead.
You can activate the compass (or gyro) as follows:
Tilt your device upward.
Tap the Compass icon in the toolbar (Android) or on the upper right of the screen (iOS).
On a device with gyro but no compass, tap the Gyro button in the toolbar to activate the gyro.
Tap the Compass (or Gyro) button again, or touch any part of the sky chart, to turn the compass (or gyroscope) off. You can enable or disable the "tilt up to use" compass feature in the Settings > Appearance & Behavior screen. You can turn off "Tilt to Use Compass" if you find that you're accidentally activating the compass too often, or if you prefer to activate it from the compass icon found on the upper right corner of the sky chart.
Please note: in SkySafari Plus and Pro, the compass and gyroscope cannot be used when you are orbiting another object in the solar system. You can only use the compass when you are viewing from Earth. See the Orbit button Help for more information.
## Using the Compass
SkySafari uses the compass to center the sky chart on the direction you're holding your phone. You can also use it to find objects in the sky. To do this, first turn on the compass. Then tap Search, and enter the name of the object you're looking for. When the Object Info view appears, tap the Locate button at the bottom of view. An arrow appears, leading you toward your selected object. Follow the arrow with your phone to center the object on the screen. When the object is centered, the arrow disappears, and your phone will be pointing toward the object's position in the sky.
In SkySafari Plus and Pro, the compass and altimeter will be turned off if you connect to a telescope, or lock on the telescope's position in the sky chart. The sky chart cannot be centered on the telescope's position, and centered on the coordinates reported by the compass, at the same time.
A note on accuracy: the solid-state compass built into most mobile devices is not very accurate, and easily affected by interference. It can easily be wrong by ten degrees or more. The compass may be useful for locating bright objects in a general part of the sky, but it's certainly not accurate enough to point a telescope.
## Using Augmented Reality
SkySafari can shows you an Augmented Reality (AR) view of the sky. AR blends the simulated sky chart with a real view of your surroundings, captured from your device's camera. Currently, AR is only available on iOS devices, and requires iOS 10 or later.
To use AR in SkySafari, first activate the compass, as described above. A small AR icon appears below the compass icon on the upper right of the screen. Tap the AR icon to turn on your device's camera. If this is the first time you've used AR, your device will ask for your permission to use the camera. To turn off AR mode, tap the AR icon again. When AR is active, the on-screen buttons will be hidden.
During daylight hours, point your device's camera at an area of clear blue sky. SkySafari will fill that area with a star chart. At night, point your camera at a black sky area. SkySafari will attempt to draw the star chart only over the sky area, and not walls, buildings, trees, etc.
You can increase or decrease the amount of sky chart blending with the camera's video image by swiping up or down. At maximum blending, the star chart is drawn everywhere above the horizon, not just in clear sky areas.
You may find that the simulated objects in the sky chart don't quite line up correctly with the real objects shown by the camera. This is due to the inaccuracy inherent in your device's digital compass. To fix this problem, swipe the chart left-to-right. This way, you can align objects drawn in the chart with their images shown by the camera. The Moon is a great object to align on. The planets and brightest stars should also be visible in the camera and can be used for AR alignment.
## Using the Gyro
Unlike the compass/accelerometer, the gyroscope measures your device's orientation relative to a known starting point. The gyro does not measure your device's true orientation relative to north/south/east/west or up/down in the sky. So to find your way around the sky with the gyroscope, you'll need to use a slightly different process.
First, locate a known reference object in the sky, like the Moon. Then search for the same object in SkySafari, and center it on the screen. With the object centered, hold your device toward the object in the sky. Then tap the gyro button (with a finger in your other hand!) to turn on the gyro. Now, as you move your device around, the gyroscope follows its orientation relative to the object you used as a starting point. As you move the device around, the sky chart on the screen follows to match the view in the sky behind it.
As with the compass, you can use the gyroscope to find an object in the sky. Start with a known object in the sky, then find and center the same known object in SkySafari's sky chart, and turn on the gyroscope - just as described above. Then search for the unknown object you're trying to find in SkySafari. When the Object Info view appears for that object, tap the Center button. SkySafari show an arrow that leads toward the object; follow the arrow with your phone to find the object in the sky.
# Image Gallery Help
Source: https://userguide.skysafariastronomy.com/app-specific/image-gallery-help
Documentation for Image Gallery Help
Image Gallery Help
You can import images into SkySafari Pro from a variety of sources. In addition, you can plate-solve most astronomical images and place them correctly sized and oriented in the sky chart.
## Download Optional Plate-Solve Data
Note: By default, SkySafari can plate-solve images with a field of view up to about 2°. The extra reference star data is optional, but highly recommended to expand the range of image fields of view SkySafari can solve.
Download each database by tapping Menu in the Toolbar and selecting Settings → On-Demand Data.
Plate-Solve 1° Images: For images with a field of view of 1° or larger.
Plate-Solve 0.25–1° Images: For images with a field of view between 0.25° and 1°.
Importing Images
Tap Observe in the Toolbar and select Image Gallery. Then tap Import to add an image.
AstroBin — Import an image from AstroBin using its image ID (hash). You can paste an image URL and SkySafari will automatically extract the ID. Then select Import From AstroBin. Most AstroBin images are already plate-solved; select Import into SkySafari to add it to your gallery and view it in the sky chart.
Photo Library — Import an image from your device’s photo library. After selecting an image, tap Import This Image. We recommend associating a target with the image and plate-solving it if you want it embedded in the sky chart.
Remote URL — Import an image from a remote URL.
Local File — Import an image from your device’s file system.
## Image Gallery View
When you open the gallery, four sections appear at the top:
All Photos — Displays all images in your gallery, sorted in chronological order.
Favorites — Displays only the images you’ve marked as favorites.
Recent — Displays only the images you’ve added recently.
Displayed — Displays only images that have been plate-solved and embedded in the sky chart.
Use the Search bar beneath these sections to find images by object name.
## Viewing an Image
Tap any image in the gallery to view it. Use a two-finger pinch gesture to zoom in and out.
Display — For plate-solved images, toggles the image’s visibility in the sky chart.
Favorite — Marks the image as a Favorite for easy access.
Share — Shares the image with others or on social media.
Delete — Permanently removes the image from the gallery.
Edit — Allows basic manual post-processing adjustments.
Solve — Initiates a plate-solve request for the image.
Info — Provides additional information about the image, including the option to show it in the sky chart.
## Plate Solving
When you import an image into the Image Gallery for the first time, SkySafari gives you the option to plate-solve it locally on your device. If you choose not to solve it at that moment, you can do it later by opening the image in the gallery and tapping the Solve icon in the bottom toolbar.
You can monitor plate-solve progress by returning to the main Image Gallery and tapping the plate-solve status icon in the top-right corner. The Image Plate Solving view shows all recent plate-solve attempts and their status (success or failure). You can also Cancel an active plate-solve from this screen.
## Remote Plate Solving
If on-device plate-solving fails, Premium subscribers can attempt a Remote Solve through our partner service, StarPx.
# Night Vision Help
Source: https://userguide.skysafariastronomy.com/app-specific/night-vision-help
Documentation for Night Vision Help
## Overview
Night Vision Help
The Night option, accessible by tapping Menu in the Toolbar, changes SkySafari's appearance to a red-on-black theme designed to help preserve your night vision. Night Vision is best used in the dark, out under the stars. Tap this selectionn once to switch to the Night Vision theme; tap again to restore SkySafari to its previous appearance.
For many people, the screen is still too bright to effectively preserve their night vision - even when SkySafari is using its Night Vision theme.
The iOS version of SkySafari has a Screen Brightness setting (also found in the Settings > Appearance & Behavior view) which lets you adjust the screen brightness. However, this setting only works within SkySafari. To turn down the screen brightness for all apps on your devices, use the screen brightness slider in the main Settings app on your device.
An even better solution is to place a piece of red film over the screen. A brand called Rubylith works particularly well. A "hardware" approach to preserving your night vision works better than any software solution because it will enforce a red appearance across all applications, not just SkySafari.
# Equipment Help
Source: https://userguide.skysafariastronomy.com/app-specific/observe-help/equipment-help
Documentation for Equipment Help
Equipment Help
Use the Equipment settings to enter information about your telescopes, eyepieces, finder scope, and/or camera(s). SkySafari uses this information to display the field of view of your equipment. And you can select this equipment when logging observations of objects.
The My Equipment settings can be accessed by tapping the Observe button in the toolbar.
SkySafari comes with an extensive built-in list of equipment. You can use these, and/or add your own equipment. To edit any equipment list, tap the Plus button to the right of the category name.
## Telescopes
This section lists your telescopes. To add a new telescope, tap the Plus button to the right of the Telescope category name. Select equipment from the existing list or select to "Create Custom" and then enter the following:
Model Name: descriptive name for the scope, for example "Celestron 8-in SCT f/10".
Manufacturer: name of the company that made the scope, for example "Celestron".
Aperture: diameter of the telescope's primary mirror or lens, in mm. An 8-inch telescope has an aperture of 200 mm. This is usually printed on the telescope tube.
Focal Length: focal length of the telescope's primary mirror or lens, in mm. This may be printed on the telescope tube. It is the same as the aperture times the telescope's f/ ratio. For example, an 8" telescope with an f/10 focal ratio has a focal length of 2000 mm (=200 mm x 10).
When you're done entering your telescope's information, tap the Add To My Equipment button. To cancel without entering anything, tap the Telescopes back button to return to the Equipment list.
## Eyepieces
This section lists your eyepieces. To add a new eyepiece, tap the Plus button to the right of the Eyepieces category name. Select eyepieces from the existing list or select to "Create Custom Eyepiece" and then enter the following:
Model Name: descriptive name for the eyepiece, for example "Celestron 25mm Plossl".
Manufacturer: name of the company that made the eyepiece, for example "Celestron".
Focal Length: focal length of the eyepiece, in mm. This is usually printed on the side of the eyepiece.
Apparent Field of View: the angular width of the visible circle (in degrees) that you see looking through the eyepiece. Often found printed on the eyepiece barrel.
## Binoculars & Finders
This section lists your binoculars and finder scopes. To add a new binocular or finder, tap the Plus button to the right of the Binoculars & Finders category name. Then select equipment from the existing list or select to "Create Custom" and then enter the following:
Model Name: descriptive name for the binocular or finder. For example "Celestron 10x50".
Manufacturer: name of the company that made the binocular or finder, for example "Celestron".
Aperture: diameter of the objective lens, in mm. Binoculars and finders are commonly described as "Magnification x Aperture". So for 10x50 binoculars, the aperture would be 50 mm.
Magnification: the enlarging power of the binocular or finder. For 10 x 50 binoculars, the magnification would be 10x.
True Field of View: the true angular width (in degrees) of the visible circle that you see looking through the finder scope or binoculars. For example, 10x finder scope with an apparent field of view of 60° would have a true field of view of 6°. May be printed on the actual equipment or found in its user manual.
## Cameras
This section lists your camera equipment. You can enter information for film cameras, CCDs, DSLR cameras, or any device with a rectangular image sensor - or more than one image sensor. For CCD cameras that have a separate guide sensor and main image sensor, enter the information for each sensor separately.
Model Name: descriptive name for the camera. For example "SBIG ST-2000".
Make: name of the company that made the camera, for example "Canon".
Sensor Width: total width of the film frame or active image sensor area, in mm.
Sensor Height: total height of the film frame or active image sensor area, in mm.
Sensor X-offset: horizontal offset of the film or sensor center from the camera's optical axis, in mm. This is usually zero, except for off-axis guide chips. Check your camera specs for details.
Sensor Y-offset: vertical offset of the film or sensor center from the camera's optical axis, in mm. Again, this is usually zero, except for off-axis guide chips. Check your camera specs for details.
## Barlows and Focal Reducers
This section lists your barlows and focal reducers. To add a barlow or focal reducer, tap the Add Barlow or Focal Reducer button below the list. Then select equipment from the existing list or select to "Create Custom Barlow or Focal Reducer" and then enter the following:
Model Name: descriptive name for this piece of equipment. For example "2x Barlow".
Manufacturer: name of the company that made the barlow or focal reducer, for example "Celestron".
Magnification: the change in magnification that this barlow or focal reducer imparts. Enter a number greater than 1 for magnification or between 0 and 1 for reduction.
## Editing Equipment Lists
You can rearrange and delete items in an equipment list. Here's how.
On iOS, tap and hold the name of the equipment item to drag it to the new position. Swipe left on the equipment item name and a delete action is presented to you. Tap the Delete action.
On Android, tap the Edit link at the top of the screen. Up/down arrows and a trash can appear at the bottom of the screen. Then tap the equipment item you want to move or delete. Use the up/down arrows to move the item in the list, or tap the trash can to delete it. When finished, tap the End Edit link at the top of the screen.
# Events Help
Source: https://userguide.skysafariastronomy.com/app-specific/observe-help/events-help
Documentation for Events Help
Events Help
The Event Finder is a powerful search engine that finds astronomical events visible tonight and far into the future. The finder dynamically generates a listing of moon phases, eclipses, Jupiter satellite events, meteor showers and planetary phenomenon such as conjunctions, elongations and oppositions to name but a few of the events that can be displayed.
You open the Event Finder by tapping Observe in the Toolbar and selecting Events.
Events by Date: By default, the list of events is created for a week in advance from today’s date. Tap Start Date and End Date to search for astronomical events on a specific date range. After setting your dates, SkySafari dynamically updates and generates a list of events.
Note: Creating a list of events for a long date range requires a greater amount of time to calculate. For faster searches limit the date range to a few months. Knowing what specific objects you are interested in will also speed up searches.
Viewing Events: Tap on an event in the list of generated events to simulate the event in SkySafari.
## Filter & Sort Options
Select the “filter & sort” icon located on the upper right of the Events screen to open a view with additional options.
## Event Visibility
Current Location is selected by default and displays only those events that are visible from your current location.
Visible From anywhere lists all events, including those not visible from your current location.
Tonight lists events visible from your current location and suppresses events that are only visible during daylight.
## Event Types
The event type options allow you to select what type of astronomical events to search for and display.
Most of the listed Event Types have special options associated with them. Taping on an event type name will display the options for that event type. For example, selecting “Lunar Phase” will display options for selecting the type of lunar phases to include in your search.
Some of the event types might not be familiar to you:
Lunar Phase Events: New Moon is the Moon as it appears on Earth when it is positioned between Earth and the Sun at the beginning of a cycle of lunar phases. The new moon rises and sets with the Sun.
During First Quarter Moon and Last Quarter Moon half of the Moon’s disc is illuminated.
Full Moon is the Moon’s appearance from Earth when it is positioned directly opposite the Sun. The Moon is full two weeks after new moon. The full moon rises at sunset and sets at sunrise when Earth is between the Moon and the Sun.
Lunar and Solar Eclipse Events: A Total Lunar Eclipse is an eclipse of the Moon where Earth crosses between the Sun and Moon, blocking sunlight from reaching any of the Moon’s surface. During a total eclipse, the Moon darkens slightly and takes on a reddish colour.
A Partial Lunar Eclipse is an eclipse of the Moon where Earth crosses between the Sun and Moon, blocking sunlight from reaching only part of the Moon’s surface. This part will appear darker than the rest of the Moon.
A Penumbral Solar Eclipse is an eclipse of the Moon where Earth crosses between the Sun and Moon, but only partially blocks the Sun’s light. Because some light still reaches all parts of the Moon, the Moon does not darken noticeably during a penumbral eclipse.
A Partial Solar Eclipse is an eclipse of the Sun where the Moon covers only part of noticeably during a partial eclipse.
A Total Solar Eclipse is an eclipse of the Sun where the Moon completely covers the Sun.
An Annular Solar Eclipse is an eclipse of the Sun where the Moon passes directly in front of the Sun, but does not completely cover it. At the eclipse’s peak, a ring of sunlight still shines around the Moon’s edges.
A Hybrid Solar Eclipse is an eclipse that starts as an annular solar eclipse, then becomes a total solar eclipse and at the end returns to an annular solar eclipse. The total eclipse phase of a hybrid eclipse tends to be short in duration.
Planetary Moon Events: A Shadow Transit is the passage of a moon’s shadow across the face of its parent planet.
Shadow transits are a favourite event among planetary observers.
A Transit is the passage of a moon over the face of its parent planet.
An Occultation is the disappearance—eclipse—of one moon behind its parent planet.
An Eclipse occurs when a moon passes through the shadow of its parent planet.
Planetary Events: A Solar Transit is the passage of one planet across the face of the Sun. These rare events require the use of a safe solar filter designed specifically for viewing the Sun.
Greatest Elongation is the greatest angular distance to the east or west of the Sun reached by a planet, usually Mercury and Venus. When a planet is at its eastern elongation, it sets after the Sun and is at its best visibility in the evening sky. When a planet is at its western elongation, it sets before the Sun and is at its best visibility in the morning sky.
There are two types of Solar Conjunctions. An Inferior Conjunction is the passage of Mercury or Venus between Earth and the Sun. The outer planets cannot pass between Earth and the Sun and therefore cannot come to inferior conjunction. A Superior Conjunction is the position of a planet when it is on the far side of the Sun (and in conjunction with the Sun).
A Quadrature occurs when the angle between the Sun and a planet is 90 degrees or a quarter of a circle. Eastern Quadrature is defined when a the planet is 90 degrees to the east of the Sun, and Western Quadrature when a planet is 90 degrees to the west of the Sun.
Solar Opposition is the position of a planet when it is opposite the Sun in the sky.
Appulses: An appulse is the close apparent approach of one celestial object to another in the sky. Use this option to find the precise moments when the Moon or planets pass near the Pleiades, other planets, or stars of a selected magnitude. This feature also includes transit and occultation search capabilities.
ISS Transits: Find the precise moments when the International Space Station passes in front of the Sun or Moon as seen from your location. This option calculates upcoming solar and lunar transits of the ISS and provides timing and visibility details for each event.
Jupiter GRS Transits: Predict transits of Jupiter's Great Red Spot (GRS). A transit occurs when the GRS crosses Jupiter’s meridian (the centerline of the planet). If the timings are off, you can adjust the GRS longitude value in Settings > Solar System > Jupiter GRS Longitude.
Meteor Shower Events: When the Earth’s orbit crosses the path of debris left behind by a comet, there is a meteor shower as the dust particles burn up in Earth’s atmosphere. Only the major meteor showers that average more than 10 meteors per hour are displayed.
# Observations Help
Source: https://userguide.skysafariastronomy.com/app-specific/observe-help/observations-help
Documentation for Observations Help
Observations Help
View and search all of your logged object observations. Observations can be viewed and created in the Observations view by tapping Observe on the Toolbar and selecting Observations.
## Creating an Observation
To create a new observation, tap the Create Observation button at the bottom right of the Observations view. Select your object and tap "Create Observation" when you get to its Object Info view. The object will be added to your list of observations. Tap the object name in the list to edit the details of your observation.
You can also create an observation by selecting an object in the sky chart. Tap & Hold the object to bring up the selection menu. From the menu tap the observation icon (looks like an eye). Alternatively, you can add an observation to an object by tapping the More icon at the bottom of the Object Info view for that object and selecting "Create New Observation".
## Filter & Sort Observations
Tap the Filter & Sort icon on the upper right to access additional options. You can select to "Only Show Observations With a Session" and sort them by "Start Date", "End Date" or "Proper Name".
## Deleting An Observation
On iOS, swipe the observation name left to delete or tap the Edit button at the top of the screen. Then tap - (minus) icon on the left side of the item to delete it. Tap the End Edit button at the top of the screen when you're finished.
On Android, tap the Edit link at the top of the screen. A 'trash can' icon will appear at the top of the screen. Then tap the item you want to delete. Tap the trash can to delete it. When finished, tap the End Edit link at the top of the screen.
# Observing Lists Help
Source: https://userguide.skysafariastronomy.com/app-specific/observe-help/observing-lists-help
Documentation for Observing Lists Help
Observing Lists Help
Observing lists keep track of objects you wish to observe, and record a log of your observations. Items in the list can be edited to show the time and date you observed the object, and let you enter notes about your observations of the object. You can share observing lists with friends by email, or send them to yourself for safekeeping.
You create and access observing lists by selecting "Observing Lists" from the Observe button in the Toolbar.
## Creating Observing Lists
By default, SkySafari comes with a single, empty observing list called "My Favorites". To create additional observing lists, tap the Create List button at the bottom of the Observing Lists view. You can import an existing list from the online Observing List Repository or select Create New List to create a new "Untitled list". Give your new list a title and description by tapping the Edit button; you can then add objects to the list.
## Adding Objects and Observations
You can add an object to an observing list, log a new observation of an object, or show your existing observations of the object, by selecting any object in the sky chart and then using a tap & hold gesture over the object to bring up the Selection menu. This gives you three options:
Add to Observing List (icon is a stack with a plus) - lets you add the object to the observing list.
Create New Observation (icon is an eye) - lets you log a new observation of the object.
Show Observations (icon is a log sheet) - shows all of your existing observations of the object.
Choose the Add to Observing List icon from the selection menu options. If you have more than one observing list, SkySafari lets you choose which list to add the object to. If you have only a single list, this choice is skipped.
## Editing, Sorting and Highlighting the List
You can rearrange and delete items in an observing list. Here's how.
On iOS, tap the Edit button at the top of the screen. Then tap and drag the "grip" icon on the right side of the list item to move it around the screen. Tap and drag the - (minus) icon on the left side of the item to delete it. Tap the End Edit button at the top of the screen when you're finished.
On Android, tap the Edit link at the top of the screen. Up/down arrows and a trash can appear at the bottom of the screen. Then tap the observing list item you want to move or delete. Use the up/down arrows to move the item in the list, or tap the trash can to delete it. When finished, tap the End Edit link at the top of the screen.
Using the controls near the top of the screen you can filter the list to show (1) all objects, (2) those with observations attached to this list or (3) objects that have yet to be observed.
You can sort all objects in the list by tapping the Sort & Filter icon on the top right of the screen. Then choose a property to sort on. For example, to sort objects in the list by magnitude (brightest to faintest), choose "Visual Magnitude" from this menu.
To visually show where objects in the selected list appear in the sky chart, tap the Highlight Objects switch above the list. When an observing list is highlighted, a small list icon appears at the bottom of the sky chart above the toolbar. Click this icon to quickly return to the highlighted list.
To delete an entire observing list, use the same techniques to edit the list of observing lists.
## Using an Observing List
Tap an observing list in the Observing List view to see the objects it contains. Tapping and holding (long-press) an object in the list provides a choice of:
Showing the Object Info for that object
Creating/Editing your observations of the object
Centering the object in the sky chart
When editing an observation of an object, you can enter the following information about your observation:
Session - a period devoted to a particular observing time. A Session can be associated with multiple observations. For example, you might want to organize a single night's object observations into a Session. New Sessions can be created in the Sessions view by tapping Observe on the Toolbar and selecting Sessions.
Date and Time - "started at" and "ended at" times when you actually observed the object.
Observing List - the list the observation is associated with. Each observation can only be associated with one observing list. If the chosen observing list doesn't contain the object you are creating an observation for, you will be asked if you want to add it to the list.
Seeing - a measure of how steady the air was when you observed the object using the Antoniadi scale. Use the slider to choose from I (Perfect seeing) to V (very bad seeing).
Limiting Magnitude - a measure of the faintest stars (apparent magnitude) that can be detected with the unaided eye near the zenith. Use the slider to choose from magnitude 0 to 8.
Sky Quality - a measure of the brightness of the sky in magnitudes per square arcsecond (Mpsa). Use the slider to choose from a Mpsa of 17 (sky is brilliantly lit) to 23 (excellent dark skies).
Comments - any particular notes about the object that you want to save. For example, "Jupiter's Great Red Spot was unusually red tonight."
Equipment - the combination of equipment you used to observe the object (telescope, eyepiece, binoculars, etc.) You can enter the equipment by tapping the "Choose Equipment" button and selecting from SkySafari's equipment list. See the Equipment Help for information on how to add equipment to the list.
You can go back and edit your observations of an object at any time.
## Downloading Deep Sky Survey Images
You can batch download a Deep Sky Survey (DSS) image for all the objects in an observing list.
Tap the More button (resembles 3 dots) above the objects in an observing list. In the menu that opens, there is a Download DDS Images For List button. Tap this button to download a DSS image for all the objects in your list.
## Sharing Observing Lists
Above the objects in an observing list, tap the More button. Then select Email List (iOS) or Share (Android) to create an email message with the observing list as an attachment. Enter the recipient's email address in the "To:" field, and edit the message body as desired. Tap Send to send the observing list to the recipient.
When the recipient receives the email on their mobile device, they can import the attached observing list into their copy of SkySafari. To do this, the recipient should touch the observing list attachment until a view appears with the option to open in SkySafari. Selecting SkySafari imports the observing list into the recipient's copy of SkySafari.
You can email your observing lists with or without observations. This lets you send a "clean" copy of an observing list to someone else.
# Observing Sessions Help
Source: https://userguide.skysafariastronomy.com/app-specific/observe-help/observing-sessions-help
Documentation for Observing Sessions Help
Observing Sessions Help
A Session lets you collect your observations into groups that span a few hours, or a few nights. For example, you might want to organize a single night's observations from your favorite dark sky site into a session. Sessions can be viewed and created in the Sessions view by tapping Observe on the Toolbar and selecting Sessions.
## Creating A New Observing Session
To create a new session, tap the Create New Session button at the bottom of the Sessions view. Then fill out the information:
Session Name - lets you add a title to your session.
Site - tapping provides a choice of locations to choose from, including your current location.
Start Time - tap to set the start time of your session. By default the time is set to when the session was created.
End Time - tap to set the end time of your session.
Comments - any particular notes about the site that you want to save. For example, "Astronomers and mosquitos don't mix at this site."
## Filter & Sort Sessions
Tap the Filter & Sort icon on the upper right to access additional options. You can select to show "Only Sessions With Observations" and sort them by "Date" or "Number of Observations in Session".
## Deleting An Observing Session
You can delete Observing Sessions if the session is not referenced by observations. Here's how.
On iOS, swipe the session name left to delete or tap the Edit button at the top of the screen. Then tap - (minus) icon on the left side of the item to delete it. Tap the End Edit button at the top of the screen when you're finished.
On Android, tap the Edit link at the top of the screen. A trash can appear at the bottom of the screen. Then tap the item you want to delete. Tap the trash can to delete it. When finished, tap the End Edit link at the top of the screen.
# Observing Sites Help
Source: https://userguide.skysafariastronomy.com/app-specific/observe-help/observing-sites-help
Documentation for Observing Sites Help
Observing Sites Help
You can add your own favorite observing sites to SkySafari. Sites can be created in the Observing Sites view by tapping Observe on the Toolbar and selecting Observing Sites.
## Creating A New Observing Site
To enter a new site, tap the Create Observing Site button at the bottom right of the Observing Sites view. You can set the new site to your current location or choose a location from a list or map at the bottom of the view. To create a custom location, fill out the information:
Name - lets you add a title to your site.
Latitude - the geographic latitude of your observing site on the Earth's surface.
Longitude - the geographic longitude of your observing site on the Earth's surface.
Elevation - the elevation of your observing site above sea level in meters.
Standard Time Zone - the time offset in hours from Greenwich Mean Time (GMT).
Tap Save when finished.
## Deleting An Observing Site
You can delete Observing Sites if the site is not referenced by an Observing Session. Here's how.
On iOS, tap the Edit button at the top of the screen. Then tap - (minus) icon on the left side of the site name to delete it. Tap the End Edit button at the top of the screen when you're finished.
On Android, tap the Edit button at the top of the screen. Then select the site you want to delete to highlight it and then tap the trash can icon on the top to delete it. Tap the Save button at the top left of the screen when you're finished.
# Planner Help
Source: https://userguide.skysafariastronomy.com/app-specific/observe-help/planner-help
Documentation for Planner Help
Planner Help
## Observing Planner
In SkySafari Plus or Pro, you can search for objects based on properties other than their name(s) or catalog number(s). For example, you could search for all galaxies in Virgo brighter than magnitude 10, or all asteroids more than 45 degrees above the horizon. Please Note: this feature is not available in the basic version of SkySafari.
To search for objects this way, select 'Planner' after tapping the Observe button in the toolbar. Then select the following:
Object Types: the kind(s) of objects you want to find - for example, Stars, Open Clusters, Bright Nebulae, Galaxies, Planets, or Comets. You can choose more than one object type.
Object Visibility: limits the search for objects to a specific location or time range.
Restrict to Ranges: the limits for properties of objects you want to find. Enter the minimum value in the left, and the maximum on the right. For example, to find objects with a magnitude between 4 and 5, enter "4" on the left and "5" on the right under Magnitude. If you leave any field blank, no limit will be applied to that value. For example, you can search for all objects closer than 10 light years by leaving the left side blank (no minimum distance), and entering 10 on the right side (maximum distance 10 light years) under Distance.
Restrict to Catalogs: limits the search for objects to specific catalogs. For example, to find objects in the Messier catalog only, enter "Messier" in the field. To limit the search to multiple catalogs, enter comma separated lists of catalog names (e.g. "Messier, NGC, Sharpless"). If you leave the field blank, all catalogs will be searched.
Restrict to Constellation: the constellation where you want to find objects. For example, to find only objects in Orion, choose Orion from the constellation list. If you want to find objects in any constellation, turn the wheel to "All Constellations" (the default).
Finally, tap Search at the bottom of the view. Your results will be displayed in an object list, just as if you'd searched for them by name. From this view you can select to make your search results into an observing list by tapping Make Into Observing List.
To reset all of your advanced search parameters to their defaults, tap Reset All.
# Scope Display Help
Source: https://userguide.skysafariastronomy.com/app-specific/observe-help/scope-display-help
Documentation for Scope Display Help
Scope Display Help
Use the scope display settings to customize the display of the telescope's field of view in the sky chart. The Scope Display settings can be accessed by tapping the Observe button in the toolbar.
Note: By default the field of view indicators only display on the sky chart when you connect to a computerized scope. To change this behavior so that field of view indicators draw on the sky chart without connecting to a scope, tap the Options icon ((looks like two sliders) on the top right of the screen and check the option named "Show Even if Not Connected to Telescope".
Any of the field of view indicators may be shown or hidden by touching its on/off switch.
## Reflex Sight Indicators
Crosshairs: Lets you turn crosshairs on or off, which precisely show the field of view center, and indicate the directions of movement of the telescope mount axes.
Telrad Circles: Shows the field-of-view indicators of a Telrad as red circles in the sky chart. These circles are 0.5°, 2°, and 4° across, always centered on the telescope's field of view.
## My Field of View Indicators
Here you will see the field of view indicators you've added. Tap the Create FOV button on the lower right of the screen to get started.
From Equipment: Choose the telescope, eyepiece, finder scope, and/or camera(s) that you're using. SkySafari will automatically compute the FOV of your equipment and display it in the sky chart when the telescope is connected.
Please Note: if you choose binoculars or a finder scope, you cannot choose an eyepiece or camera. Binoculars and Finders have built-in eyepieces with fixed fields of view. You can use any eyepiece or any camera with any telescope, however.
You can enter your equipment into SkySafari using the My Equipment settings which are accessed by tapping the Observe button in the toolbar. See the My Equipment Help for more information.
Circular FOV: You can also enter a custom circular FOV (in degrees) if you don't want to choose any equipment from the list.
Rectangular FOV: Enter a custom rectangular FOV (in degrees).
## Editing FOV Indicators
You can edit the color, field rotation angle, and orientation for individual FOV Indicators by tapping on the name of an FOV indicator in the Scope Display screen.
Indicator FOV: Tap to edit the FOV indicator values. Radius in degrees for circular indicators, width x height in degrees for rectangular indicators or select a different equipment combination for equipment-based indicators.
Display Name: Edit the display name of an FOV indicator.
Indicator Color: Lets you set the color of an FOV indicator. Tap the color button to bring up the color picker.
Field Rotation Angle: Lets you set the scope field-of-view rotation angle of your non-circular FOV indicator. When zero, "up" in the scope's field of view is north in the sky (for equatorial mounts) or "up" in the sky (for alt-azimuth mounts).
Sky Chart Rotatable FOV: In addition to setting the rotation angle numerically, you can now adjust it directly on the sky chart. Double-tap a non-circular FOV indicator to reveal a rotation handle, allowing you to manually rotate the FOV to any desired orientation. Double-tap again to hide the handle and lock the FOV at the angle you’ve chosen. This provides a faster, more intuitive way to fine-tune the field rotation during observing or imaging sessions.
Field Orientation: Allows you to set the orientation of your non-circular FOV indicator relative to Horizon Coordinates (Alt Az), Equatorial Coordinates, or the Default (takes on the orientation of the mount of your selected scope preset). For example, an Horizon (Alt Az) orientation will align your FOV indicator with the Zenith, while an Equatorial orientation will align your FOV Indicator with the North Celestial Pole (NCP.)
## Global Field of View Display Options
Tap the Options icon on the top right of the screen to access additional settings for your field of view indicators.
Draw Labels: Shows names of the equipment combination that defines a particular field of view in the sky chart.
Cardinal Directions: Shows cardinal direction (N/E/S/W) markers around the edges of the field of view indicator. These help you follow the cardinal celestial directions in a small telescopic field of view. As the Earth rotates from west to east, objects appear to travel through the eyepiece from east to west.
Show Even if Not Connected to Telescope: Lets you display the field-of-view indicators even if SkySafari is not actually communicating with your telescope controller. In this case, the field of view indicators will always be displayed at the center of the sky chart. You may use this feature, for example, to "preview" how a star cluster may appear in a particular eyepiece or finder scope.
# Scope Control Help
Source: https://userguide.skysafariastronomy.com/app-specific/scope-control-help
Documentation for Scope Control Help
## Scope Control View
The Scope Control view lets you connect to your telescope and control it. Before connecting, select your telescope type and communication options by tapping Menu > Settings in the Toolbar and then selecting Presets under the TELESCOPE section. By default, SkySafari's uses a "Demo" telescope. This is a dummy virtual telescope that lets you to use the controls without having a real scope connected. To connect to a real telescope, choose the telescope type and communication parameters in Presets.
Please note: you can't use SkySafari's telescope controls when you are orbiting another object in the solar system. To use them, first return home to Earth. See the Orbit button Help for more information.
## Connecting and Disconnecting
After setting up your telescope configuration in the Settings > Presets, tap the Scope button in the Toolbar.
Connect: This button opens a connection to your telescope and brings up the Scope Control view at the bottom of the screen. If you are using an Android device with a paired bluetooth serial adapter, then SkySafari will use bluetooth for telescope communication. Otherwise, SkySafari will use Wi-Fi for wireless telescope communication.
Note: Tap & hold the Connect button to bring up a menu that allows you to quickly select a saved Scope Preset.
Connect and Align: This button only shows when connecting to a Celestron WiFi telescope. Tapping it will connect and then guide you though an alignment process. The type of alignment can be chosen in the Telescope > Presets > Add Preset > Other screen.
Once you've connected, you will see a red disconnect icon on the far left of the Scope Control view. Tapping it will end your telescope control session.
Before tapping the Connect button, make sure you've selected the correct telescope type and communication options in the Settings. Make sure the scope is powered on, and any necessary alignment procedures are completed. Consult your telescope manual for details on the scope's alignment procedure.
After connecting, the sky chart is centered where SkySafari thinks the scope is pointing, as reported by your telescope. If this is wrong, your telescope is probably not star-aligned correctly.
While you're connected to a telescope, the Compass or Gyro button will be turned off. The sky chart cannot be centered on the telescope, and centered on the compass, at the same time.
## Slewing and Aligning
Once your telescope is connected, arrow buttons appear on the sides of the screen (unless you have selected the "Tilt Device to Slew" option in the Settings under the Telescope section). The arrow buttons let you move the scope directionally. A motion rate control appears, to let you control how fast the directional motion occurs.
The other buttons in the Scope Control panel are now active, too:
GoTo/Stop: This button issues a "GoTo" command to your telescope, which will physically slew it to the coordinates of the currently-selected object in the sky chart. To select an object in the sky chart, tap on it, or use the Search view.
While a GoTo is in progress, this button's title changes to "Stop", and pressing it will issue a command to stop the currently-in-progress GoTo. You can use this as an "emergency stop" if the telescope is in danger of hitting something, or if you have accidentally slewed to the wrong object.
Note that not all telescopes support GoTo commands, and that you cannot GoTo an object which is below the horizon.
Align: This synchronizes the scope to coordinates of the selected object. The bullseye indicator in the sky chart shows where the telescope thinks it is pointing. If that appears incorrect, the scope and the software must be synchronized. To do this:
Physically point the scope at a real star in the sky, using SkySafari's arrow buttons or the scope control panel. Center the object in the eyepiece.
Select that same object in SkySafari to make it the current target object. Do this by tapping the object in the sky chart, or by searching for it by name.
Tap the Align button.
Lock/Unlock: Tapping this button keeps the sky chart centered on the telescope's position. Moving the telescope will cause the sky chart to move, following the telescope's motion.
More: This button (which resembles 3 vertical dots) brings up a new control that allows you to "GoTo" a specific RA/Dec and to Reverse N/S or E/W.
## Notes on Alignment
Note: for Celestron NexStar, Orion/SkyWatcher SynScan, iOptron GOTONova, and ServoCAT telescope controllers, tapping the Align button stores the offset between the telescope's reported position and the selected object's position. It subtracts that offset from the telescope's reported position whenever the telescope is within 10 degrees of the object you Aligned on. In other words, SkySafari performs a "local sync" around the alignment target. If you move the telescope to a very different part of the sky, you may want to Align on a target in that part of the sky. Also note that the telescope's RA/Dec reported by SkySafari will differ from the RA/Dec reported by its hand controller (since SkySafari is applying the alignment offset to the telescope's reported position.)
Note: for encoder-based "Push-To" systems, like the Tangent Instruments BBox, Celestron Astro-Master, JMI NGC-MAX, and Orion Intelliscope, SkySafari lets you perform a multi-star alignment (up to 10 stars). This eliminates the need to level your telescope mount base. Simply set up your telescope, point it at the first alignment star, select that star in SkySafari, and tap "Align". Repeat the process with a second alignment star, choosing "Align" rather than "Restart Alignment" when asked. Your encoders should now be aligned to the sky. You can continue to align on additional stars; SkySafari only uses the most recent 10 stars aligned on. If you want to forget the pervious alignment stars and align as your first star, choose "Restart Alignment".
Make sure your two alignment stars are at least 10 degrees apart; 90 degrees apart is ideal. SkySafari will warn you if your alignment stars are too close together, or if their positions don't match - for example, if you've accidentally selected the wrong alignment star in SkySafari, or you're not really pointing the telescope at that star in the sky.
SkySafari remembers the telescope's alignment until you quit the app, so you should not have to realign if you disconnect (or are accidentally disconnected) from the encoder control box. However, if you accidentally kick the telescope mount, or otherwise destroy your alignment, you can realign without having to quit SkySafari. To start over, point the telescope at a star, select the same star in SkySafari, and tap Align. When given the option, align on the star as the "First Star". That will reset SkySafari's alignment process and start it over with the star you just selected.
## Going to a specific RA/Dec
Normally GoTo will take you to the coordinates of the selected object. To go to specific coordinates rather than the selected object, tap the More button in the scope control view and select GoTo RA/Dec. A small RA/Dec panel will pop up onscreen. Fill in the RA and Dec you want and tap GoTo in the panel. The coordinates may be entered using either decimal format or as HH MM SS.S for RA and DD MM SS.S for Dec.
## Celestron Wi-Fi
When using the Celestron Wi-Fi scope type, it is important that you do not align using the hand controller before you connect. Doing so will cause the hand controller and SkySafari to fight over who's in control and cause incorrect behavior.
Telescopes with StarSense AutoAlign: SkySafari automatically detects a telescope with StarSense AutoAlign. The StarSense camera must be plugged into the telescope's AUX port before powering and connecting to the telescope. Set the telescope to its home position (using index marks if applicable). After tapping "Connect & Align", tap OK to begin auto alignment.
Once alignment is complete, follow the prompts to align the StarSense camera to the telescope. Select and go to a bright star. Center the star in the telescope, tap Align, then tap Calibrate. The star's new position on the camera will be displayed as a set of coordinates (640, 480 is default center), and this is automatically saved in SkySafari for future alignments. Once the camera is calibrated, follow the prompts to start a new StarSense Auto alignment. You only need to do this the first time you are using StarSense with the telescope in SkySafari.
All-Star Polar Alignment: After completion of star aligning an equatorial mount, there will be an ASPA button shown in the final panel. This starts the All-Star™ Polar Alignment procedure which guides you through the process of polar aligning your mount.
# Search Help
Source: https://userguide.skysafariastronomy.com/app-specific/search-help
Documentation for Search Help
## Search Help
The search view lets you search for objects, by typing their names, or by choosing them from lists.
## Search
At the top of the list view is a search field. Enter all or the first part of an object's name; then tap the Search button to display a list of matching objects. For example, if you search for "Saturn", SkySafari will find both the planet Saturn and the Saturn Nebula.
You can search for an object using any of its catalog designations. For example, the Andromeda Galaxy can be found as M31, NGC 224, UGC 454, PGC 2557, MCG 7-2-16 or CGCG 535-17. Likewise, the double star Porrima can be found as Gamma Virginis, 29 Vir, HR 4826, SAO 138917, BD -00 2601, HIP 61941, STF 1670, ADS 8630 or WDS 12417-0127.
All of the objects matching your search will be displayed in the list of results. Objects below the horizon are dimmed, but still selectable.
Choose a specific object from that list to bring up the **Object Info** view for that object. If there is only one object which matches the name you entered, the **Object Info** will be shown immediately, without a list of search results (since that list would contain only one item!)
## Common Object Lists
This section contains lists of the most commonly-known objects in the sky (e.g. planets, stars, deep sky objects, etc). Choose a list to display the most commonly-known objects in that category. For example, the **"Planets" list** shows the major planets in our solar system; the **"Brightest Stars" list** shows the brightest stars in the sky; the **Messier Objects** list shows the most famous 110 star clusters, nebulae, and galaxies, etc.
Objects currently above the horizon are listed with a brighter text color. Objects below the horizon are dimmed, but you can still select them. Choose a specific object from this list to bring up the **Object Info** view. This view displays basic information about the object, and contains buttons to center it in the sky chart or in your telescope's field of view.
**Tonight's Best** is a list of the best objects that will be visible between tonight's dusk and tomorrow's dawn. The objects in this list change depending on your location, and on the date. An object must reach at least six degrees above the horizon between astronomical dusk and dawn to be included in this list.
In SkySafari's basic version, **Tonight's Best** list includes only brightest stars and planets visible to the naked eye, and the brightest and best-known deep sky objects that can be seen with a pair of binoculars. SkySafari Plus and Pro add the best double and variable stars, star clusters, nebulae, and galaxies visible in small backyard telescopes. A few objects of extreme astrophysical or historical importance are also in the list, even if they're difficult or impossible to see in a backyard telescope - like Barnard's Star, Halley's Comet (at least until 2061), and Eris - the "dwarf planet" which dethroned Pluto as the solar system's outermost planet.
Objects in the list are sorted by their transit times, giving you a natural order in which to observe them. If you are viewing **Tonight's Best** list during daylight hours, many objects toward the end of the list may not have risen yet, and so are dimmed in the list. Similarly, if you are viewing **Tonight's Best** list in the early hours before dawn, objects near the start of the list may have already set, and so are also dimmed.
## Sorting and Highlighting List Objects
In SkySafari Plus and Pro, you can change the way objects in a list are sorted. You can do this both with common object lists, and custom observing lists.
To change the way a list is sorted, first tap **Filter & Sort** at the top right of the screen. Then choose the value to sort the objects by. You can sort objects by their name, catalog number(s), magnitude, distance, constellation, rise/transit/set times, or coordinates in the sky. Under Filter By, select '**Visible Tonight**' to only display objects visible during darkness.
You can highlight objects in a list, to show their distribution in the sky. You can do this both with common object lists, and custom observing lists.
To highlight a list of objects, turn on the **Highlight Objects** switch at the top of the list. Objects in that list will then be highlighted with blue circles in the sky chart. The objects will be highlighted even if they are fainter than the sky chart's current magnitude limit, so you can easily find them.
Only one object list can be highlighted at a time. If you turn on the **Highlight Objects** switch for one list, SkySafari will turn it off for all other object lists.
When the **Highlight Objects** switch is turned on, a small list icon appears in the sky chart, right above the middle of the toolbar. Tapping this icon gives you the following choices:
* **Show List**: Returns you to the currently-highlighted object list, right at the point you last viewed the list.
* **Unhighlight List**: Turns off the list highlighting.
* **Select Next Object**: Selects and pans to the next object in the list following the currently selected object.
* **Surprise Me**: Selects and pans to a random object in the list that is currently above the horizon.
* **Edit Selected Object**: Opens a separate view to edit the object's in the list.
* **Show List in Galaxy View**: Opens the Galaxy View, showing where the list objects are located in and around the Galaxy. This feature is only available in SkySafari Plus and Pro.
## Custom Observing Lists
In SkySafari Plus and Pro, you can create a custom observing list. To create a custom list of objects, first tap the **"More" button** (looks like 3 vertical dots) at the top of the list. Then tap a button labelled **Make Into Observing List**. You can tap this button to convert your list of search results, or the common object list, or the **Tonight's Best** list, into a custom observing list.
Custom observing lists keep track of objects you want to observe, and record logs of your observations. By default, SkySafari comes with a single, empty observing list called "My Favorites". To create additional lists, tap the "**Observe**" button in the Toolbar, select **Observing Lists** and then tap the **Create List** button.
Please Note: this feature is only available in SkySafari Plus and Pro. For more information on observing lists, see the [**Observing Lists Help**](/app-specific/observe-help/observing-lists-help) section.
You can delete an observing lists by swiping left over the list name and selecting '**Delete**'.
To move or delete items inside an observing list, tap the "**Edit**" button at the top of the screen. Then tap and drag the "grip" icon on the right side of the list to move it around the screen. Tap and drag the - (minus) icon on the left side of the list to delete it. Tap the End Edit button at the top of the screen when you're finished.
# 3D Galaxy View
Source: https://userguide.skysafariastronomy.com/app-specific/selection-help/3d-galaxy-view
Documentation for 3D Galaxy View
## Galaxy View Help
Please Note: This feature is only available in SkySafari Plus and Pro.
Galaxy View helps you visualize the 3-D location of stars and deep sky objects. Using paired face-on and edge-on views of the Galaxy, it shows you where that cluster or nebula is actually located relative to the rest of the Galaxy - a three-dimensional perspective. The face-on image is an artist's rendition based on recent data from the Spitzer Space Telescope looking down from above the north galactic pole
Objects in the left, face-on view are always drawn overlaid on the galactic disk so they will be visible. This does not imply the object is actually in the northern galactic hemisphere. You should consult the right, edge-on view to see which hemisphere the object is actually in.
If Galaxy View is shown from the Object Info, the current object's location in the Galaxy is shown. You can also show the Galaxy view from the highlighted list's icon along the bottom of the chart. In this case, all objects in the highlighted list are show in the view. In either case, if an object is outside the current field of view, a blue line is drawn in the direction it will be found.
Share: Takes a snapshot of the view that may then be shared with others through Email, Facebook, iCloud Photo Sharing, etc.
Auto Zoom: If the selected object is outside the viewable area, this will will zoom out to make the object visible. If the selected object is very close to the Sun at the current zoom level, the command will zoom in to display the object better in relation to the Sun.
More - Tap this button to bring up additional options, including:
Show Spiral Arm Labels: Labels the various spiral arms in the Galaxy.
Show Constellation Sectors: Divides the Milky Way galaxy in the neighborhood of the Sun into sectors, where each sector corresponds to the Milky Way constellation you would see when looking in that direction. Showing the constellation sectors allows you to better understand which part of the Milky Way galaxy you are looking at when observing within a particular Milky Way constellation.
For example:
When looking at the Milky Way in Sagittarius and Scorpius, you are looking at the next spiral arm inward from the Earth toward the galactic core at galactic longitude 0°. This spiral arm is appropriately called the Sagittarius Arm.
Cygnus lies at 90° galactic longitude and looks lengthwise along our own spiral arm which is called the Orion Spur. This is looking in the direction toward which the Galaxy is rotating.
When viewing the Milky Way in Auriga and Orion you are looking directly away from the galactic center, back through our own spiral arm. This is in the direction of galactic longitude 180°.
Finally, the southern hemisphere constellation, Vela, lies near galactic longitude 270° and looks down an inter-arm gap in the direction from which the Galaxy is rotating as a whole.
Center On Sun: Centers the view on our Sun's location in the Galaxy.
Center On Selected Object: Centers the view on the selected object's location in the Galaxy.
Reset: Resets the view to a zoom level where the whole Galaxy is visible.
# Center Object Help
Source: https://userguide.skysafariastronomy.com/app-specific/selection-help/center-object-help
Documentation for Center Object Help
## Center Help
The Center button centers the selected object in the sky chart. Tap-and-hold (or double tap) your selected object to bring up a popup menu. Tap the Center icon to center the object in the sky chart.
Note: You can also tap-and-hold anywhere on the sky chart to center the selected object. Enable this feature in Settings > Appearance & Behavior by selecting "Center Selection on Long Press".
## Center Button in Toolbar
You can customize the Toolbar under Settings > Appearance & Help > Configure Toolbar to include the Center icon as one of the defaults. Use this button if the selected object has moved off screen, and you want to re-center it in the field of view. The selected object will stay centered if you zoom in or out, or animate the sky chart using the Time Flow controls.
If you are using your device's gyroscope or compass/altimeter, then tapping the Center button will not center the selected object directly. Instead, an arrow appears, leading you toward the selected object. Move your phone in the direction of the arrow to center the object in the field of view. When the object is centered, the arrow disappears, and your phone will be pointing toward the object's position in the sky.
The Settings > Appearance & Behavior > Chart Animations switch provides animated panning to objects that you select and center in the sky chart. If turned off, the chart jumps instantly to objects when you center them. Please Note: this feature is only available in SkySafari Plus and Pro.
# Graph Object Help
Source: https://userguide.skysafariastronomy.com/app-specific/selection-help/graph-object-help
Documentation for Graph Object Help
## Graph Help
The Graph offers a way of examining changes in an object's qualities over time. The horizontal axis of the Graph always plots time, while the vertical axis plots altitude in the sky.
## Plotting An Object On The Graph
Tap an object in the sky chart you wish to plot and then select the 'info' button. In the Object Info view, select "Graph Object" from the More button located at the bottom right of the screen. The Graph will open displaying a plot of your object.
You can also add the "Graph" option to the Tap & Hold object select menu. You can turn this option on or off using the "Configure Selection Menu" under Settings > Appearance & Behavior.
## Using the Graph
Only one selected object can be plotted on the Graph at any one time. For reference, the Moon and Sun are always plotted. You can select which objects are plotted by tapping on the switches found on the upper left of the graph.
Pinch with two fingers to zoom in or out on the graph.
The horizontal axis plots time, with the current time at the Graph's center. Swipe left or right with one finder to adjust the time.
Tap X when you are finished with the Graph.
# Measure From Help
Source: https://userguide.skysafariastronomy.com/app-specific/selection-help/measure-from-help
Documentation for Measure From Help
## Measure Help
## Measuring Distance
In SkySafari Plus and Pro, you can measure the distance between objects. Start by selecting an object you want to measure the distance from. Then 'tap and hold' the selected object to bring up the selection menu. Tap the "Measure" icon (looks like a ruler) from the menu. Tap on the object you want to measure to, and a line between your selected object and the object you initially selected will appear. The angular distance in the sky between the objects is shown at the end of the line, as well as the physical distance between those objects in space (if known).
Tap anywhere away from the distance measurement line to dismiss it or select "Measure" again from the 'tap and hold' selection menu.
# Object Info Help
Source: https://userguide.skysafariastronomy.com/app-specific/selection-help/object-info-help
Documentation for Object Info Help
Object Info Help
The Object Info view shows a variety of information about the selected object. It also contains English-language description and images of several hundred of the brightest and best-known objects in the sky. Swipe the Object Info view left to see the object data; swipe right to return to the description.
On iPads and other tablets, images are displayed in-line with object descriptions. On phones or other devices with smaller screens, you can tap on image links embedded in the descriptions to show full-screen images.
Buttons at the bottom of the view let you center the object in the sky chart, slew or align your telescope to the object, or - in SkySafari Plus and Pro - go into orbit around the object!
## Object Data
The exact information displayed depends upon the type of object you have selected (e.g. a star, planet, deep sky object, etc).At a minimum, SkySafari displays the following information for the object you selected:
Names - the object's proper name, and any alternate names by which it is commonly known.
Catalog Numbers - the object's numerical designation(s) in the catalogs of stars and deep sky objects most commonly used by astronomers. The object's best-known catalog numbers are listed first.
Description - the type of the object, and the constellation that it appears in.
Apparent Size or Separation- how large the object appears in the sky, or the component separation for double stars; measured in arcminutes (') or arcseconds ("). The full moon appears about 30 arcminutes across. Double stars are typically separated by a few arcseconds.
Visual Magnitude - how bright the object appears in the sky; smaller numbers imply a brighter object. Sirius, the brightest star in the sky, is magnitude -1.44; the faintest stars visible to the naked eye are about magnitude +6.5.
Distance - the distance to the object, if it is known. For solar system objects, the distance is displayed in miles, kilometers, or Astronomical Units; 1 AU is the distance from the Earth to the Sun, or about 149.6 million km. For stars and deep sky objects, the distance is given in light years or parsecs. One light year, the distance light travels in a year, is about 63,300 AU. One parsec is the distance from which the Earth's orbit appears 1 arcsecond in radius, and equals about 3.26 light years, or 206,265 AU.
RA and Dec - the object's Right Ascension and Declination describe its position in the Equatorial coordinate system used with printed star atlases. The Equatorial coordinate system rotates with the Earth, so the object's RA and Dec do not change (unless the object itself is moving!)
Azimuth and Altitude - the object's coordinates in the local Horizon coordinate system describe its current position in the sky. As the Earth turns, the object appears to move across the sky, so these coordinates change even if the object itself is not moving.
Rise and Set Times - when the object appears on the horizon for the current local day. For the Sun and Moon, rise/set times are when the upper limb of the visible disk appears on the horizon. Depending on your current latitude, and the object's declination, the object may not set (e.g. Polaris seen from the northern hemisphere); or it may not rise (e.g. the Sun from Antarctica in winter). Due to varying atmospheric conditions and local horizon obstruction, rise/set times should only be considered accurate to about a minutes.
Transit Time - if the object is visible from your location on the current date, the transit time is when the object crosses the meridian and appears highest in the sky.
Angular Separation - SkySafari Plus and Pro show the object's angular separation and position angle from the Sun, from the last object you selected, and from the chart center.
In SkySafari Plus and Pro, when you're in orbit around another solar system object, the Object Info view provides all information about an object as it is seen from your perspective in orbit. For example, it gives the constellation in which the object appears, and the object's visual magnitude and distance, as seen from your simulated location in space - not as seen from Earth.
Events with a specific time have a small clock icon on the right. Tapping the clock will take you to that time and center the selected object, allow you to see the simulated event in the sky chart.
## Other Controls
Along the top of the Object Info view you might see two buttons which let you play an audio tour for the selected object or listen to a pronunciation of its name. The audio tour and pronounce feature is only available for the brightest objects. Objects might have both an audio tour and a pronunciation available but not always.
Audio Tour - provides an audio narration to learn the history, mythology, and science of the object. You can press the Audio Tour icon (looks like a speaker) to pause the narration. Please Note: this button is only present in SkySafari Plus or Pro and available as an in-app purchase (Cosmos Collection) in SkySafari.
Pronounce - helps you learn how to correctly pronounce the names of hundreds of celestial objects from different categories such as stars, constellations and planets. Tap the underlined pronounce text icon on the top right of Object view screen to listen. Please Note: this button is only present in SkySafari with the purchase of the Cosmos Collection in-app.
Along the bottom of the Object Info view are other buttons which let you center the object in the sky chart, go into orbit around it, slew your telescope to the object, or align the scope on the object.
Center - this button centers the object in the sky chart. See the Center button Help for more information.
If you are using your device's gyroscope or compass/altimeter, then tapping the Center button will not center the selected object directly. Instead, an arrow appears, leading you toward the selected object. Move your phone in the direction of the arrow to center the object in the field of view. When the object is centered, the arrow disappears, and your phone will be pointing toward the object's position in the sky.
iOS Users Please Note: for best results with the compass, turn your phone sideways to landscape mode.
Orbit - this button lets you leave Earth and orbit the object, if it's a solar system object. See the Orbit button Help for more information. Please Note: this button is only present in SkySafari Plus on Pro!
GoTo and Align - If you have connected with a GoTo telescope using your mobile device's Wi-Fi or bluetooth capability, or with SkyWire, additional buttons appear. These let you slew (GoTo) the object with your telescope, or to Align your telescope on the object. See the Scope Control view for more information about this.
Galaxy View - see detailed description below. Please Note: this button is only present in SkySafari Plus on Pro!
More - Tap this button to:
Add the object to an observing list
Log a new observation of the object
View all your logged observations of the object
See all observing lists containing this object
Download a Deep Sky Survey (DSS) image of the object
If you are adding an object to an observing list you only have one list, the object will be added to that list. If you have more than one list, SkySafari will let you choose which list you want to add the object to. See the Observing Lists Help for more information.
Please Note: This feature is only available in SkySafari Plus and Pro.
## Galaxy View
Please Note: This feature is only available in SkySafari Plus and Pro.
Galaxy View helps you visualize the 3-D location of stars and deep sky objects. Using paired face-on and edge-on views of the Galaxy, it shows you where that cluster or nebula is actually located relative to the rest of the Galaxy - a three-dimensional perspective. The face-on image is an artist's rendition based on recent data from the Spitzer Space Telescope looking down from above the north galactic pole
Objects in the left, face-on view are always drawn overlaid on the galactic disk so they will be visible. This does not imply the object is actually in the northern galactic hemisphere. You should consult the right, edge-on view to see which hemisphere the object is actually in.
If Galaxy View is shown from the Object Info, the current object's location in the Galaxy is shown. You can also show the Galaxy view from the highlighted list's icon along the bottom of the chart. In this case, all objects in the highlighted list are show in the view. In either case, if an object is outside the current field of view, a blue line is drawn in the direction it will be found.
Share: Takes a snapshot of the view that may then be shared with others through Email, Facebook, iCloud Photo Sharing, etc.
Auto Zoom: If the selected object is outside the viewable area, this will will zoom out to make the object visible. If the selected object is very close to the Sun at the current zoom level, the command will zoom in to display the object better in relation to the Sun.
More - Tap this button to bring up additional options, including:
Show Spiral Arm Labels: Labels the various spiral arms in the Galaxy.
Show Constellation Sectors: Divides the Milky Way galaxy in the neighborhood of the Sun into sectors, where each sector corresponds to the Milky Way constellation you would see when looking in that direction. Showing the constellation sectors allows you to better understand which part of the Milky Way galaxy you are looking at when observing within a particular Milky Way constellation.
For example:
When looking at the Milky Way in Sagittarius and Scorpius, you are looking at the next spiral arm inward from the Earth toward the galactic core at galactic longitude 0°. This spiral arm is appropriately called the Sagittarius Arm.
Cygnus lies at 90° galactic longitude and looks lengthwise along our own spiral arm which is called the Orion Spur. This is looking in the direction toward which the Galaxy is rotating.
When viewing the Milky Way in Auriga and Orion you are looking directly away from the galactic center, back through our own spiral arm. This is in the direction of galactic longitude 180°.
Finally, the southern hemisphere constellation, Vela, lies near galactic longitude 270° and looks down an inter-arm gap in the direction from which the Galaxy is rotating as a whole.
Center On Sun: Centers the view on our Sun's location in the Galaxy.
Center On Selected Object: Centers the view on the selected object's location in the Galaxy.
Reset: Resets the view to a zoom level where the whole Galaxy is visible.
# Orbit Object Help
Source: https://userguide.skysafariastronomy.com/app-specific/selection-help/orbit-object-help
Documentation for Orbit Object Help
## Orbit Help
The Orbit button lets you leave Earth behind, and orbit the Sun, other Solar System objects, and even nearby stars. Please Note: this feature is only available in SkySafari Plus and Pro! In the basic version of SkySafari for iOS, you can unlock the Orbit feature with an in-app purchase.
## Entering Orbit and Returning to Earth
To orbit a solar system object or another star, first select one by tapping on it in the sky chart, or by searching for one with the Search view. Then tap and hold your selected object to open the selection popup and select the Orbit button (looks like a rocket). Alternatively, you can open the Object Info view for an object and tap the Orbit button on the bottom left of the screen. In a few seconds, you'll fly through millions of miles of space into orbit near the object you selected.
If you select the Sun, you'll fly to a location 100 Astronomical Units above the Sun, where you can see the entire solar system as a whole. From there, you can select any other solar system object and fly into orbit around it.
When you want to go home to Earth, tap the small Earth icon at the bottom of the sky chart. SkySafari will fly you back to same Earthly location you left earlier.
## Navigating in Orbit
When you're orbiting a solar system object or nearby star, that object stays locked at the center of the sky chart. Swiping the chart moves you around the object. Two new buttons at the bottom right of the sky chart let you fly toward or away from the object you're orbiting. The status bar above the sky chart indicates your distance from the object.
You can magnify the field of view by pinching and zooming, just as you can when viewing from Earth. Zooming will not move you toward or away from the object you're orbiting; it simply changes the sky chart's field of view. A planet can appear very large in the sky chart because you're far away from it but highly zoomed in, or because you're zoomed out but very close to the planet. Usually the distinction is obvious, but this is one thing to note in case you become confused.
While you're orbiting another star or solar system object, you can center the sky chart on a different object by tapping it and selecting the Center Object button from the tap and hold selection menu, or by searching for it and tapping the "Center" button in the Object Info window. If you do this, swiping the chart will no longer move you around the object you're orbiting; it will simply pan the field of view. To resume orbiting the object, tap it to select it again, then tap and hold the object to either Center or Orbit.
## Using SkySafari in Orbit
When you're orbiting another star or solar system object, certain SkySafari features are not available. For example, you cannot use the compass or gyroscope, and you cannot use any telescope control features. These features are only designed to work when you're observing from the Earth's surface!
SkySafari also adjusts some display settings when you leave Earth and enter "orbit mode". For example, planet and moon orbits are automatically displayed, and constellation lines are hidden. The maximum field of view width is restricted to 90 degrees. SkySafari does these things to provide a clearer display. When you return home to Earth, your previous display settings are restored.
When you're in orbit around another star or solar system object, the Object Info view provides all information about an object as it is seen from your perspective in orbit. For example, it gives the constellation in which the object appears, and the object's visual magnitude and distance, as seen from your simulated location in space - not as seen from Earth.
When you are orbiting another star, SkySafari only displays stars in the Hipparcos catalog, and nearby stars whose distances are well known. SkySafari does not display faint Tycho or Guide Star Catalog stars, because their positions in three-dimensional space are unknown. Therefore their apparent positions when seen from outside our Solar System cannot be accurately depicted.
The Settings > Appearance & Behavior > Chart Animations switch controls the animation to and from stars or solar system objects that you wish to orbit. If turned off, you will jump instantly into orbit around objects when tapping Selection>Orbit Object instead of experiencing a few seconds of animated "flight".
# Livesky Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/account-help/livesky-help
Documentation for Livesky Help
LiveSky Help
Your SkySafari user data is backed up automatically by storing it on your device and synchronizing it with our cloud server. Although not recommended, this automatic backup can be disabled in the Storage settings.
When you first launch the app, you are assigned an automatically generated and anonymous username and password. The app uses this information to backup your data on our server. The username and password combination is stored in your device's keychain, allowing you to restore your data if the app is reinstalled.
## Sign Up For LiveSky
Sign up for LiveSky to enjoy convenient access to your SkySafari app data on any web browser from any device. Set up an account by entering your email and password in the 'Account' section of the Settings view.
After you sign up, 'Account' becomes a view of your account. From here you can change your email address, request another verification email, launch the LiveSky.com portal, or switch to another account.
SkySafari includes a free Basic Subscription to LiveSky, providing you with access to your data. An optional Premium subscription allows you to edit and manage your data, in addition to providing you with access to an online version of SkySafari that can be accessed from any browser on any device.
# Appearance And Behavior Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/display-options-help/appearance-and-behavior-help
Documentation for Appearance And Behavior Help
## Appearance & Behavior Help
The settings in this view let you control the use of color in your sky charts, and set some preferred behaviors for the app.
## Chart Appearance
These options let you control the appearance of your sky charts.
Application Theme: Select from five different color themes to customize your experience.
Color Chart: Displays stars, planets, and other objects in the sky chart using full color.
Monochrome Chart: Displays stars, planets, and other objects in the sky chart using grayscale, with light objects on a dark background.
Inverse Monochrome Chart: Displays stars, planets, and other objects in the sky chart using grayscale, with dark objects on a light background. This makes the sky chart appear as a photographic negative, and may make it easier to see for those with poor vision.
Screen Brightness: Sets the brightness level of the screen. Turning this down, especially when using the Night Vision theme, may help preserve your visual dark adaptation, as well as save battery life.
When using the Night Vision theme, the sky chart is always drawn with red objects on a dark background (or dark objects on a red background, if you're using Inverse Monochrome sky charts). Again, this is to preserve your night vision - red light affects your dark adaptation much less than white light.
Font: Sets the font size for text displayed in the sky chart.
Use English: Sets the app language to english, if your system language is different.
## Interface Elements
These options let you display additional information or control elements in the sky chart.
Show Coordinates & FOV: When turned on, the chart center coordinates and field of view are always displayed at the top of the sky chart. You can turn them off if you want to see the sky chart without any coordinates superimposed.
Display Location & Date: Displays your current location and date underneath the status bar.
Display Zoom Buttons: Toggles on a set of Zoom buttons (+ / -) in the sky chart.
Display Compass & AR Buttons: (iOS only) Toggles the Compass/AR button on the upper right of the sky chart.
Display Onscreen Info Button: Displays an Info button next to a selected object label. Tap the Info button to open Object Info. When this option is switched off, the Info button is removed and automatically added to the 'circular' selection menu that is accessed by double-taping or long-pressing an object in the sky chart.
Configure Toolbar: Lets you show/hide and rearrange the items on the main sky chart toolbar. Tap this item to show the list of toolbar items. On iOS, you can have 5 buttons visible on your phone in portrait mode, or 10 buttons visible in landscape and in tablet devices. To change the ordering of items on iOS, drag the "grip" on the right side of an item in the list to move it. On Android, tap-hold and drag the item to move it. Please Note: this feature is only available in SkySafari Plus and Pro.
## Behavior
Ambient Sound: Controls the use of ambient sound in the app. Tapping the entry will display a sound picker where you can choose from several ambient sounds or none at all. You can add you own custom ambient sounds to the list. On iOS, place them in the Documents directory using iTunes file sharing. On Android, add them to the app's SkySafari directory on the SD card.
Sound Effects: Controls the use of sound in the app. When turned on, SkySafari plays sounds in response to events such as selecting a new object, connecting to a telescope, and so on. When turned off, SkySafari does not play sounds.
Use Voice Control: Enables the use of simple voice commands in the app. When turned on, a microphone icon appears on the bottom left of the sky chart. Tap the microphone icon to issue your voice command. It will listen for up to 15 seconds after it has last tried to process a command. Please Note: this feature is only available in the iOS versions of SkySafari.
There are three types of commands accessible via voice control.
## Toolbar button commands
You can perform the equivalent of pressing toolbar buttons by saying the name of the button. This also applies for the buttons in the Scope Control panel (Connect, GoTo, Align, Lock). In addition, to change the slew rate in the Scope Control panel you can say "Rate x" where x is 1,2,3, or 4. Saying "toolbar" toggles the toolbar visibility.
## Show, Hide, Toggle commands
Although you can show the time flow panel by saying "Time", it might feel more natural to say "Show Time", "Hide Time" or "Toggle Time". This also works for Scope.
You can also do commands like "Show Settings" rather than "Settings". Show should work for most other screens or panels that can be shown from the main chart view.
## Sky Object commands
There are a series of commands that work on sky objects. These are:
"center on "
"center "
"find information for "
"find info for "
"find "
"select "
"go to "
"orbit "
"search for "
"search "
"show information for "
"show info for "
"show "
Just follow the command with the name of the object.
Note: Recognizing the name of the object is somewhat problematic and we are still trying to fine tune this. For example, sometimes saying "Messier 15" works better than "M 15". Sometimes you need to try speaking a bit slower and more distinctly. And generally saying a star name will not work. Apple's speech recognition dictionary doesn’t have all the star names in it. For these reasons, please be aware that we consider this feature "in development".
Other Commands:
"zoom in"
"zoom out"
"stop" (stops a GoTo)
"north", "south", "east", "west" changes the direction the sky chart is facing.
"done" (will dismiss most screens with a "X" button. Also will dismiss the Selection, Observe, and Tonight popups). This will also stop voice listening if there are no panels to dismiss.
"stop recording" (stops voice listening).
Tilt to Use Compass: When turned on, you can tilt your iPhone, iPad, or Android device upward to activate the compass. Once activated this way, touch the screen anywhere to turn the compass off. Turn "Tilt to Use" off if you find that you're accidentally activating the compass too often, or if you prefer to activate it from the main toolbar.
Please Note: the compass will be turned off if you connect to a telescope, or lock on the telescope's position in the sky chart. The sky chart cannot be centered on the telescope's position, and centered on the coordinates reported by the compass, at the same time. See the Scope Control view Help page for more information.
If your device does not have a compass, this option is disabled. When the compass has been activated, the coordinate system will change to Horizon coordinates (see above).
Prevent Sleep: When turned ON, this prevents the device from sleeping while SkySafari is active. This allows a continuous connection to a telescope. Please Note: this feature is only available in the SkySafari Plus and Pro.
Redden Keyboard in Night Vision: When turned ON, SkySafari reddens the keyboard when displayed in Night Vision mode. Normally this is desired but you may want to turn this OFF when using a custom keyboard that is already reddened or when using a red film over the screen.
Allow Auto Rotation: When turned on, the main sky chart and other views automatically rotate as you turn your device from portrait to landscape mode. When turned off, all views stay in portrait mode, regardless of how you are holding your device.
On the iPad, this setting overrides the hardware rotation lock. In other words, if the auto rotation setting is turned off, all of SkySafari's views will remain in portrait mode even if the iPad's hardware rotation lock is disabled. You may want to have other iPad apps auto-rotate, but keep SkySafari in portrait orientation.
# Constellations Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/display-options-help/constellations-help
Documentation for Constellations Help
## Constellations Help
The settings in this view let you control the display of constellations - the mythical figures associated with the stars that have been formalized in modern times to define precisely-bounded areas of the sky. These settings can also display asterisms, which are star patterns that are commonly known, but not formally recognized as constellations.
## Constellation Display
Show Constellations: Sets whether constellations are displayed in the sky chart. When turned off, most of the settings in this section are disabled. When turned on, multiple constellation display options can be selected below. For example, you can display traditional constellation lines overlaid with mythical figures, or modern constellation lines and IAU boundaries at the same time.
as Traditional Lines: Displays constellations as stick-figures, using traditional line patterns, connecting the brighter stars in each constellation.
as Modern Lines: Displays constellations as more complex stick figures, using modern patterns based on H.A. Rey's 1952 book, "The Stars: A New Way to See Them". These complex lines are more suggestive of the constellations' mythology.
as Mythical Figures: Displays constellations as the images of the mythological figures associated with these star patterns. SkySafari only shows constellation images near the central part of the sky chart; other constellation images will fade in and out of view as you pan around the chart. Constellation images are used with permission from Kosmic Kreations, [www.kosmickreations.net](http://www.kosmickreations.net). Original constellation artwork was created by Johan Meuris and released under the Free Art License.
as IAU Boundaries: Displays constellation boundaries in the sky chart. The constellations' official boundaries were formalized by the International Astronomical Union in 1930, using the precession epoch of 1875. Note the constellation boundaries are offset from the lines of Right Ascension and Declination in today's equatorial coordinate system, especially near the celestial poles. The offset results from the fact that precession has rotated the sky by nearly 2 degrees since 1875!
Show Zodiac Only: If this switch is turned on, SkySafari only shows the twelve constellations of the Zodiac - the constellations that the Sun appears to pass through as the Earth orbits around it each year. If this switch is turned off, SkySafari shows all constellations.
Intensity: Displays a slider which lets you control the brightness of the constellation lines and labels. Use this to adjust the visibility of the constellations, versus the stars which make them up.
Tap to Select: Sets whether constellations can be selected by tapping the area inside the constellation's boundaries on the sky chart with your finger. When turned off, constellations cannot be selected by tapping - only planets, stars, and deep sky objects.
## Constellation Names
Show Names: Sets whether the sky chart displays the names of the constellations.
Use Abbreviations: Sets whether the sky chart displays constellations names using their official IAU abbreviations (e.g. "CMa") or their fully-spelled-out Latin names (e.g. "Canis Major").
## Asterism Display
Show Asterisms: When turned on, asterism outlines are displayed in the sky chart. Asterisms are commonly-known star patterns that are not formally recognized as constellations, or that span multiple constellations. Examples include the Big Dipper, in the constellation Ursa Major; and the Summer Triangle, which contains the brightest stars in the constellations Lyra, Cygnus, and Aquila.
Show Names: When turned on, asterism names are displayed in the sky chart. You can display asterism names independently of asterism outlines.
# Deep Sky Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/display-options-help/deep-sky-help
Documentation for Deep Sky Help
## Deep Sky Objects Help
The settings in this view let you control the display of star cluster, nebulae, and galaxies - including the selection of deep sky objects that are shown, and the labelling of objects with their names or catalog numbers.
## Deep Sky Object Display
Show Objects: Draws symbols for star clusters, nebulae, and galaxies in the sky chart. When turned off, most of the other settings in this view are disabled.
Show Images: Displays images of deep sky objects in the sky chart. When turned on, Digitized Sky Survey images of several hundred best-known deep sky objects are drawn at their true size and orientation in the sky chart. Deep sky images can be displayed independently of deep sky object symbols (above), and vice-versa.
Best-Known Only: Sets whether only the best-known deep sky objects are shown in the sky chart. These objects include the Messier objects, the Caldwell objects, and any other deep sky objects with a proper or common name.
The Messier Catalog is a famous list of 110 prominent deep sky objects compiled by the 18th century astronomer Charles Messier. The Messier catalog includes some of the most prominent star clusters, nebulae, and galaxies visible from the northern hemisphere, such as the Hercules Cluster (M 13) and the Whirlpool Galaxy (M 51).
The Caldwell Catalog is a modern complement to Messier's list, compiled in 1995 by the British astronomer Patrick Caldwell-Moore. It includes additional 110 "Messier-quality" deep sky objects which Messier missed, many because they are only observable from the southern hemisphere. Together, the Messier and Caldwell lists include most of the deep sky objects easily visible in backyard telescopes from both hemispheres.
Please Note: this feature is only available in the SkySafari Plus and Pro.
Show in Wide Fields: allows deep sky objects to be displayed when the field of view is wider than 45 degrees. This option is turned off by default, since deep sky objects can only be seen through binoculars or telescopes, which have very small fields of view. However, turning this option on may let you see the distribution of (for example) galaxies across wide areas of the sky.
Please Note: this feature is only available in SkySafari Plus and Pro.
Magnitude Limit: Sets the deep sky object magnitude limit. This determines the faintest deep sky objects that are visible in the sky chart. The brighter an object, the lower its magnitude. The magnitude limit will change automatically as you zoom the sky chart in and out. When zoomed in, fainter objects are displayed.
Never Show Fainter Than: This sets the absolute magnitude limit of the faintest deep sky objects visible. When you zoom in, objects fainter than your set limit are never displayed.
Intensity: Sets the brightness used to display deep sky object symbols and names. Move the slider to vary the brightness from 0% (black) to 100% (white).
## Deep Sky Object Name Display
Show Names: Sets whether deep sky objects' names are displayed next to the objects in the sky chart.
Proper Names: Sets whether proper names are displayed for deep sky objects, when possible. When turned off, deep sky objects names are always shown using catalog numbers (e.g. "M 13") instead of proper names (e.g. "Hercules Cluster").
Name Density: Sets the percentage of deep sky objects whose names are displayed on the sky chart. At 0%, no objects have names shown; at 100%, all objects have names shown. At 80%, the brightest 80% of deep sky objects have names shown.
## Deep Sky Object Type Selection
Please Note: this section is only available in SkySafari Plus and Pro.
Globular Clusters: Sets whether globular clusters are displayed in the sky chart. These are dense concentrations of stars, typically containing tens of thousands to millions of stars. These massive clusters are among the oldest objects in our galaxy. Examples are M 13 in Hercules and M 22 in Sagittarius.
Bright Nebulae: Sets whether bright nebulae are displayed in the sky chart. These are glowing clouds of gas usually found in the disk of the Milky Way. These nebulae glow either from the reflection of light from nearby stars or from the emission of light produced by nearby stars heating the nebulae. Examples are M 42 (the Great Orion Nebula) in Orion and M 20 (the Trifid Nebula) in Sagittarius.
Dark Nebulae: Sets whether dark nebulae are displayed in the sky chart. These are opaque clouds of cold dust which obscure the light from the stars behind them. They are mostly located along the Milky Way. Examples are B 33 (the Horsehead Nebula) in Orion, and the Coal Sack in Crux.
Planetary Nebulae: Sets whether planetary nebulae are displayed in the sky chart. These are expanding shells of gas expelled from a star late in its life. A round, planet-like appearance led to the name "planetary nebulae" in the eighteenth century, though there is no actual connection with planets. Examples are M 57 (the Ring Nebula) in Lyra and M 27 (the Dumbbell Nebula) in Vulpecula.
Galaxies: Sets whether galaxies are displayed in the sky chart. Galaxies are immense star systems outside of our own Milky Way galaxy; many are larger than our own. The total number of galaxies is in the billions, and they extend to the edge of the known universe. Most galaxies are classified as spiral galaxies, elliptical galaxies, or irregular galaxies, based on their appearance. Examples are M 31 (spiral) in Andromeda, M 87 (elliptical) in Virgo, and the Small Magellanic Cloud (irregular).
# Grids And Reference Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/display-options-help/grids-and-reference-help
Documentation for Grids And Reference Help
## Grids & Reference Help
The settings in this view let you show or hide grids which display the major celestial coordinate systems, as well as the reference lines and points that those systems are based on.
## Celestial Coordinate Grid
Show Grid: Sets whether a celestial coordinate grid is displayed on the sky chart. When turned on, the following items are enabled:
with Horizon Coordinates: displays an alt-azimuth coordinate grid on the sky chart.
with Equatorial Coordinates: displays a right ascension/declination grid on the sky chart.
with Ecliptic Coordinates: displays an ecliptic longitude/latitude grid on the sky chart.
with Galactic Coordinates: displays a galactic longitude/latitude grid on the sky chart.
Please Note: Ecliptic and Galactic coordinate grids are only available in SkySafari Pro.
## Reference Lines
Celestial Equator: Sets whether the celestial equator is displayed on the sky chart. The celestial equator is the plane of the Earth's equator projected onto the celestial sphere.
Galactic Equator: Sets whether the galactic equator is displayed on the sky chart. The galactic equator is the plane of the Milky Way galaxy projected onto the celestial sphere.
Ecliptic Path: Sets whether the Ecliptic path is displayed on the sky chart. The Ecliptic is the plane of the Earth's orbit projected onto the sky. It is also the annual path of the Sun around the celestial sphere.
Meridian Line: Sets whether the meridian is displayed on the sky chart. The meridian is the projection of your longitude on Earth onto the celestial sphere. It extends from the northern horizon through the zenith to the south cardinal point on the horizon. An object is said to transit when it crosses the meridian.
## Reference Line Marks
Celestial Equator Ticks: Adds bars markers at every 10 minutes of arc and labels at each hour of arc on the celestial equator line.
Ecliptic Ticks: Adds date bar markers and labels along the ecliptic line.
Meridian Ticks: Adds bar markers at 1 degree intervals and labels every 10 degrees on the local meridian line.
## Reference Points
Celestial Poles: Sets whether the celestial poles are displayed on the sky chart. The celestial poles are where the Earth's polar axis (i.e. the line perpendicular to the plane of the Earth's equator) intersects the celestial sphere. The north and south celestial poles are currently in the constellations Ursa Minor and Octans, but they move slowly over the centuries due to precession.
Galactic Poles: Sets whether the galactic poles are displayed on the sky chart. The north and south galactic poles are where a line perpendicular to the plane of the Milky Way galaxy intersects the celestial sphere. They are currently located in the constellations Coma Berenices and Sculptor, respectively.
Ecliptic Poles: Sets whether the ecliptic poles are displayed on the sky chart. The ecliptic poles are where a line perpendicular to plane of the Ecliptic intersects the celestial sphere. The north and south ecliptic poles are in the constellations of Draco and Dorado, respectively.
Zenith & Nadir: Sets whether the zenith and nadir are displayed on the sky chart. This marks and labels the points directly overhead and underneath your feet.
Equinox Markers: Displays markers for the Vernal and Autumnal Equinoxes. The Vernal Equinox is the sun’s position relative to the stars on the first day of spring (in the Northern Hemisphere), while the Autumnal Equinox is the sun’s position relative to the stars on the first day of fall. More precisely, they are the sun’s positions in the sky at the two exact times when the plane of Earth’s equator is the same as the plane defined by Earth’s revolution around the sun.
Solstice Markers: Displays markers for the Summer and Winter Solstices. The Summer Solstice is the sun’s position relative to the stars on the first day of summer (in the Northern Hemisphere), while the Winter Solstice is the sun’s position relative to the stars on the first day of winter. More precisely, they are the sun’s positions in the sky at the two exact times when the plane of Earth’s equator is inclined at the largest angle (about 23.5 degrees) to the plane determined by Earth’s revolution around the sun.
# H R Diagram Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/display-options-help/h-r-diagram-help
Documentation for H R Diagram Help
## H-R Diagram Help
Please Note: This feature is only available in SkySafari Plus and SkySafari Pro.
The Hertzsprung-Russell Diagram (H-R Diagram) is a special kind of graph that tells us about a star's age and its mass. Each dot on the diagram represents a star. The diagram plots absolute magnitude on the vertical axis versus star spectral class on the horizontal axis by default. Young stars that are still burning hydrogen in their cores are found in the "Main Sequence", the curve along the left side of the graph. The larger and heavier a star is, the farther it will be to the upper left along this curve. Older stars in the "red giant" phase of their lives are no longer on the main sequence, and will be found in the upper right corner of the H-R diagram. Finally, "white dwarf" stars will be found along the bottom left of the H-R diagram, beneath the main sequence.
The stars plotted in the H-R diagram are the same stars shown onscreen in the sky chart. You can tap on any star in the H-R diagram, and SkySafari will draw a marker around this star, allowing you to identify it. This makes it easy to identify stellar oddballs, such as white dwarfs, supergiants, and extremely massive main sequence stars. Conversely, you can tap any star in the sky chart, and SkySafari will highlight this star's position (with a marker) on the H-R diagram.
Tip: The H-R diagram is fully dynamic. If you scroll around the screen or change your field of view, the stars shown onscreen will change, and the H-R diagram will update to plot these new stars.
## Adjusting the H-R Diagram
The H-R Diagram can be moved around the sky chart by touching the middle and dragging it to a new location. It can also be resized by dragging on the corners or edges.
Visibility
Show H-R Diagram: Toggles the H-R Diagram on/off in the sky chart.
Show Region Labels: Labels the areas of red giants, white dwarfs, and main sequence stars.
Show Main Sequence: Draws a line representing stars on the main sequence. Any star that appears close to this line is probably a main sequence star (a star that is still burning hydrogen fuel).
Show Grid Lines: Draws gridlines from the vertical and horizontal axes.
## X-Axis
Color: Uses a star's B-V color as the field along the horizontal axis. Hotter stars have a lower B-V value.
Temperature: Plots star temperature (in thousands of degrees Kelvin) along the horizontal axis. Degrees Kelvin are equal to degrees Celsius + 273.
Spectrum: Plots star spectral class along the horizontal axis. There are seven main types of stars. In order of decreasing temperature, O, B, A, F, G, K, and M. The Sun is a G type star (specifically G2V), a yellow dwarf and a main sequence star.
## Y-Axis
Absolute Magnitude: Absolute magnitude tells us how bright objects would appear if they were all at the same distance (the distance we use is arbitrary, but has been chosen to be 10 parsecs). As with visual magnitude, a lower absolute magnitude means a brighter body.
Visual Magnitude: Tells us the brightness of a star in the sky as seen by an observer on Earth. The brighter an object appears, the lower its magnitude value. The Sun, at visual magnitude of −26.7, is the brightest object in the sky.
Luminosity: Amount of light and energy emitted by a star.
Note: White dwarfs are very dim and few are in SkySafari's star catalogs (the Hipparcos/ Tycho catalogs), so few will be found in SkySafari's H-R diagram.
# Horizon And Sky Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/display-options-help/horizon-and-sky-help
Documentation for Horizon And Sky Help
## Horizon & Sky Help
The settings in this view let you control the display of the local horizon, and the sky background. In SkySafari Plus and Pro, note that the horizon is not visible if you are not displaying the sky chart using horizon coordinates! Use horizon coordinates to show the horizon.
## Horizon Display
Show Horizon & Sky: Turns the local horizon and sky background display off or on. When turned off, most of the other settings in this section are disabled.
as Transparent with Line: With a transparent horizon, objects below the horizon are visible, as if the Earth were transparent. The horizon line itself is still visible.
as Translucent Area: With a translucent horizon, objects below the horizon are visible, but so is the Earth.
as Opaque Area: With an opaque horizon, no objects are visible below the horizon.
as Panoramic Image: With a horizon panorama, the horizon is drawn as a realistic image that moves as you pan and zoom around the chart. You can select a specific panorama from the list in the section below.
Show Cardinal Points: Sets whether the cardinal points are displayed along the horizon line. Cardinal points label the north, east, south, and west directions on the horizon.
Show Daylight: When turned on, the sky color changes with the cycle of day and night. When turned off, the sky background color is always black.
Show Horizon Glow: When turned on, the horizon is drawn with a soft glow that increases with daylight to simulate atmospheric haze. When turned off, the horizon is always drawn against a clear sky background.
Horizon Altitude: Lets you determine when objects rise and set above a specific altitude. Selecting this item leads to a slider that lets you set the horizon altitude used to compute rise/set times, and to draw the horizon line.
Objects below the altitude you select will be considered below the horizon. You can use this to quickly identify objects which - even though technically above zero degrees altitude - are still too low in the sky to be easily observed. It's usually not worthwhile to observe objects below an altitude of 10 - 15 degrees, since they are lost in atmospheric haze.
Please note: this feature is only available in SkySafari Plus and Pro.
## Horizon Panoramas
This section lists the horizon panoramas that are available in SkySafari. The currently-selected panorama is shown with a check mark. The panorama is only displayed if you've selected Panoramic Image display option above. Choosing any item from the list of horizon panoramas will automatically select this option!
## Creating Your Own Horizon Panorama
You can create your own horizon panorama - for instance, an image of your own back yard, or your favorite observing site. You can then import it into SkySafari, to show the sky as it realistically appears from your location.
To do this, first create a panorama using your digital camera, and a panorama-stitching program like Adobe Photoshop, Canon PhotoStitch, or DoubleTake for Mac OS X. Resize your panorama image to dimensions of exactly 2048 pixels wide x 1024 pixels tall.
The vertical sweep of the image represents 180 degrees of altitude, from the zenith at +90° (top of the image), to the horizon at 0° (middle of the image) to the nadir at -90° (bottom).
The horizontal direction on your image represents 360° of azimuth, starting with north (0°) at the left edge, east (90°) one quarter of the way across, south (180°) halfway across, west (270°) three fourths of the way across, and finally wrapping around again to north (360° or 0°) at the right edge.
When you're done photoshopping, save your panorama as a 32-bit (8 bits per channel) RGBA color image file in PNG format. Make sure your image contains an alpha or transparency layer that accurately indicates the parts of your panorama that are opaque (the ground, trees, buildings, etc.) versus the parts that are transparent (the sky).
## Importing Your Horizon with the Files App
If you are using SkySafari on an iPhone, iPad, or iPod touch, you can import your horizon panorama using the Files app. To do this, open the Files app and navigate to the SkySafari folder located under the "On my iPhone/iPad" category. Now add your horizon PNG image file. Close and reopen SkySafari.
If everything works correctly, your image will then appear in the list of horizon panoramas in SkySafari. You can select and display it just like SkySafari's other built-in horizon panoramas.
If your horizon panorama doesn't appear in SkySafari's list, make sure it's in PNG format, and that its name ends with ".png". Make sure its dimensions are 2048 x 1024, and that it is a 32-bit (8 bit per channel) RGBA color image with an alpha (transparency) layer. If all else fails, you can email your image to Simulation Curriculum's technical support, and we can try to debug it for you.
## Importing Your Horizon with SD Card
If you are using SkySafari for Android, you can import your horizon panorama using your SD card. To do this, connect your Android device with a USB cable to a computer. Then mount your Android's SD card on your computer, so it appears as a disk. Look for a SkySafari, SkySafari Plus, or SkySafari Pro folder on the root (top level) of your SD card, depending on which version of SkySafari you own. Then copy your horizon PNG image file into the Horizon Panoramas folder within this folder.
For example, if you own SkySafari Pro, copy your panorama to the following directory on your SD card:
## \/SkySafari Pro/Panoramas/
If everything works correctly, your image will then appear in the list of horizon panoramas in SkySafari. You can select and display it just like SkySafari's other built-in horizon panoramas.
If your horizon panorama doesn't appear in SkySafari's list, make sure it's in PNG format, and that its name ends with ".png". Make sure its dimensions are 2048 x 1024, and that's a 32-bit RGBA color image with an alpha (transparency) layer. If all else fails, you can email your image to Simulation Curriculum technical support, and we can try to debug it for you.
# Milky Way Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/display-options-help/milky-way-help
Documentation for Milky Way Help
## Milky Way Help
The Milky Way is the visible concentration of stars, star clusters, bright gas clouds, and dark dust lanes that lie along the plane of our galaxy in the sky. The settings in this view control how the Milky Way is displayed in the main sky chart.
## Milky Way Display
Show Milky Way: Turns the Milky Way off or on. When turned off, the Milky Way is not drawn, and most of the other settings in this section are disabled.
as Framed Outline: shows the Milky Way's boundaries as a thin outline.
as Filled Area: shows the Milky Way region filled with a solid gray color.
as Realistic Image: shows a digital all-sky panorama of the Milky Way in visible light, provided by Axel Mellinger.
as Hydrogen Alpha Image: shows the sky in Hydrogen Alpha light, with an image produced by Doug Finkbeiner. This full-sky H-alpha map is a composite of the Virginia Tech Spectral line Survey (VTSS) in the north and the Southern H-Alpha Sky Survey Atlas (SHASSA) in the south. The view shows the distribution of glowing ionized hydrogen gas clouds in our galaxy's star-forming regions.
as 2MASS Infrared Image: shows the sky at near-infrared wavelengths, using data from the 2-micron all-sky survey.
as WISE Infrared Image: shows the sky in mid-infrared wavelengths, using data from NASA's WISE (Wide-Field Infrared Explorer) satellite. The colors in this image represent specific wavelengths of infrared light. Cyan (blue-green) represents light emitted predominantly from stars and galaxies at a wavelength of 3.4 microns. Green and red represent light mostly emitted by dust at 12 and 22 microns, respectively.
as IRAS Infrared Image: shows the sky at a far-infrared wavelength of 100 microns, using data from the IRAS (Infrared Astronomy Satellite) and COBE (Cosmic background Explorer) spacecraft. Galactic dust clouds are visible at these wavelengths.
as Planck Microwave Image: shows the distribution of cosmic microwave background radiation, as measured by the Planck satellite. Constructed from observations of the sky at wavelengths spanning 850 microns to 1 cm (353 GHz to 30 GHz).
as 408 MHz Radio Image: shows the sky observed at a frequency of 408 MHz by the Haslam radio survey. The Haslam 408 MHz map is derived from 4 separate surveys.
as 1420 MHz (21 cm) Radio Image: shows the sky observed at a frequency of 1420 MHz or a wavelength of 21 cm (the frequency of HI, neutral hydrogen) by the LAB (Leiden/Argentine/Bonn) radio survey.
as ROSAT X-Ray Image: shows the sky at soft X-ray wavelengths, as observed by the ROSAT satellite. The maps cover approximately 98% of the sky in the 1/4 keV, 3/4 keV, and 1.5 keV bands. In this 3-color image, red is 0.1 - 0.4 keV, green is 0.5 - 0.9 keV, and blue is 0.9 - 2.0 keV.
as Fermi Gamma-Ray Image: shows the sky at gamma rays frequencies, as observed by the Fermi spacecraft. This view shows how the sky appears at energies greater than 1 billion electron volts (GeV) according to five years of data from NASA's Fermi Gamma-ray Space Telescope. For comparison, the energy of visible light is between 2 and 3 electron volts.
Please Note: The options to show the Milky Way in Hydrogen Alpha thru Gamma Ray wavelengths are only available in SkySafari Pro.
## Milky Way Intensity
Intensity: Sets the brightness level of the Milky Way when shown as a filled area or realistic image.
Fade in Small Fields: When turned on, the Milky Way's intensity will fade to zero as the field of view decreases from 10 to 1 degrees wide. It is often not useful to show the Milky Way in very small fields of view.
# Notifications Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/display-options-help/notifications-help
Documentation for Notifications Help
## Notifications Help
SkySafari can notify you about transient events, bright satellites, and planet risings. These notifications are generated for next 24 hour period each time SkySafari is brought to the foreground. If you go for a day or so without using SkySafari, the notifications will stop.
Do Not Disturb: When turned on, no notifications will be delivered during the specified time period.
Planet risings: When turned on, SkySafari will notify you of Sun, Moon and planet risings.
Bright Satellites and Flares: When turned on, SkySafari will notify you when the International Space Station (ISS), the Tiangong Space Station, or the Hubble Space Telescope (HST) is rising for a visible pass. It will also notify you when an Iridium satellite is about to flare.
Transient Events: When turned on, SkySafari will notify you of moon phases, eclipses, planetary moon events, meteor showers and planetary phenomenon such as conjunctions, elongations and oppositions to name but a few of the events you can select to be notified about.
# Solar System Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/display-options-help/solar-system-help
Documentation for Solar System Help
## Solar System Help
The settings in this view control the display of planets, moons, and other "minor bodies" in the solar system (asteroids and comets), as well as artificial Earth-orbiting satellites.
## Planet & Moon Display
Show Lunar Feature Outlines: Displays outlines of selected Moon features, such as craters.
Lunar Surface Features: use this setting to select which lunar outline features are displayed when the “Show Lunar Feature Outlines” option is enabled.
Show Planets: Displays planets and moons in the sky chart. When turned off, most of the other settings in this section are disabled.
with Grids: Shows planets and moons with surface coordinate grids. Planet grids show the orientation of the planet's equator and rotational axis. The equator and prime meridian are drawn as bold lines; other longitudes/latitudes are shown with lighter lines.
with Axes: Shows the rotational axes of planets and moons. Their north poles are drawn as bold lines; south poles are shown with lighter lines. Note: this option is only available in SkySafari Plus and Pro.
with Phases: Shows planets and moons with their night sides shaded in a darker color. When turned off, planets shown as fully illuminated, without any night side shading.
with Atmospheres: Shows the atmospheres of Venus, Earth, and Titan. To see these objects' surfaces unobscured by clouds, turn off this option. Note: this option is only available in SkySafari Plus and Pro.
with Surfaces: Shows planet and moons with realistic surfaces (and ring systems) using NASA planetary mission image data. This option can slow performance when zoomed in a planet's disk, but generates a very pretty view.
with Surface Labels: Shows labels for named planetary surface features like craters, mountains, maria, and canyons. Spacecraft that have landed on other solar system objects, and cities on Earth, are indicated with a green dot and label. Only the largest features are labelled when a planet's disk appears very small; to see more labels for smaller features, zoom in on the planet. Note: this option is only available in SkySafari Plus and Pro.
with Names: Shows names next to planets and moons.
with Minor Moons: Shows the "irregular" satellites of the outer planets, as well as other minor moons discovered by spacecraft exploration. All of these are small, asteroid-sized objects that are only visible in large professional telescopes. Many of these objects have highly inclined, elliptical, and/or retrograde orbits; most are suspected to be captured into temporary orbits around their primary planets. Note: this option is only available in SkySafari Pro.
Jupiter GRS Longitude: use this setting to adjust the longitude of Jupiter's Great Red Spot (GRS) by adjusting the slider at the bottom of the screen. This value is used to predict transits of the GRS - when this massive storm crosses Jupiter’s meridian (the centerline of the planet). Calculated value is used by default. To reset back to the default calculate value, move the slider all the way to the left.
## Minor Body Display
Show Asteroids: Sets whether asteroids are displayed in the sky chart.
Show Comets: Sets whether comets are displayed in the sky chart.
Show Satellites: Sets whether satellites are displayed in the sky chart.
with Names: Sets whether the sky chart displays names next to asteroids, comets, and satellites.
## Orbits, Paths & Shadows
Note: this section is only available in SkySafari Plus and Pro.
Show Planet Orbits: Shows orbital paths of the major planets around the Sun. Since the planets orbit in the nearly the same plane as the Earth (the Ecliptic plane), their orbits appear near the Ecliptic line - the Earth's orbit as seen from the Earth - in the sky. Note: this option is only available in SkySafari Plus and Pro.
Show Moon Orbits: Shows orbital paths of the moons around their primary (parent) planet. You may need to zoom in on a planet to see its moon orbits; Mercury and Venus have no moons!
Selected Object Orbit: Shows the orbit of the selected planet, moon, asteroid, comet, or satellite. You need to select such an object and turn on this option to show its orbit.
Orbit Node Markers: Shows markers at key points (nodes) in the orbit of solar system objects. The ascending node of the orbit— the point at which the planet crosses up through the ecliptic plane—is marked with a solid wedge. The descending node of the orbit—the point at which the planet crosses down through the ecliptic plane—is marked with a hollow wedge. The point at which the orbit comes closest to the parent body—the pericenter—is marked with a bar.
Selected Object Path: Shows the apparent path of a solar system object across the sky, with its position at specific dates labelled. The solar system object must be selected, and you must be viewing it from the Earth's surface, in order to see the path.
Earth & Moon Shadow Circles: Shows the Earth's shadow (when viewing from Earth) or the Moon's shadow (when viewing from the Moon). When this option is turned on, the Earth's (or Moon's) umbral and penumbral shadows are shows as concentric circles. Inside the smaller umbral shadow, the Sun is totally hidden; inside the larger penumbral shadow, the Sun is only partially blocked. This can be helpful for simulating lunar and solar eclipses, and illustrating the difference between total and partial eclipses.
## Brightness & Size
Please Note: this section is only available in SkySafari Plus and Pro.
Magnitude Limit: This item lets you set the faintest planets, moons, asteroids, comets, and spacecraft that the sky chart will display. You can use this item to filter out the many hundreds of faint asteroids and comets that are not observable in backyard telescopes - or you may want to show them all!
Planet Magnification: This slider lets you magnify the Solar System's major planets by a factor of up to 10,000x their true size. The planets are very small compared to the space between them. This option is useful for showing comparative views of the planets from different perspectives.
Moon Magnification: This slider lets you magnify the moons of the planets by a factor of up to 100x over their true size. Since most moons are very small compared to their primary planet, this option lets you exaggerate them to make easier comparative views.
## Update Minor Body Orbit Data
SkySafari normally updates its database of asteroid, comet, and satellite orbits once per week. In SkySafari Plus and Pro, you can tap this button to download new asteroid, comet, and satellite orbit data any time your iOS or Android device is connected to the internet. SkySafari will download the following files:
* Bright Asteroids - from the Minor Planet Center
* Observable Comets - from the Minor Planet Center
* Visual Satellites - from [Celestrak.com](https://celestrak.com/)
These downloads should take 10 - 30 seconds if you are connected to the internet by Wi-Fi, and a 1 - 3 minutes if you are connected by a cellular data network. If successful, SkySafari will report the number of asteroid, comet, and satellite orbits that it has updated. If that number is zero, it probably means SkySafari can't connect to the on-line data sources for this information (because the server is down, or because you are not connected to the internet, etc).
Updating your orbit data every month or so is a good idea. It will ensure that SkySafari's position predictions are accurate. This is especially true for satellites, whose orbits change rapidly due to atmospheric drag, and due to perturbations from the Earth's non-spherical gravity field.
Updating also ensures that as new objects are launched - or discovered! - SkySafari will be able to show them to you.
Please Note: this feature is only available in SkySafari Plus and Pro.
# Stars Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/display-options-help/stars-help
Documentation for Stars Help
## Stars Help
The settings in this view let you control the display of stars, including the number of stars that will be shown, the size and color of the star symbols, and the labelling of stars with their names or catalog numbers.
## Star Display
Show Stars: Sets whether stars are displayed in the sky chart. When turned off, most of the other settings in this view are disabled.
Magnitude Limit: Sets the star magnitude limit. This determines the faintest stars that are visible in the sky chart. The brighter a star, the lower its magnitude. The faintest stars visible to the naked eye are about magnitude 6.5. Very bright stars can have negative magnitudes; the brightest star in the sky, Sirius, is magnitude -1.44.
The magnitude limit will change automatically as you zoom the sky chart in and out. When zoomed in, fainter stars are displayed.
Never Show Fainter Than: This sets the absolute magnitude limit of the faintest stars visible. When you zoom in, stars fainter than your set limit are never displayed.
## Star Label Display
Show Names: Sets whether star names are displayed next to some stars in the sky chart.
Proper Names: Sets whether proper names are displayed for stars when possible. When turned off, stars' names are displayed using their catalog numbers (e.g. "α CMa") instead of their proper names (e.g. "Sirius").
Greek Symbols: Sets whether greek symbols are displayed for stars which have Bayer letters. When turned off, greek letters are spelled out in English, e.g. "Alpha CMa" instead of "α CMa".
Name Density: Sets the percentage of stars whose names are displayed on the sky chart. At 0%, no stars have names shown; at 100%, all stars have names shown. At 20%, only the brightest 20% of stars have names shown. When the star name density is at 10% or below, only stars with Bayer letters, Flamsteed numbers, or proper names will be labelled.
Double Stars: displays double stars with their component identifiers (A, B, C, D, etc.) as well as their visual separations in arcseconds ("). For binary stars with known orbits, this option also displays the orbital path of the secondary component relative to the primary. Turn this option on, then zoom in on Sirius or Alpha Centauri, and take a look!
Please Note: this feature is only available in SkySafari Plus and Pro.
## Star Symbol Options
Symbol Size: Sets the size of the star symbols. Use small, subtle star symbols to give the screen the appearance of the night sky.
Color Intensity: Sets the displayed intensity of the color difference between stars of different spectral types.
# Settings Files Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/settings-files-help
Documentation for Settings Files Help
Settings Files Help
Settings Files let you save all of your sky chart options so you can restore them at a later date. You can email settings files to yourself or download them from LiveSky, and even send them to your friends, so that they can easily reproduce a sky chart which you have created.
To do all of these things, select the Save Settings item from the main Settings view.
## Saving New Settings Files
To create your own settings files, tap the "Create Settings" button and select "Save Current Settings". SkySafari will make another "snapshot" of all the app's current settings, and save them as a settings file called "Current Settings." If the date and time are set to something other than current time, the simulated date is used as the file name: for example, "April 15, 2014 Settings" for a settings file that reproduces the total lunar eclipse of April 15th, 2014. You can edit the settings file name to something else if you want a more descriptive title.
Description: SkySafari generates a default description for your settings file, to give you an idea of what's inside it. You can edit this description as well. When you are satisfied with the name and description, tap the "X" button in the upper right side of the status bar to return to the list of settings files that you have already saved - your new settings file is added to the list!
## Restoring Settings Files
To restore a saved settings file, tap its name in the list of settings files. SkySafari will display the file's description, to let you make sure this is the file you want. If so, tap the "Apply Settings" button - and all of the app's settings will be replaced with those from the settings file. You can tap the "Cancel" button if you don't want to do this! We suggest you "Preview Settings" first.
## Editing and Emailing Settings Files
You can view and edit a previously-saved settings file's name and description by selecting the files name and tapping the Edit button at the top of the screen. You can overwrite the settings inside the file with a copy of the app's current settings, by tapping the "Update with Main Chart Settings" button. You might want to do this if, for example, you wanted to tweak the settings inside that file, without having to save them to an entirely new file.
You can email a saved settings file to yourself, or to anyone else, by tapping the "Email This Settings File" button. When the email is received, the e-mail app on the recipient's phone will launch their copy of SkySafari, import the settings file into their list of saved settings, and let them restore the settings you sent them - all in a single step!
## Rearranging and Deleting Settings Files
You can rearrange and delete settings files. Here's how.
On iOS, tap the Edit button at the top of the screen. Then tap and drag the "grip" icon on the right side of the settings file to move it around the screen. Tap and drag the - (minus) icon on the left side of the settings file to delete it. Tap the End Edit button at the top of the screen when you're finished.
On Android, tap the Edit link at the top of the screen. Up/down arrows and a trash can appear at the bottom of the screen. Then tap the settings file you want to move or delete. Use the up/down arrows to move the item in the list, or tap the trash can to delete it. When finished, tap the End Edit link at the top of the screen.
## Reverting Chart to Factory Defaults
You can restore SkySafari to its initial state at any time by tapping the Revert Chart to Factory Defaults button found at the bottom of the main Settings screen.
# Settings Help Menu
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/settings-help-menu
Documentation for Settings Help Menu
# Settings Help
## Time & Coordinates Help
1. [Date & Time Help](time-and-coordinates-help/date-and-time-help)
2. [Location Help](time-and-coordinates-help/location-help)
3. [Coordinates Help](time-and-coordinates-help/coordinates-help)
4. [Precession Help](time-and-coordinates-help/precession-help)
5. [Formats Help](time-and-coordinates-help/formats-help)
## Display Options Help
1. [Appearance & Behavior Help](display-options-help/appearance-and-behavior-help)
2. [Notifications Help](display-options-help/notifications-help)
3. [Horizon & Sky Help](display-options-help/horizon-and-sky-help)
4. [Solar System Help](display-options-help/solar-system-help)
5. [Stars Help](display-options-help/stars-help)
6. [Deep Sky Help](display-options-help/deep-sky-help)
7. [Milky Way Help](display-options-help/milky-way-help)
8. [Constellations Help](display-options-help/constellations-help)
9. [Grids & Reference Help](display-options-help/grids-and-reference-help)
10. [H-R Diagram Help](display-options-help/h-r-diagram-help)
## Telescope Control Help
1. [Scope Presets Help](telescope-control-help/scope-presets-help)
2. [Scope Settings Help](telescope-control-help/scope-settings-help)
3. [Celestron Wi-Fi Help](telescope-control-help/celestron-wi-fi-help)
## User Data
1. [Saved Settings Help](settings-files-help)
2. [Storage Help](storage-help)
## Account Help
1. [LiveSky Help](account-help/livesky-help)
# Storage Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/storage-help
Documentation for Storage Help
Storage Help
## Cloud Storage
SkySafari allows you to store and back up your data (observing lists, observations, saved settings files etc) to our LiveSky server. Backup To LiveSky is on by default. Tap the switch to the off position to disable automatic backups.
## DSS Cache Storage
The DSS Cache Storage setting controls how much space is allotted to storing Deep Sky Survey images downloaded from the Object Info view or when images are batch downloaded for an Observing List. When the allotted storage is filled, older images are automatically discarded.
Please Note: DSS Cache Storage is only available in SkySafari Plus and Pro.
# Celestron Wi Fi Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/telescope-control-help/celestron-wi-fi-help
Documentation for Celestron Wi Fi Help
## Celestron Wi-Fi Help
SkySafari controls compatible Celestron Wi-Fi telescopes. Compatible models include NexStar Evolution models with built-in Wi-Fi, and others that use Celestron's SkyPortal Wi-Fi accessory.
Use these settings to specify your Celestron Wi-Fi telescope preferences, including slew speeds and tracking.
You must connect to your telescope's Wi-Fi network before you can access these settings. Make sure your telescope type is set to Celestron Wi-Fi.
## Utilities
Battery Status (NexStar Evolution only) - Displays the battery voltage, whether it's discharging or charging, and High, Medium, or Low status.
Tray Lighting (NexStar Evolution only) - Adjusts the LED brightness of the accessory tray light. Light can be turned off by sliding all the way to 0.
Logo Lighting (NexStar Evolution only) - Adjusts the LED brightness of the Celestron power logo and the Wi-Fi logo. Can be dimmed down or shut off for dark sky locations.
Move To Switches (CGE and CGE Pro only) - Moves the telescope to the home switch position.
GoTo Home Position (EQ mounts only) - Moves the telescope to the home position.
Set Home Position (EQ mounts only) - Sets the telescope home position.
## Tracking Rate
Sidereal - Compensates for the rotation of the Earth to keep objects centered in the eyepiece of your telescope. This is the default tracking rate and used for all stars and deep space objects.
Lunar - Used for tracking the Moon and when observing the lunar landscape.
Solar - Used for tracking the Sun when solar observing with a proper filter.
Off - Turns tracking off. The motors in the telescope will stop unless you are slewing.
Track RA Only (EQ mounts only) - When EQ alignment is complete, SkySafari will track in both RA and Dec to keep your object centered in the field of view, even if the mount is not perfectly polar aligned. Disable this default setting if you want the mount to track in RA only. Disabling this setting is also recommended if you are imaging with an autoguider.
## Go To
Simply enter the Right Ascension (RA) and Declination (Dec) coordinates to go to a specified area of sky. This is the quickest way to go to a custom object, such as a new comet or object of interest provided by a star chart or online reference.
## Alignment
Align Using SkyAlign (Altazimuth mounts only) - The default and recommended alignment for altazimuth mounts is SkyAlign. Alignment is achieved by centering and aligning 3 bright stars in the telescope's eyepiece.
Align Using Manual Align (Altazimuth mounts only) - Identify, select, and center 3 named stars in the telescope's eyepiece. The star is selected on your screen and centered with the on-screen direction arrows.
Align Using EQ Align (for EQ mounts only) - Identify, select and center 4 named stars, 2 on each side of the meridian. Similar to Manual Align for altazimuth mounts but requires 4 stars instead of 3, and 2 must be selected on each side of the Meridian
Align Using Wedge Align - Use this align method if you have an altazimuth mounted telescope equipped with a wedge. Alignment requires selecting and centering 4 stars, 2 on each side of the meridian.
Align StarSense Auto (only visible when StarSense is connected) - This is the default alignment option when a StarSense AutoAlign camera is plugged into the telescope. This method allows you to automatically align the telescope using StarSense.
Align StarSense Manual (only visible when StarSense is connected) - Choose StarSense Manual to manually move the telescope to any 3 (for Alt-Az) or 4 (for EQ) points in the sky to achieve an alignment. You do not have to select stars. Simply move the telescope with the in-app direction arrows to 3 different locations in the sky, following the prompts and tapping "Align" for each selected location. This method is useful for aligning in an area with a heavily obstructed sky.
Calibration Info (Only visible when StarSense is connected) - Displays the current center coordinates of the StarSense camera. Default is 640, 480. This number typically changes after a new StarSense camera calibration is performed.
StarSense Wedge Align Enabled - Toggle this on if you are using an altazimuth telescope on a wedge with StarSense. Alignment is still automatic.
Hibernate Enabled - Allows the telescope alignment to be saved when shutting the telescope off or disconnecting and exiting the app.
## Slew Buttons at Slow Speeds
Telescope direction can be reversed up/down and left/right to change the apparent motion of the star in the telescope's eyepiece in the three lowest slew speeds. Reverse Up and Down is default enabled so the star moves in the same direction as the direction button.
Reverse Left and Right - Reverses left and right directions in the three lowest slew speeds.
Reverse Up and Down - Reverses up and down directions in three lowest slew speeds.
## Slew Limits
Some telescopes may require limiting the altitude angle to prevent striking the mount. Altazimuth mounts default to a 70° altitude slew limit as some mounts cannot point all the way to zenith. This can be changed manually to set the best slew limit for your telescope. Note that the slew limit does not work until the telescope is aligned with the sky, or the slew limit assumes the telescope is pointing horizontally.
Maximum - Sets the maximum (or highest) limit, 0-90°. Use to prevent the telescope from striking the mount when oversized accessories are attached
Minimum – Sets the minimum (or lowest) limit, 0-90°. This setting is useful when trying to avoid obstructed horizons.
Software RA Limits (for EQ mounts only):
RA Minimum - Sets how many degrees approach to the east the mount can slew before stopping. -20° will slew 20° past the meridian.
RA Maximum - Sets how many degrees approach to the west the mount can slew before stopping. -20° will slew 20° past the meridian.
## Anti-Backlash
All mechanical gears have a certain amount of backlash or play between the gears. This is seen as a delay in the time it takes for the telescope to move after a direction arrow is pressed, especially when changing directions. Anti-backlash compensates for backlash by inputting a value which quickly rewinds the motors just enough to eliminate the play between gears.
Altitude - Sets the backlash compensation value for altitude (up/down motion of the telescope), 0-99.
Azimuth - Sets the backlash compensation value for azimuth (left/right motion of the telescope), 0-99.
RA (EQ mounts only) - Sets the backlash compensation value for right ascension (east/west motion of the telescope), 0-99.
Dec (EQ mounts only) - Sets the backlash compensation value for declination (north/south motion of the telescope), 0-99.
## Advanced
Advanced settings in allow you to adjust other features on your telescope.
External Power (NexStar Evolution only) - Set the maximum potential current draw from the power supply. Default is 2.0A for the included power supply. Any setting higher than 2.0 requires a higher capacity power supply which is not included with the telescope. The telescope has built-in fail safes if the External Power setting is incorrectly set, but we recommend always using a suitable power supply for the given setting.
When used with a higher capacity power supply, this setting allows you to charge the internal battery at the fastest speed while using the telescope, and while optionally charging your smart device from the USB charger.
USB Charging (NexStar Evolution only) - Sets the USB charger on the mount to always on or auto. Default is Auto, meaning the charger will shut off to save battery life when the battery is low. On will force the charger to stay on at all times, even when the battery is low.
RA Guide Rate - Percentage of sidereal speed for the autoguider 0-99.
Dec Guide Rate - Percentage of sidereal speed for the autoguider 0-99.
Max Slew Rate Enabled - Allows the max slew speed to be adjusted from default.
Max Slew Rate - Increasing speed will draw more current from your power source. Decreasing speed will operate more quietly.
# Scope Presets Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/telescope-control-help/scope-presets-help
Documentation for Scope Presets Help
Scope Presets Help
In order to communicate with your telescope, you will need either:
ASCOM Alpaca - A server adhering to the ASCOM Alpaca restful API, that manages your telescope.
INDI - A server adhering to the INDI XML-based protocol that manages your telescope.
SkyFi - a Wi-Fi adapter that relays wireless communication from your mobile device to the serial port on your telescope. Our original SkyFi 2 used Ad Hoc networking which most Android devices do not support. All iOS devices support Ad Hoc Wi-Fi, and therefore can use the SkyFi 2. Our latest SkyFi 3 creates a standard Wi-Fi hotspot that will work with all devices.
A bluetooth serial adapter, which relays communication from SkySafari through your Android's bluetooth radio hardware to your telescope's serial port. Does not work with iOS devices.
A network enabled scope or controller.
## Adding And Managing Scope Presets
By default, SkySafari comes with a single telescope preset called "Demo", which represents a simulated telescope. To add additional presets, tap the Add Preset button at the bottom of the Scope Presets view. Follow the setup sequence described below to configure the telescope connection.
To select an active preset to use to connect to a scope, simply tap on a preset in the list and accept it as the new active preset. For SynScanLink connections, please make sure you have the SynScan or SynScan Pro app installed to communicate with the SkyWatcher scope.
To delete a preset or edit an existing one, tap the Edit in the top right of the view and then tap the delete icon to remove a preset or the disclosure icon on the right to edit it.
## Adding a Preset: Connection Type Selection
ASCOM Alpaca - Tap this button to connect to your scope through an ASCOM Alpaca server.
INDI - Tap this button to connect to your scope through an INDI server.
SkyFi - Tap this button to connect to your scope through a SkyFi adapter.
Other - Tap this button to connect to your scope through a bluetooth serial adapter, Celestron WiFi, SynScanLink, or a network enabled controller.
## Adding a Preset: Equipment Selection
Note: This step is not necessary and is skipped for ASCOM Alpaca and INDI connections.
Once you have selected the correct Mount Type and Scope Type, tap the Next button in the top right corner to continue.
Mount Type - selects your telescope's type of mounting:
Equatorial Push-To - a non-motorized mount whose right ascension axis is pointed at the celestial pole. The mount must be manually turned around this axis to follow the diurnal motion of the sky.
Equatorial GoTo (Fork) - a motorized equatorial mount that automatically follows the diurnal motion of, and can automatically slew to, targets in any part of the sky. Has one or two fork arms that suspend the telescope between them. The Meade LX-200 and Celestron NexStar (when used with an equatorial wedge) are examples.
Equatorial GoTo (German) - a motorized, polar-aligned mount that requires reversing the telescope tube to the east or west side of the mount when the telescope passes through the meridian. Examples include the Losmandy and Takahashi mounts.
Alt-Az. Push-To on Equ. Platform - a non-motorized mount that must be manually pushed to targets in different parts of the sky. However, it sits on a motorized platform that is aligned with the Earth's polar axis, so the mount follows the diurnal motion of the sky when it is not being pushed.
Alt-Az. Push-To - a non-motorized alt-azimuth platform with fork arms that suspend the telescope between them. It is moved manually by pushing the telescope tube. It sits flat on the ground, so its "up-down" and "left-right" axes of motion align to the local horizon and zenith. This includes most Dobsonian telescopes.
Alt-Az. GoTo - a motorized alt-azimuth platform with fork arms that suspend the telescope between them, and can slew to any set of coordinates in the sky on command. Includes the Meade LX-200 and Celestron NexStar when used in the alt-azimuth configuration.
Scope Type - selects the type of telescope you want to control. SkySafari can control any of the telescopes in the list. SkySafari supports many encoder systems that can read out the telescope position but not actually move the telescope. The Celestron AstroMaster and JMI NGC-MAX are examples of such encoder systems.
## Adding a Preset: Communication Settings
Note: If you are connecting to your telescope through SynScanLink, or you are running SkySafari on Android and have picked a non-network capable telescope, this step is not necessary and will be skipped.
## For an ASCOM Alpaca or INDI Server Connection
Select Auto-Detect (for Alpaca Only) then tap Scan Network For Devices to list available telescopes.
Or select Manual Configuration and enter the IP Address and Port Number on which the server is hosted. Then tap Check IP and Port For Devices to list available telescopes
After telescopes have been discovered and are listed at the top of the view, select the telescope with which you want to connect and tap the Next button in the top right corner.
## For a SkyFi Connection
Select Auto-Detect, enter the SkyFi name, then tap Scan Network For Devices to list available SkyFi adapters.
Or select Manual Configuration and enter the IP Address and Port Number on which the SkyFi adapter is available. Then tap Check IP and Port For Devices to verify the connection.
SkyFi Settings Web Page - If you have a SkyFi wireless adapter, this item displays its settings/configuration web page. You must be connected to SkyFi's wireless network in order to see this web page.
After a SkyFi adapter has been discovered, tap SkyFi Settings Web Page to display its settings/configuration web page.
Once a SkyFi adapter has been discovered and configured, tap the Next button in the top right corner.
## For a Celestron WiFi Connection
You will see choices for Setup and Control and for Communication. The Setup and Control screen will have options that vary depending upon the exact Celestron WiFi scope you are connected to. The Communication screen will allow you to change whether you connect to the WiFi directly or by using access point mode where the WiFi scope has joined your local network. Once setup is complete, tap the Next button in the top right corner.
## For Other Connections
If you are using SkySafari for Android, select Connect via Bluetooth to communicate with your telescope using a bluetooth serial adapter. The adapter must be turned on, physically connected to your telescope's serial port, and paired with your Android device. Otherwise select Connect via Wi-Fi to communicate with your telescope using a network connection.
To use a network connection, enter the IP Address and Port Number on which the telescope is available. Then tap Check IP and Port to verify the connection.
## Adding a Preset: Additional Options
Preset Name - Allows you to enter a custom name for your scope preset.
Set Time and Location - If turned on, SkySafari will send the time and location from your mobile device to the telescope when establishing a connection. This will overwrite your telescope's previously-set time and location. For some telescopes, this may invalidate your alignment. For older Meade LX-200 telescopes, this may also cause a delay of up to 15 seconds when connecting.
Note: This option is disabled with Celestron WiFi scopes. The time and location is always sent in this case. For SynScanLink, this option is also disabled as time and location information is managed by the SynScan app.
Readout Rate - The readout rate is how often SkySafari requests the telescope's position from the mount. If you set this rate to "4 per second", then SkySafari will request the telescope's position (and update it on screen) four times every second.
If the telescope communication drops often, the rate of position requests may be too high for the telescope to respond properly. Setting a lower rate of 1 or 2 readouts per second may improve reliability. The best readout rate may require some trial and error to find. A lower readout rate will update the telescope's position in the sky chart less frequently, and may make SkySafari's telescope controls feel sluggish.
Timeout Seconds - The timeout is how long SkySafari will wait for a response from the mount. 3.0 seconds is a reasonable wait time, but older scopes may work better with longer timeouts.
If your telescope mount has encoders which provide a digital readout of the scope's position, additional text fields will appear here. These let you specify the encoder resolution.
RA/Azm - The number of steps per revolution for the encoder attached to the telescope's Right Ascension axis (or Azimuth axis, if you have an alt-azimuth mount).
Dec/Alt - The number of steps per revolution for the encoder attached to the telescope's Declination axis (or Altitude axis, if you have an alt-azimuth mount).
Get Automatically - If turned on, SkySafari will attempt to read these values from your encoders when it connects to the telescope controller. If turned off, you can enter the encoder steps per revolution manually; then SkySafari will send the values you entered to the encoders when connecting to the telescope. You can do this if (for example) your mount is using gears or pulleys to increase the effective encoder resolution.
Depending how your encoders are installed, their position readouts may increase when they are turned clockwise, or increase when they are turned counterclockwise. If the encoder position readouts increase when they are turned counterclockwise, enter a negative value for the number of steps per revolution. You may need to determine the correct + or - sign by trial-and-error. If you push your telescope left (or up), but the telescope field-of-view indicator on the sky char moves right (or down), the sign is probably wrong.
# Scope Settings Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/telescope-control-help/scope-settings-help
Documentation for Scope Settings Help
Scope Settings Help
Tilt Device to Slew - when enabled, SkySafari uses the accelerometers in your device to gently translate the movements of your hands into slew commands for your telescope. The "tilt-to-slew" telescope control allows you to tilt and pan your device while keeping your eye on the eyepiece. To use this feature, enable "Tilt Device to Slew" in the Settings and connected to the scope. Then tap the small button on the right of the star chart. If the device is not already held level, it will prompt you to level the device before tilt-to-slew will engage. Once engaged, you just tilt the device to move in a particular direction. The amount of tilt does not control the slew rate. We found that was too hard to control. Instead use the rate control to adjust the rate.
Save Log File - If turned on, SkySafari saves a log of its communication with the telescope on your mobile device. SkySafari creates a new log file every time you connect to the telescope. The log file records every command that SkySafari sends to the telescope, and every response from the telescope. The log file name contains the date and time you began the telescope session, for example:
## 2014-01-31-12-34-56.txt
This log file can be emailed to Simulation Curriculum for troubleshooting telescope communication problems. You can transfer the telescope communication log from your iOS or Android device using the Files app (iOS) or app file sharing (Android).
iOS users - transfer the scope communication log to your computer with the Files app. Connect your iPhone or iPad, open the Files and look for a folder named 'SkySafari \[version name]'. Tap the scope log file and use the Share icon.
Android users - After the scope is disconnected, the scope communication log is written to the Documents folder accessible through your Files app. Share the log file with yourself or email it directly to Simulation Curriculum from your device.
# StarSense Explorer
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/telescope-control-help/starsense-explorer-help
Documentation for StarSense Explorer
## Overview
Any Starsense Explorer push-to telescope owner can secure a supermassive upgrade over the free bundled SSE app. We developed the technology and created the original app...now join us for the preferred StarSense Explorer experience in SkySafari 8 Pro.
# Coordinates Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/time-and-coordinates-help/coordinates-help
Documentation for Coordinates Help
## Coordinates Help
The settings in this view let you select the coordinate system used to display the sky chart, and precisely center the chart on a particular set of coordinates. You can also set the field of view width and orientation, and display or hide the chart center coordinates and field of view while you are swiping or pinching the chart.
## Coordinate System
This sets the coordinate system used by SkySafari. These are the options that you can choose here:
Horizon - In the Horizon (or "Alt-Az") coordinate system, altitude is how high in the sky something is, and azimuth is the direction around the horizon. This system is used to show an object's position in the sky relative to your local horizon line.
Equatorial - In the Equatorial (or "RA-Dec") coordinate system, RA stands for Right Ascension, and Dec stands for Declination. These coordinates are akin to longitude and latitude on the Earth. The Equatorial system is aligned with the Earth's equator and rotates with the Earth, so the coordinates of objects in the sky do not change as the Earth turns. Hence, Equatorial coordinates are commonly used with printed star atlases.
Ecliptic - In the Ecliptic coordinate system, longitude and latitude in the sky are akin to longitude and latitude on the Earth. Ecliptic coordinates are defined by the plane of the Earth's orbit. The "equator" in this coordinate system is called the Ecliptic path or simply the Ecliptic. It is a great circle traced by the Sun as the Earth orbits around it over the course of a year. Most objects in the solar system orbit the Sun in nearly the same plane as the Earth, so they usually appear near the ecliptic in the sky. Ecliptic coordinates are the "natural" coordinates for the solar system, and are used extensively to describe the motion of planets, comets, and asteroids. Please Note: Ecliptic coordinates are only available in SkySafari Plus and Pro.
Galactic - In the Galactic coordinate system, longitude and latitude in the sky are akin to longitude and latitude on the Earth. Galactic coordinates are defined by the plane of our Milky Way galaxy. Galactic longitude is measured westward along the galactic equator from 0° at the galactic center, located in the constellation Sagittarius, to 360°. Galactic latitude is measured from 0° on the galactic equator to +90° at the north galactic pole and -90° at the south galactic pole. Galactic coordinates are used most commonly for dynamical studies of stars, star clusters, galaxies, and other objects outside the solar system. Please Note: Galactic coordinates are only available in SkySafari Pro.
The horizon is only visible in the sky chart when using Horizon coordinates. In other coordinate systems, it would appear as a confusing distraction that tilts and move as the Earth rotates - so SkySafari hides it.
## Chart Center Coordinates
Center Azm/RA/Lon: Sets the azimuth at the center of the sky chart. North is 0°, East is 90°, South is 180° and West is 270°. You may enter a new azimuth to precisely set the chart's center.
Center Alt/Dec/Lat: Sets the altitude at the center of the sky chart. At 0° the chart is centered on the horizon, at +90° it is centered directly overhead at the zenith, and at -90° it is centered directly under your feet.
When using Equatorial or Ecliptic coordinates, the chart center RA and Dec are always assumed to be for the precession epoch specified in the Precession settings.
When using Horizon coordinates, the chart center altitude is assumed to be apparent (i.e. it includes the effects of atmospheric refraction) if the Refraction option is turned on in the Precession settings. If this option is turned off, the chart center altitude is assumed to be the true (un-refracted) altitude.
## Field of View
Field Width Angle: Sets the sky chart's field of view width angle using a slider control.
The largest field of view SkySafari can display is 180 degrees, letting you see the entire sky at once. As the field of view increases past 90 degrees, the horizon becomes curved, due to the distortion caused by projecting the entire celestial sphere onto the flat screen. In SkySafari Plus and Pro, the maximum field of view is 90 degrees when you are orbiting another solar system object. See the Orbit button Help for more information.
If you hold your mobile phone at arm's length, about 2 feet from your eyes, its 2-by-3 inch screen has an apparent size of 4.8 by 7.2 degrees. So, if you set the field of view width to 4.8 degrees (portrait mode) or 7.2 degrees (landscape mode), and hold your phone out at arm's length, the view on your phone should appear at the same scale as the real sky.
The smallest field of view SkySafari can display is 0.1 arcminutes, or 6 arcseconds - about the average size of the planet Mars as seen from Earth. One arcsecond is the best resolution a typical 8" backyard telescope can achieve under good observing conditions.
For comparison, the Sun and Moon appear about 1/2 degree (or 30 arcminutes) across. The smallest angle the unaided human eye can resolve is about 1/30th of a degree, or 2 arcminutes - about 1/15th the width of the full Moon. At its closest approach to Earth, the planet Venus appears about 1 arcminute across; Jupiter typically appears appears 45 arcseconds across.
Flip Horizontally: "On" flips the sky chart display horizontally to match the view in a telescope whose optical design results in a mirror-image view.
Flip Vertically: "On" flips the sky chart display vertically to match the view in a telescope whose optical design results in an upside-down view.
# Date And Time Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/time-and-coordinates-help/date-and-time-help
Documentation for Date And Time Help
## Date & Time Help
The Date & Time view can be set to use the current time, or can be set to a specific time/date in the past or future. When set to current time, the chart view will update every second to show the current positions of objects in the sky.
Use Current Time keeps SkySafari's simulated time in sync with your mobile device's built-in system clock. When turned on, the sky chart updates to match real time every second.
## Setting the Time and Date
To change the simulated date:
Touch the Time and Date tab at the top of the screen, then use the calendar to set the desired date.
Time Presets:
There are buttons below the picker to allow you to quickly set the time to specific events such as Sunset, Moonset, etc. The exact time of these events will differ based on your location and the simulated date.
Automatic Daylight Saving Time: This switch turns the automatic daylight saving time (DST) correction on and off. When the switch is on, SkySafari automatically determines whether DST is currently in effect based upon the date and your simulated location. SkySafari displays a message below the switch, telling you whether it thinks DST is currently in effect for your simulated date and location.
Governments often change the rules for daylight saving time, so SkySafari's automatic DST option may not work. If this happens, you may turn off Automatic DST, and instead add one hour to your Time Zone setting, in the Location view.
## Advanced Options
In SkySafari Plus and Pro, there are additional options for setting the date and time. The standard iOS or Android date/time controls do not easily let you pick a date more than 100 years in the past or future, or to the nearest second. Select the Advanced tab to see some new options which let you enter the time more precisely and in alternate ways.
With the Advanced tab, you can enter a date up to 10,000 years from the present in SkySafari Plus, and up to 500,000 years from the present in SkySafari Pro. You can also enter the time to the nearest second.
Julian Date is another method to set the date. Widely used in astronomy, the Julian Date is the number of days since January 1, 4713 B.C. The Julian Date begins at Greenwich noon, not midnight. Noon (i.e., 12h UTC) on 1 January 2000 A.D. is Julian Date 2451545.0. Julian dates do not observe any time zones or daylight saving time changes.
Local Sidereal Time is displayed near the bottom of the Date settings view. This indicates the hour of right ascension that is currently on your local meridian, and is sometimes used for aligning a telescope.
# Formats Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/time-and-coordinates-help/formats-help
Documentation for Formats Help
## Formats Help
The settings in this section let you control how SkySafari displays dates, times, and celestial coordinates throughout the program.
## Time
These settings let you choose the format which SkySafari uses to display the time of day.
12 hour displays the time with an AM or PM.
24 hour displays the time in "military" format.
HH:MM displays the time to the nearest minute.
HH:MM:SS displays the time to the nearest second.
## Date
These settings let you choose the format which SkySafari uses to display the calendar date.
Mmm DD, YYYY displays the month abbreviation and day, separated by a comma from the year.
YYYY, Mmm DD displays the year, separated by a comma from the month abbreviation and day.
MM/DD/YYYY displays the month, day, and year all separated by slashes.
DD/MM/YYYY displays the day, month, and year all separated by slashes.
YYYY/MM/DD displays the year, month, and day all separated by slashes.
YYYY/DD/MM displays the year, day, and month all separated by slashes.
## Geographic Longitude, Latitude
These settings let you choose the format which SkySafari uses to display geographic longitude and latitude on the Earth's surface.
DDD MM SS.S, DD MM SS.S displays geographic longitude and latitude to the nearest tenth of an second.
DDD MM, DD MM displays geographic longitude and latitude to the nearest minute.
DDD.DDDDDD, DD.DDDDDD displays geographic longitude and latitude in decimal degrees to the nearest millionth.
## Azimuth, Altitude
These settings let you choose the format which SkySafari uses to display azimuth and altitude, which describe an object's position in the local horizon coordinate system.
DDD MM SS.S, DD MM SS.S displays azimuth and altitude to the nearest tenth of an arcsecond.
DDD MM, DD MM displays azimuth and altitude to the nearest arcminute.
DDD.DDDDDD, DD.DDDDDD displays azimuth and altitude in decimal degrees to the nearest millionth.
## Right Ascension, Declination
These settings let you choose the format which SkySafari uses to display right ascension and declination. These coordinates describe an object's position in the equatorial coordinate system.
HH MM SS.SS, DD MM SS.S displays Right Ascension to the nearest hundredth of a second, and Declination to the nearest tenth of an arcsecond.
HH MM SS.S, DD MM displays Right Ascension to the nearest tenth of a minute, and Declination to the nearest arcminute.
HH.HHHHHH, DD.DDDDDD displays Right Ascension in decimal hours to the nearest millionth, and Declination in decimal degrees to the nearest millionth.
## Ecliptic Longitude, Latitude
These settings let you choose the format which SkySafari uses to display longitude and latitude in the ecliptic coordinate system.
Please Note: Ecliptic coordinates are only available in SkySafari Plus and Pro.
DDD MM SS.S, DD MM SS.S displays ecliptic longitude and latitude to the nearest tenth of an arcsecond.
DDD MM, DD MM displays ecliptic longitude and latitude to the nearest arcminute.
DDD.DDDDDD, DD.DDDDDD displays ecliptic longitude and latitude in decimal degrees to the nearest millionth.
## Galactic Longitude, Latitude
These settings let you choose the format which SkySafari uses to display longitude and latitude in the galactic coordinate system.
Please Note: Galactic coordinates are only available in SkySafari Pro.
DDD MM SS.S, DD MM SS.S displays galactic longitude and latitude to the nearest tenth of an arcsecond.
DDD MM, DD MM displays galactic longitude and latitude to the nearest arcminute.
DDD.DDDDDD, DD.DDDDDD displays galactic longitude and latitude in decimal degrees to the nearest millionth.
# Location Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/time-and-coordinates-help/location-help
Documentation for Location Help
## Location Help
SkySafari needs to know your location on Earth in order to correctly plot the location of objects in the sky. You can set this in the Location view from the main Settings screen. Four pieces of information are necessary:
Latitude - distance north or south of the equator
Longitude - distance east or west of the prime meridian
Elevation - altitude above sea level
Time Zone - local time offset in hours from Universal Time (UT), previously called Greenwich Mean Time (GMT)
A time zone west of (behind) Universal Time is negative; time zones east (ahead) of UT are positive. Always enter the time zone offset for Standard Time only. Don't enter a Daylight Saving Time offset here. SkySafari will automatically correct for Daylight Saving Time. See the Date & Time Settings Help.
Please note: in SkySafari Plus and Pro, you cannot change your location settings while you are orbiting another object in the solar system. These settings only refer to your location on Earth. To change them, you first need to return home to Earth. See the Orbit button Help for more information.
## Use Current Location
Tap the "Use Current Location" button to automatically determine your location from your iPhone's Location Manager, your Android device's GPS, or other information supplied by your internet service. When the location information is obtained this way, your location name is automatically filled in as "Current Location".
## Choose Location From Map
If your mobile device is connected to the internet, you can use Apple or Google maps to choose a new location. This is useful if you want to see the sky from a place other than where you are located right now. You could use this feature to view the sky as it will appear from a distant city or island where you're planning to take a vacation.
After you tap this option, a map view will appear, centered on your current location. You can pinch or swipe this map to zoom or move around, just as you would with the built-in Maps app on your device.
Tap and hold to drop a pin on your desired location. Then tap "X" at the top of the screen. SkySafari will use the longitude, latitude, and location name where your pin dropped. SkySafari will make a best guess about the elevation and time zone, since this information is not available from Apple or Google Maps.
## Choose Location From List
If your mobile device is not connected to the internet, and GPS or Wi-Fi-based location services are not available, you can choose a location from a searchable list of thousands of cities in SkySafari's database.
Tap "Choose from List" to see a list of locations, sorted by country. Choose a country to see a list of cities in that country. Choosing a city will automatically fill in the data for you. You can also search for any location in SkySafari's database by entering its name in the search field above the list of countries. If SkySafari finds more than one location which matches the name you entered, it will list all matching locations, and you can choose the one you want. If there is only one matching location, SkySafari will use it.
The location database in SkySafari's basic version includes every city worldwide with more than 100,000 inhabitants - a total of over 4,000 cities. In SkySafari Plus and Pro, the location database includes all cities with more than 10,000 people, plus more than 500 observatories, star parties, NASA centers, and other "astronomical" locations - a grand total of over 30,000 locations in all!
## Save as User-Defined Observing Site
Tap the "Save as User-Defined Observing Site" to store a manually-entered location for retrieval later. To retrieve a user-defined site, tap the "Choose Location from List" button, then choose the "User-Defined Observing Sites" group. You must name your location something other than "Current Location" before saving it as user-defined. If you choose the same name as an existing user-defined site, that existing site will be overwritten with the new longitude, latitude, etc. currently displayed in the Location view.
You can delete user-defined sites as follows:
Tap the "Choose Location from List" button. Choose the "User-Defined Observing Sites" group at the start of the location groups list.
Tap the "Edit" button at the top of the user-defined sites list.
Tap the small, red, round, "-" button for each user-defined site you wish to delete. Confirm by tapping the red "Delete" button that appears to the right.
When you are finished, tap the "X" button at the top of the user-defined sites list.
Please Note: the User-Defined Observing Sites feature only available in SkySafari Plus and Pro.
# Precession Help
Source: https://userguide.skysafariastronomy.com/app-specific/settings-help/time-and-coordinates-help/precession-help
Documentation for Precession Help
## Precession Help
The settings in this view let you set the precession epoch of the Equatorial coordinate system that SkySafari uses to report the right ascensions and declinations of objects. It also gives you precise control over the corrections SkySafari makes when computing the positions of objects in the sky.
## Precession & Nutation
Precession is a very slow "wobble" in the direction of the Earth's rotational axis, which takes about 25,800 years to complete. The Earth's axis defines both the Equatorial (or RA-Dec) coordinate system. Because of precession, an object's right ascension and declination change over time - not because the object is moving, but because the coordinate system is moving.
Use Current Epoch: if turned on, SkySafari will always report right ascensions and declinations for the current year ("epoch"). If turned off, SkySafari will report RA and Dec for the precession epoch entered below.
Precession Epoch: the precession epoch (or year) for which equatorial coordinates should be reported, if "Use Current Epoch" is turned off. Star atlases and ephemeris predictions (e.g. as in the Astronomical Almanac) often use a fixed epoch, such as 2000.0, for reporting RA and Dec.
Include Nutation: a small wobble in the orientation of the Earth's axis superimposed on its overall precessional motion. Nutation causes a small change in an object's position, typically amounting to about 8-10 arc seconds.
## Other Corrections
Aberration: a systematic shift in star positions caused by the Earth's velocity through space. It is a result of Einstein's theory of special relativity. Aberration causes objects to appear to shift in the direction that the Earth is moving by about 20 arc seconds, and affects all objects in the same part of the sky equally.
Proper Motion: a slow change in the positions of the stars due to their physical motion through space. For all except the nearest stars, proper motion is only a small fraction of an arc second per year. When this option is turned on, a star's proper motion in right ascension and declination is displayed adjacent to its coordinates in the Object Info window.
Light Time: adjusts the positions of objects in the solar system for the finite velocity of light. We see Saturn in the sky not where it is right now, but instead where it was about 90 minutes ago, because light from Saturn requires about 90 minutes to travel to Earth.
For most objects, the effect of light time amounts to only a few arc seconds. Where light time makes a noticeable difference is in the positions of the outer planets' moons, and especially in planetary rotation.
Dynamic Time: also called Terrestrial Dynamic Time (TDT), this is the standard for precise time keeping in astronomy. It differs from Universal Time (UTC or GMT) because the Earth's rotation is slowing irregularly, due to the gravitational influence of the Moon. We periodically add or subtract "leap seconds" to/from our civil time scale, to keep it in sync with where the Earth is actually pointing. The accumulated difference between UTC and TDT is called Delta T, and its current value is about 67 seconds. Delta T affects the local time when an astronomical event is observed on Earth.
If you turn on the Dynamic Time option, SkySafari adds Delta T to the civil time obtained from your Android device before computing the positions of solar system objects. If your leave Dynamic Time off, SkySafari will assume that there is no difference between UTC and TDT. This is technically incorrect, but it may be useful to compare SkySafari's results against another reference (such as the Astronomical Almanac) which tabulates an ephemeris of planetary positions against Dynamic Time instead of Universal Time.
Refraction: a distortion in an object's apparent altitude caused by the Earth's atmosphere, which bends light as it passes through. Refraction can amount to over 1/2 degree near the horizon, but decreases rapidly as an object's altitude increases. Refraction only affects an object's apparent altitude, not its azimuth, right ascension, or declination.
# Sky Chart Help (iOS)
Source: https://userguide.skysafariastronomy.com/app-specific/sky-chart-help-ios
Documentation for Sky Chart Help (iOS)
## Sky Chart Help (iOS)
The Sky Chart view is the primary screen of SkySafari. It consists of the Time, Compass/AR, and Status bar at the top, the main Sky Chart in the middle, and a Toolbar along the bottom.
The first time your run SkySafari, the program will ask if you want to retrieve your current location using the GPS or network location capabilities built into your iOS device. You can always change these settings later, using the main **SkySafari Settings** view.
## Status Bar
The coordinates of the chart's center, and the width and height of the field of view, are displayed in the Status Bar at the top of the chart. You can turn this feature on or off using the "**Show Coordinates & FOV**" option in the **Settings > Appearance & Behavior** screen.
**Popup Control Panels:** In SkySafari Plus and Pro, tap the coordinates or field of view in the Status Bar at the top of the sky chart to display a popup control panel which quickly lets you change these items.
Tap the coordinates at the left corner of the Status Bar to quickly look north/south/east/west, to adjust the star or deep sky magnitude limits, or to change coordinate systems. Tap the field of view at the right corner of the Status Bar to quickly change the field of view from 1 to 180 degrees, show field-of-view indicator rings, and flip the chart horizontally or vertically to match the view seen through your telescope's eyepiece.
After you tap on an object, the Status Bar also shows an object's common name, catalog number, magnitude, type, and constellation for a few seconds. You can get this information back by tapping the status bar again. You can still get more detailed information on the object by tapping the "Info" button to the right of the selected object's name.
## Time and Compass/AR
Tap the time button on the upper left top of the screen to display a set of controls to adjust time. Tap compass button on the upper right top of the screen to enable compass mode and augmented reality (AR).
## Toolbar
The Toolbar contains buttons to bring up commonly-used functions and other secondary views used by SkySafari. These include **Menu**, **Tonight**, **Observe**, **Scope**, and **Search**.
Tip: Users of SkySafari can also customize and change the icon order in **Settings > Appearance & Behavior > Configure Toolbar.** The toolbar for tablet users will contain additional buttons.
## Sky Chart
The sky chart shows an accurate depiction of the sky. The information displayed is highly configurable, and may be changed in the **SkySafari Settings** views. The sky chart shows the location of the stars, planets, and deep sky objects (star clusters, nebulae, and galaxies) as seen from your date, time, and location.
Tap an object on the sky chart to select it. If there are multiple objects next to each other and the first tap selects the wrong object, tap again and SkySafari will select an alternate object nearby. Once you have selected an object, tap the "Info" button next to the selected object's name to bring up the **Object Info** view. The **Object Info** view shows numerical data for the object, as well as English-language descriptions and images for many of the brighter objects in the sky. Tap and Hold (long-press) the selected object to open a popup menu with additional options and quick actions such as **Center** and **Orbit.**
The horizon is shown with a customizable image that you change in the **Settings > Horizon & Sky** view. The cardinal directions (east, northeast, north, etc.) marked. Note: if the sky chart is using Equatorial coordinates (see **Settings > Coordinates**) the horizon will not be shown.
## Swiping and Zooming
Touch the chart and drag to change the direction you are looking in the sky. You can pinch with two fingers to change the field of view shown in the chart. You can zoom the field of view from 180 degrees, showing you the whole sky at once, down to 0.1 arcminutes - about the average size of Mars as seen from Earth.
You can also zoom in or out by touching and holding your finger down on the + and - sign in the lower right corner of the sky chart. Enable this feature by selecting the "**Show Zoom Buttons**" option in the **Settings > Appearance & Behavior** screen. This is much easier than pinching and zooming, especially when you're trying to zoom in or out by a factor of 100x or more!
## Location & Time
You can display the current date, time and location used by the application to depict the night sky. You can turn this feature on or off using the "**Display Location & Date**" option in the **Settings > Appearance & Behavior** screen.
## Sky Chart Help (Android)
The Sky Chart view is the primary screen of SkySafari. It consists of the **Time and Location** bar at the top, the main Sky Chart in the middle, and a Toolbar along the bottom.
The first time your run SkySafari, the program will ask if you want to retrieve your current location using the GPS or network location capabilities built into your Android device. You can always change these settings later, using the main **SkySafari Settings** view.
## Status Bar
The status bar at the top of the screen displays the current date, time and location used by the application to depict the night sky. These values may be changed in **SkySafari Settings**.
After you tap on an object, the status bar also shows an object's common name, catalog number, magnitude, type, and constellation for a few seconds. You can get this information back by tapping the status bar again. You can still get more detailed information on the object by tapping the "Info" button to the right of the selected object's name.
## Toolbar
The Toolbar contains buttons to bring up commonly-used functions and other secondary views used by SkySafari. These include **Search**, **Time Flow**, **Compass**, **Night Vision**, **Calendar**, and **Scope**.
Tip: You can swipe the Toolbar left or right to access more items. You also change the icon order or add/remove icons using **Settings > Appearance & Behavior > Configure Toolbar.**
## Sky Chart
The sky chart shows an accurate depiction of the sky. The information displayed is highly configurable, and may be changed in the **SkySafari Settings** views. The sky chart shows the location of the stars, planets, and deep sky objects (star clusters, nebulae, and galaxies) as seen from your date, time, and location.
Tap an object on the sky chart to select it. If there are multiple objects next to each other and the first tap selects the wrong object, tap again and SkySafari will select an alternate object nearby. Once you have selected an object, tap the "Info" button next to the selected object's name to bring up the **Object Info** view. The **Object Info** view shows numerical data for the object, as well as English-language descriptions and images for many of the brighter objects in the sky. Tap and Hold (long-press) the selected object to open a popup menu with additional options and quick actions such as Center and Orbit.
The horizon is shown with a customizable image that you change in the **Settings > Horizon & Sky** view. The cardinal directions (east, northeast, north, etc.) marked. Note: if the sky chart is using Equatorial coordinates (see **Settings > Coordinates**) the horizon will not be shown.
## Swiping and Zooming
Touch the chart and drag to change the direction you are looking in the sky. You can pinch with two fingers to change the field of view shown in the chart. You can zoom the field of view from 180 degrees, showing you the whole sky at once, down to 0.1 arcminutes - about the average size of Mars as seen from Earth.
You can also zoom in or out by touching and holding your finger down on the + and - sign in the lower right corner of the sky chart. Enable this feature by selecting the "**Show Zoom Buttons**" option in the **Settings > Appearance & Behavior** screen. This is much easier than pinching and zooming, especially when you're trying to zoom in or out by a factor of 100x or more!
## Location & Time
You can display the current date, time and location used by the application to depict the night sky. You can turn this feature on or off using the **"Display Location & Date"** option in the **Settings > Appearance & Behavior** screen.
# Smarteye And Camera Configuration Help
Source: https://userguide.skysafariastronomy.com/app-specific/smarteye-and-camera-configuration-help
Documentation for Smarteye And Camera Configuration Help
## SmartEye & Camera Configuration Help
To communicate with your camera, your device must support one of the following:
ASCOM Alpaca: The camera must be ASCOM Alpaca compatible. This includes devices such as the ZWO SeeStar.
SmartEye: Pegasus Astro SmartEye devices are supported through our custom, built-in integration.
## Adding And Managing SmartEye & Cameras
To add a preset, tap the Add Preset button at the bottom of the SmartEye & Camera Configuration view. Follow the setup sequence described below to configure the camera connection.
To select an active preset to use to connect to a camera, simply tap on a preset in the list and accept it as the new active preset.
To delete a preset or edit an existing one, tap the Edit button in the top right of the view and then tap the delete icon to remove a preset or the disclosure icon on the right to edit it.
## Adding a Preset: Camera Device Type Selection
SmartEye - Tap this button to connect to SmartEye.
ASCOM Alpaca Camera - Tap this button to connect to your ASCOM Alpaca compatible camera.
ZWO SeeStar Camera - Tap this button to connect to a ZWO SeeStar camera.
## Adding a Preset: Communication Settings
Select Auto-Detect then tap Scan Network For Devices to list available cameras.
Or select Manual Configuration and enter the IP Address and Port Number on which the server is hosted. Then tap Check IP and Port For Devices to list available cameras.
After cameras have been discovered and are listed at the top of the view, select the camera with which you want to connect and tap the Next button in the top right corner.
## Adding a Preset: Additional Options
Preset Name - Allows you to enter a custom name for your camera preset.
Timeout Seconds - The timeout is how long SkySafari will wait for a response from the camera. 5.0 seconds is a reasonable wait time, but some cameras may work better with different timeouts.
Readout Rate - The readout rate is how often SkySafari requests data from the camera.
## Accessing Your Camera After Configuration
From the Sky Chart, tap Menu in the toolbar and select Camera Control. Then tap Connect.
## ALPACA Camera Control View
Depending on your specific ALPACA camera, the following options may be available:
Description – The name/model of your camera as reported by the device.
Driver Info – Any driver or firmware information provided by the camera.
ALPACA Version – The Alpaca protocol version supported by the camera.
Gain – Adjusts the camera’s electronic amplification. Higher gain brightens the image but increases noise.
Exposure Time – Sets how long the sensor collects light for each frame.
Readout Mode – Selects the camera’s readout configuration (e.g., normal, fast, etc, if supported.
Sub-Exposure Duration(s) – For cameras that allow it, this sets the number of individual sub-exposures to be taken.
Cooler – Enables or disables the camera’s cooling system, if present.
Cooler Power – Shows the current power level being used by the cooling system.
Sensor Temperature – The current temperature of the camera sensor.
Set Temperature – The target temperature you want the camera’s cooler to maintain.
# Social Stargazing Help
Source: https://userguide.skysafariastronomy.com/app-specific/social-stargazing-help
Documentation for Social Stargazing Help
Social Stargazing Help
Stargazing is often done on your own but looking up at the stars reminds us that we are all part of a larger interconnected universe. SkySafari brings social stargazing to mobile devices with two new features to help you connect with other like minded people.
Note: An Internet connection is required to use these features.
## OneSky
OneSky connects you with other users around the world in real-time. This feature highlights objects in the sky chart being observed and displays the number of users observing the object. Astronomy can be a solitary endeavour, OneSky creates a sense of community by connecting users to each other through shared observation.
## Connecting to OneSky
Tap Menu in the Toolbar and select OneSky. The Sky Chart will show highlighted circles around objects that are currently being observed by other SkySafari users in real-time. Objects with more than 1 observer will display the number of observers under the circle.
For an object to be highlighted in OneSky, a user a) did a telescope goto on the object, b) selected the object in the sky chart in the current session, or c) is pointing at the object with the built in compass/gyros. In addition, your OneSky object selections are only included if it's dark at your set location, with the exception of the Moon and Venus.
Notice the OneSky icon on the lower left of your screen. The OneSky icon number counter displays the number of observers currently observing/selecting objects in SkySafari.
## OneSky Options
Tap the OneSky icon to bring up a popup menu with additional options.
Center Most Popular: Centers the view on the object with most observers.
Center Next: Selects the next most popular object being observed.
Show/Hide Labels: Displays or hides labels for the highlighted objects.
Make Into Observing List: Creates a list of all the OneSky objects being observed. You can find the list in “Observing Lists” by tapping Observe in the Toolbar.
Turn off OneSky: Toggles off the OneSky feature.
Privacy
We take your privacy seriously. All shared observing data is completely anonymous and statistical in nature. OneSky only records what you're looking at (not where or who you are). If you would like even this anonymous data excluded from our OneSky statistics, you can select to "Exclude My Observing From OneSky" under Settings > Privacy.
## SkyCast
SkyCast allows you to guide a friend or group around the night sky through their own copy of SkySafari. After initiating SkyCast, you can generate a link and conveniently share it with other SkySafari users via text message, apps or social media accounts.
When your friend receives and clicks on the SkyCast link, they will be guided to the object you are currently observing with the aid of location arrows. SkyCast keeps track of how many people have joined your live skycasting session.
## Starting and Ending a SkyCast
Tap Menu in the Toolbar and select SkyCast. In the popup that appears, select SkyCast to begin SkyCasting. The Share view on your device will appear, allowing you to share a link with people in your contacts or with other apps on your device.
When your contact receives the link, they just need to tap on it and their copy of SkySafari will launch, showing them a view with arrows directing them to your selected object. When you select a new object, your contacts sky chart view will update to point to the new object.
Notice the SkyCast icon on the lower left of your screen. The SkyCast icon number counter will update from 0 as your contacts join your SkyCast.
Tap the SkyCast icon to bring up a popup menu with options to end a skycasting session or to send out the link again.
Note: Take it to the next level and broadcast your own personal star show by sharing your SkyCast link alongside a FaceTime Link.
## Following a SkyCast
To follow a SkyCast you will need your own copy of SkySafari 7 installed on your device. When you receive a skycast invitation, simply click on the link and SkySafari will open automatically. Onscreen arrows point you to the object the SkyCaster has selected in their sky chart. As the SkyCaster selects different objects, the arrows will guide you to the new object.
To stop following a skycast, tap the SkyCast icon on the lower left of your screen and select Stop Following.
# Time Flow Help
Source: https://userguide.skysafariastronomy.com/app-specific/time-flow-help
Documentation for Time Flow Help
## Time Flow Help
Overview
The **Time** button located on the upper left in the main Sky Chart displays a set controls which let you flow the date and time dynamically, or adjust it step-by-step. Tap the **Time** button to show these controls; tap it again to hide them.
When visible, the time flow controls contain the following items:
**Current Time Label**: The chart's current date and time is shown at the top of the panel. If there is an underlined segment, this indicates the time unit that will be used for flowing or stepping time (e.g. by days, hours, minutes, etc).
**Calendar**: Tap the Calendar icon on the top right to quickly set a specific date/time. Select Set Time when done.
**Time Flow Arrows**: In the middle of the panel is a set of VCR controls lets you start and stop the flow of time. Tap the rightmost arrow to start the flow of time continuously forward; tap the leftmost arrow to flow time continuously backward. Tap either of these arrows to stop the flow of time.
**Time Step Arrows**: The central arrows adjust the time by a single step, equal to the time unit you have selected underneath. For example, if you selected 1 day as the time unit, tapping the center right arrow will move the time forward by 1 day - but it will not continuously run the time by one day.
Now Button: Stops the flow of time, and returns to the current date and time indicated by your device's internal clock.
**Time Units Button**: The button in the lower left shows the time unit you will change time by when stepping or flowing time. Tapping the button brings up a panel where you can change the time unit. The unit can also be quickly changed by tapping the corresponding part of the time label at the top of the panel. For example you would tap the day part of the time and date to change the unit to days.
In SkySafari Plus and Pro, the number in the time unit button is a multiplier applied to the time unit. You can tap the button to enter a new multiplier with the numeric keypad - for example: 10 minutes, 7 days, 86164 seconds (one sidereal day), or 24.659784 hours (one Martian day). Please Note: in the basic version of SkySafari, you can only use time steps of exactly one minute, one day, etc.
You'll find that different astronomical phenomena are best simulated using different time units. For example:
**Second -** best for showing the motion of fast-moving satellites.
**Minute -** best at showing the daily rise-and-set motion of the Sun, Moon, and Stars.
**Hour -** best for showing the motion of Jupiter and Saturn's moons.
Day - best for showing the motion of the planets against the background stars, as they (and we!) orbit the Sun.
**Year -** best for showing the orbital motion of binary star systems like Sirius and Alpha Centauri.
Time flow is temporarily paused when another view (like Search, Object Info, or Settings) is present. You can also use the Settings to change the simulated date and time. If you are using SkySafari to control a telescope, we do not recommend using Time Flow while the telescope is connected - simulating a view other than the current date and time may result in pointing the telescope at the wrong place in the sky!
# Tonight At A Glance Help
Source: https://userguide.skysafariastronomy.com/app-specific/tonight-at-a-glance-help
Documentation for Tonight At A Glance Help
## Tonight at a Glance
The Tonight at a Glance view provides a concise summary of what is happening in the sky. It shows important information for the Sun, Moon, curated calendar events, transient events, planets, deep sky objects and selected satellites. Tapping an entry will take you to the object or event so you can learn more about it.
## Sun & Moon Info
The Sun section provides at a glance information about the time of sunrise/sunset and dusk/dawn (darkness). The Moon section displays the phase of the moon along with rise/set times.
Note: For the Sun, Moon and planets, the rise and set times are calculated for the current day. For example Sun rise time is when it rose, or will rise today, not necessarily for the next Sun rise.
## Calendar
The Calendar provides a curated list of stargazing events for each day of the month. Tap on a calendar event to simulate it in SkySafari.
To access the full Calendar, tap the "View All" button in the Calendar section. Then tap the VIEW icon next to a particular event to view a custom sky map which illustrates the event.
## Events
The Event Finder is a powerful search engine that finds astronomical events visible tonight and far into the future. Tap on an event to simulate it in SkySafari.
## Solar System
A list of the brightest solar system objects visible from your location tonight. Tap on an object to simulate it in SkySafari.
Note: For satellites, the rise and set times refer to the next visible pass of the satellite.
## Deep Sky
A list of the brightest deep sky objects visible from your location tonight. List is sorted by transit times. Tap on an object to simulate it in SkySafari.
## Featured Articles
Curated articles and special features written by trusted experts. SkySafari Premium subscription required (See Settings - Subscription). Available on iOS only for now.
## Light Pollution Map
Interactive world map of artificial light pollution. Information on the color scale can be found here. SkySafari Premium subscription required (See Settings - Subscription). Available on iOS only for now.
## Observing Conditions
Convenient access to hourly forecasts designed for observers. SkySafari Premium subscription required (See Settings - Subscription). Available on iOS only for now.
# Astronomical Coordinates
Source: https://userguide.skysafariastronomy.com/general/basic-concepts/astronomical-coordinates
Documentation for Astronomical Coordinates
## The Celestial Sphere
Ancient astronomers perceived the sky as a large sphere with the Earth at its center. They thought the stars were attached to the surface of this great sphere, and as it rotated once each day, the stars would rise and set as they were carried across the sky. We know today that this sphere is not real. The stars and planets are at great distances from the Earth, and their apparent daily motion across the sky results from the Earth's spinning on its axis.
But this image of a celestial sphere which surrounds the Earth at some great distance remains a useful concept. Astronomers have created a grid of reference lines and points on the celestial sphere to describe the position of each star, planet, and galaxy. Every object has a numerical address in the sky and that address is on the celestial sphere.
From mid-northern latitudes, we see the stars moving across the sky from east to west. As the Earth rotates, the sky appears to pinwheel about the North Celestial Pole. In the early evening a star rises in the east; at midnight it is on the meridian; and by sunrise it sets the west. The stars and planets seem to be attached to a great dome that encircles the Earth. The dome rotates once each day.
There is a part of the sky that is always visible. Stars near the North Celestial Pole dip near the horizon, but they do not set; these stars are called circumpolar. One bright star, Polaris, is located less than one degree from the pole. As the sky turns, Polaris, being near the center of rotation, is almost stationary. This makes it a beacon, marking the direction of north for all observers and mariners in the Northern Hemisphere. Polaris is the North Star. Stars far to the south move on small arcs above the southern horizon. They are visible for only a short time. Stars very far south near the South Celestial Pole do not rise at all, and they remain unseen for observers in the north.
## Altazimuth Coordinates
Altazimuth coordinates describe the position of an object with reference to the local horizon. Azimuth is the angular measure along the horizon, beginning at 0° in the north, through 90° in the east, 180° at south, and 270°in the west.
Altitude is measured from 0° at the horizon to 90° at the zenith point directly overhead. A star with an azimuth of 225° and an altitude of +30° would be located 30° above the southwestern horizon. Points below the horizon have negative altitudes. Altitude is sometimes called elevation, but in astronomy, elevation refers to the distance one is above sea level (in feet or meters). The zenith is the point overhead. An object at the zenith will have an altitude of +90°. The nadir is the point opposite the zenith with an altitude of -90°. When you point directly upward, you are pointing toward the zenith. Point downward, and you point toward the nadir.
The meridian is the line that passes through the celestial poles and the zenith. It is the projection of the observer's Earth longitude on to the sky. A transit occurs when an object passes across the meridian. At this time, the object is at its maximum altitude in the sky.
The altazimuth coordinates of an object are local coordinates. They apply at a particular location and at a particular time. At a different location on the Earth, an object will have different altazimuth coordinates. The altitude and azimuth of a star are constantly changing as the Earth rotates, but the star’s right ascension and declination on the celestial sphere remain fixed.
## Equatorial Coordinates
We use a coordinate system of longitude and latitude to locate any point on the surface of the Earth. On the celestial sphere we use a similar system called equatorial coordinates, which are based on the Earth’s poles and equator.
The Earth's axis of rotation, extended outward from the North Pole, intersects the celestial sphere at a point called the North Celestial Pole. The star closest to this point, Polaris, is often called the North Star. A similar extension from the South Pole marks the South Celestial Pole. The Celestial Equator is the projection of the Earth's equator onto the celestial sphere.
All points along the celestial equator are equidistant from the north and south celestial poles. To define the location of an object on the celestial sphere, we first determine its angular position along the celestial equator. This coordinate is right ascension, and it is analogous to longitude on the Earth. As Greenwich marks the zero of longitude on Earth, the vernal equinox is the zero of right ascension in the sky. This point marks the Sun's position on the first day of spring, when the Sun crosses the celestial equator. The vernal equinox is currently in the constellation of Pisces, but the position changes slowly over time.
Right ascension is measured eastward in units of time (hours, minutes, and seconds), starting at 0 hours and continuing to 24 hours. An hour of right ascension is equal to 15 degrees on the celestial equator (1/24 of 360 degrees) The second equatorial coordinate is declination, and it measures an object’s angular distance from the celestial equator. Declination is measured in degrees, from 0° at the celestial equator to 90° at the celestial poles. Objects above the celestial equator have positive declination, and those below have negative declination. The North Celestial Pole is +90° and the South Celestial Pole is -90°. The position of any object in the sky can be defined by the coordinates of right ascension (abbreviated RA) and declination (abbreviated Dec). All celestial objects have an address on the celestial sphere - their RA and Dec. For example, the bright star Sirius is found at RA 6h 44.8m and Dec -16° 42’.
## Ecliptic Coordinates
As the Earth orbits the Sun, we see the Sun moving against the background stars. The Sun's apparent annual path against the background stars is the ecliptic. The planets are always seen within about 18 degrees of the ecliptic, since they orbit the Sun in nearly the same plane. The ecliptic is the zone of planets, as well as the Sun.
As the Sun moves along the ecliptic, it passes through particular groups of stars. Ancient observers organized these groups into constellations and named them after particular animals. This group of twelve constellations is the zodiac, which derives from the Greek word for "animals". These constellations came to be known as the sign of the zodiac, and they are a center piece of astrology.
The zodiac includes some prominent constellations such as Taurus the bull and Leo the lion. The majority are more obscure and not easy to identify. Astronomers eschew the term "signs of the zodiac" because of its astrological overtones. For astronomers, the constellations along the ecliptic are the "zodiacal constellations".
In this special region, we see the complex motions of the planets as they orbit the Sun. If you observe the sky at sunrise over several weeks, you can see how the Sun moves eastward against the background stars. A constellation that is near the eastern horizon at sunrise will be considerably higher in the sky at sunrise one month later. The Earth' position in its orbit has changed, and from this new perspective we see the Sun against new background stars. This annual cycle creates the changing pattern of constellations that are visible at night. It is the natural calendar of the sky.
Like the Sun, the planet's positions are continually changing with respect to the background stars. The word ‘planet’, in fact, derives from a Greek word meaning "wanderer". In comparison, the stars are essentially fixed on the celestial sphere. They are in truth moving, but because of great distances their relative positions appear nearly fixed for many centuries. The positions of the planets are often described using ecliptic coordinates. These are similar to equatorial coordinates, except that they are defined by the plane of the ecliptic, rather than by the Earth's equator. Because of the tilt of the Earth's axis, this coordinate system is inclined 23.5° to the equatorial system. The vernal equinox is the starting point of ecliptic longitude. It is measured in degrees eastward along the ecliptic. Ecliptic latitude is measured in degrees north or south of the ecliptic. The north ecliptic pole is in the constellation Draco.
# Basic Concepts Menu
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Documentation for Basic Concepts Menu
# Basic Concepts
1. [Astronomical Coordinates](astronomical-coordinates)
2. [Equinoxes](precession-of-the-equinoxes)
3. [Date and Time](date-and-time)
4. [The Seasons](the-seasons)
5. [Phases and Tides of the Moon](phases-and-tides-of-the-moon)
6. [Solar and Lunar Eclipses](solar-and-lunar-eclipses)
7. [Motion of the Planets](motion-of-the-planets)
8. [Orbital Mechanics](orbital-mechanics)
9. [Star Names and Catalogs](star-names-and-catalogs)
10. [Properties of the Stars](properties-of-the-stars)
11. [Star Formation and Evolution](star-formation-and-evolution)
12. [How Stars Die](how-stars-die)
13. [Double, Multiple, and Binary Stars](double-multiple-and-binary-stars)
14. [Variable Stars, Novae, and Supernovae](variable-stars-novae-and-supernovae)
15. [Deep Sky Objects](deep-sky-objects)
16. [Star Clusters](star-clusters)
17. [Nebulae](nebulae)
18. [The Milky Way](the-milky-way)
19. [Galaxies](galaxies)
# Date And Time
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Documentation for Date And Time
Date and Time
The positions of the Sun, Moon, planets, and stars change over the course of days, months, and years. Thus the units we use to mark the passage of time are intimately tied to astronomy. A day is (roughly) the length of time the Earth takes to turn on its axis, i.e. for the Sun to return to the same position in the sky. A month is (roughly) the time the Moon takes to orbit the Earth, i.e. to return to the same position relative to the Sun. A year is (roughly) the length of time the Earth takes to orbit the Sun, i.e. for the Sun to return to the same position among the stars.
## History of the Calendar
The first calendar to be based on the motion of the Sun was used by the ancient Egyptians. The Egyptian year had twelve months of thirty days each, plus an extra five days at the end of the year, given over to celebrations of the birthdays of the gods. Thus the Egyptian year was exactly 365 days long. But in fact, the amount of time the Earth takes to orbit the Sun is actually 365.24219... days. So the Egyptian calendar gradually fell out of step with the actual seasons, in a cycle that would take 1460 years to complete.
To solve this problem, Julius Caesar introduced the concept of leap-years in 44 BC. Every fourth year, an extra day was added to the month of February, thus making the length of the year 365.25 days. The Julian calendar prevailed for more than 1500 years, and is the predecessor of the calendar system the Western world uses today.
But 365.25 days is still not the correct length of the year, and by the 16th century the Julian calendar had gotten significantly out of sync with the seasons. The Gregorian calendar, with its more complicated leap-year rules - every 4th year, but not every 100th year, except for every 400th year - was introduced by Pope Gregory XIII. It officially replaced the Julian calendar on 10 October 1582 and has now come into universal use. However, the changeover from the Julian to the Gregorian calendar did not happen until 1752 in England and the American colonies, until 1873 in Japan, and until 1927 in Turkey.
## Julian Date
This calendar confusion presents some difficulties for astronomers - months have different lengths; different years have different numbers of days; and different calendar systems were used at different times in different parts of the world.
To rectify this problem, the 18th-century English astronomer John Herschel introduced a simple, uniform time scale to use for astronomical calculations. This is the Julian Date (JD), and it is simply the total number of days elapsed since 12 noon at Greenwich on 1 January 4713 B.C. The start of the third millennium, i.e. Greenwich midnight on 1 January 2000, corresponds to JD 2451544.5. Julian dates always start at noon (not midnight) Greenwich time, and do not observe any time zones or daylight saving time changes.
One advantage of the Julian date is that the time interval between two events is found by simple subtraction. Moreover, there are mathematical formulae to convert any Julian date to a Julian or Gregorian calendar date. If you reckon time using the Julian date, there is no Y2K problem. For all of these reasons, the Julian Date is still widely used by astronomers today.
## Universal Time and Time Zones
At any given time, half of the Earth is in sunlight and the other half is in darkness. If the Sun is overhead in Beijing, it is midnight in New York.
In 1884 an international conference divided the Earth into 24 time zones. The meridian of Greenwich, England was set as the zero of longitude, and the time zones were measured in steps of 15 degrees from this location. The exact boundaries of each zone would vary depending on local politics and circumstance.
Time zones of the world.
Universal Coordinated Time (UTC) - also called simply Universal Time (UT), or Greenwich Mean Time (GMT) - is simply the local time in the time zone of Greenwich, England. UT is traditionally counted on a 24 hour clock, and is widely used by astronomers and navigators.
## The Equation of Time and the Analemma
Our activities are regulated by the cycle of day and night; it is natural to use the Sun as our time keeper. We could define a day as the period between successive passages of the Sun across the meridian. But this does not work well, since the length of the day varies continuously throughout the year. Due to the slight eccentricity of the Earth's orbit and the tilt of the Earth's axis, the Sun's eastward motion against the background stars is not uniform. The Sun moves faster in the winter and slower in the summer. The 24 hour day would in some months be 10 minutes less, and in other months 15 minutes more. This is one reason that classic sundials provide very crude time keeping.
To resolve this difficulty, astronomers defined a "fictitious sun" that moves at a uniform rate throughout the year. This is called the mean sun, and it gives us a uniform day. The mean solar day is the average length of the apparent solar day over the course of one year. Mean solar time is what we use in our daily lives. The difference between apparent solar time (using the real Sun) and mean solar time is the called the equation of time. Over one year, it can be as large as 16 minutes.
The Equation of Time is commonly plotted on globes of the Earth, where it appears as a figure "8" curve. This figure is known as the analemma; it shows the Sun's position at exactly 1-day intervals over an entire year. The Sun moves north and south across the celestial equator because of the inclination of Earth's polar axis to its orbital plane (the Ecliptic). The Sun shifts in longitude because of the difference between the mean and apparent solar time.
## Solar and Sidereal Time
A sidereal day is the period of the Earth's rotation in relation to the stars, rather than the Sun. A sidereal day is about 4 minutes shorter that a mean solar day. Over 24 hours, the Earth moves about 1/360 of its orbit. We see the Sun drifting eastward against the stars by 1/360 of a circle, which is 1 degree. In terms of the Earth's rotation, this amounts to 4 minutes in 24 hours. If the Sun and a star are at the same position in the sky, after one rotation of the Earth, the Sun will have drifted eastward, and the star will pass overhead 4 minutes before the Sun. The sidereal day is approximately 1436 minutes, 4 minute less than 24 hours.
The Sidereal Day.
Sidereal time is "star time". It is the elapsed time in hours, minutes, and seconds since the vernal equinox crossed the meridian. The right ascension that is currently on the zenith is the local sidereal time.
## Dynamic Time
Universal Time is necessary in civil life to set our clocks, and to make the trains run on time. UT is based on the rotation of the Earth. But the Earth's rotation is uneven, and gradually slowing down - by about 1 second per year - so the average length of the day is increasing unevenly. This forces us to insert or remove leap seconds every so often to make UTC line up with where the Earth is actually pointing. Hence UTC is not a uniform time scale, and it is therefore not suitable for accurate astronomical calculations.
Precise astronomical calculations require a dynamical time scale that is free of the irregularities caused by the Earth's rotation. Terrestrial Dynamic Time (TDT) or simply Dynamic Time is based on atomic clocks, and it is the standard for precise time keeping in astronomy. The difference between UTC and TDT is known as Delta T (ΔT):
ΔT = TDT - UTC
The value of Delta T can only be deduced empirically, by observations of the Moon or extra-galactic radio sources. If the positions of the Sun and Moon are known to high accuracy at a specific dynamic time, the corresponding Universal Time (based on the irregular rotation of the Earth), and hence ΔT, can be calculated. It is currently about 70 seconds. Extrapolating backward, ΔT was as large as 4-1/2 hours around 500 B.C. - we know this value because of observations of eclipses that were recorded by the ancient Greeks.
# Deep Sky Objects
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Documentation for Deep Sky Objects
Deep Sky Objects
With the invention of the telescope, astronomers could now see objects much smaller and fainter than anything previously visible to the human eye. When turned toward the sky, the telescope revealed hundreds of faint, fuzzy new objects, not quite starlike, that had never been observed before. These objects are collectively referred to by amateur and professional astronomers alike as "deep sky objects".
Deep sky objects include star clusters, bright and dark nebulae, galaxies, and clusters of galaxies. Many of these objects have nicknames derived from the constellation where they are located, or from their appearance in a small telescope. Examples include the Great Andromeda Galaxy and the Lagoon Nebuls. A few deep sky objects have proper names handed down from historical times, like the Pleiades star cluster.
As with stars, astronomers have developed classification systems and catalogs for deep sky objects. This section lists some of the most important and well-known.
## The Messier (M) Catalog
Charles Messier (1730-1817) was a French astronomer whose particular interest was comet hunting. In 1758, Messier was one of the first to see the predicted return of HaIley's comet. In all, Messier discovered 15 new comets and claimed another six that were actually first seen by others.
In 1758, Messier was deceived by the comet-like appearance of the Crab Nebula in his telescope, and made a special note to avoid it. From that point onward, he began cataloging fuzzy-looking objects in the sky that might be mistaken for comets.
Messier produced his first list, containing 45 objects, in 1771; a second list in 1780 added another 23 objects; and his final list of 1781 (published in 1784) had 103 objects. Not all were Messier's own discoveries; several had been seen first by other observers, notably the Swiss comet-hunter Jean Philippe Loys de Cheseaux (1718-1751). Another six objects were added to Messier's list by the French astronomer Pierre Francois Andre Mechain (1744-1804).
Charles Messier’s famous list of 110 deep sky objects includes some of the brightest star clusters, nebulae, and galaxies in the sky. Astronomers still refer to objects by their Messier, or M, numbers; for example, M 1 (the Crab Nebula) and M 31 (the Andromeda Galaxy).
The English astronomer Sir William Herschel (1738-1822) was a contemporary of Charles Messier. He and his son, John Herschel (1792-1871) were also great discoverers of deep sky objects. The Herschels had better instruments, however, and were able to resolve many star clusters that appeared merely nebulous to Messier, and discovered many more hundreds of objects themselves. John Herschel's so-called "General Catalogue" of nebulae and star clusters was published in 1846. As an admirer of Messier's list, Herschel carefully avoided giving his own numbers to any object catalogued by the Frenchman. As a result, Herschel's catalog of deep sky objects is rarely used today.
## The New General Catalog (NGC) and Index Catalog (IC)
The NGG and its supplements, the IC I and II, were originally compiled by J. L. E. Dreyer in the 1880s. Dreyer was born in Denmark, but emigrated to Ireland in 1874 to work at Lord Rosse's great observatory in Parsonstown. Lord Rosse had built successively larger telescopes through the late 1830's and early 1840's. Rosse, his son, and his observers - such as Dreyer - spent years discovering and examining the known nebulae in the northern sky with the famous "Leviathan of Parsonstown".
During these observations, it became clear to Dreyer that it was time to update Herschel's so-called "General Catalogue" of nebulae and star clusters. There were simply too many new nebulae being discovered, and too many different lists to consult. Preparing observing lists or simply finding if a nebula had been previously discovered had become a time-consuming chore. The "New General Catalogue of Nebulae and Star Clusters" appeared as Volume 49, Number 1 of the Memoirs of the Royal Astronomical Society in 1888, and contained more than 8,000 objects. Dreyer subsequently expanded the NGC with two Index Catalogues (IC I in 1895, and IC II in 1908), adding nearly 5,000 objects.
The NGC and IC remain today one of the best-known catalogs of deep sky objects in astronomy. They are one of the most comprehensive general deep sky catalogs, as they include objects of all kinds - open and globular star clusters, diffuse and planetary nebulae, supernova remnants, and galaxies of all types. They also contains some erroneous entries corresponding to single or double stars, or to no object at all.
The NGC and IC include both the northern and southern celestial hemispheres, and objects in them are numbered in order of right ascension. Nearly all 110 Messier objects, and almost every deep sky objects that can be seen in backyard telescopes, has an NGC or IC number. The Andromeda galaxy (M 31) is NGC 224; the Orion Nebula (M 42) is NGC 1976.
## Specialized Catalogs
Many other lists and catalogs of deep sky objects exist, but these are mostly specialized to a particular kind of object (e.g., galaxies). Some of the most commonly-used specialty catalogs are described below.
Open Clusters are often referred to by their numbers on the lists of Trumpler (Tr), 1930; Melotte (Mel), 1915, and Collinder (Cr), 1931. Examples include Tr 24 in Scorpius, the Coma Star Cluster (Mel 111), and the "Coathanger" cluster in Vulpecula (Cr 399).
Bright nebulae are frequently referred to by their numbers in the Sharpless catalog of HII regions (Sh 2), 1959; the Lynds Catalog of Brignt Nebulae (LBN), 1965; and the Cederblad Catalog of Bright Diffuse Galactic Nebulae (Ced), 1946. Examples include the Cave Nebula (Sh 2-155) and Cederblad 214 in Cepheus.
Dark nebulae are most famously cataloged in E. E. Barnard's 1927 "Catalogue of 349 Dark Objects in the Sky". The Horsehead Nebula, for instance, is Barnard 33. Numbers from the The Lynds Catalog of Dark Nebulae (LDN), 1962, are also often used for dark nebulae. The Horsehead Nebula, B 33, is LDN 1630.
For planetary nebulae, the Perek-Kohoutek Catalog of Galactic Planetary Nebulae (PK) is a compilation of all planetary nebulae known in the Milky Way Galaxy in 1964. There is an online version of the second edition (2000). The Ring Nebula, M 57, is also known as PK 63+13.1 in the Perek-Kohoutek catalog. The 1992 Strasbourg-ESO Catalog of galactic Planetary Nebulae (PN G) is another well known catalog of these objects.
Many galaxies not listed in the NGC or IC are referred to by their number in the Uppsala General Catalogue of Galaxies (UGC), published by P. N. Nilson in 1973. Nilson also published the "Catalogue of Selected Non-UGC Galaxies" (UGCA) as an extension of the UGC in 1974. One of the largest and most comprehensive databases of galactic data is the Principal Galaxy Catalog (PGC), published by G. Paturel in 2003. It contains data for nearly 1 million galaxies, and is still being updated.
# Double Multiple And Binary Stars
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Double, Multiple, and Binary Stars
Our Sun is a single star. However, most stars in the universe are not. The majority of all stars orbit around other stars under the influence of their mutual gravitational attraction. Two stars which orbit around one another form a double or binary star system. Three stars which interact with each other form a trinary star system. In fact, over 50% of all stars are members of such multiple star systems.
There are essentially five types of multiple star systems, classified according to how the multiple nature of the system was discovered. Some multiple systems belong to more than one category.
## Optical Double Stars
Not all stars which appear close together in the sky are physically related. They could actually be at different distances from us, but simply appear along the same line of sight. Such stars are called optical double stars. A good example is Mizar (ζ UMa or Zeta Ursae Majoris) and Alcor (80 UMa) in the handle of the Big Dipper. Mizar and Alcor are physically unrelated stars at distances of 78 and 81 light years from us, respectively, but appear less than 1/4 degree apart in our sky. The ancient Greeks used the ability to resolve Mizar and Alcor as a test of visual acuity.
Mizar and Alcor, an optical double star in the Big Dipper.
Optical double stars do not have much physical significance.
## Visual Binary Stars
Visual binary star systems are those in which in which the components are physically bound to each other by gravity, and can both be seen visually. The brightest component in the system is labelled with the suffix "A", the second-brightest as "B", and so on. Systems with two, three, four, five, or six components have been identified.
Visual binary systems tend to be relatively close to us, so that the individual stars can be resolved. Their components are also widely separated physically, by distances of tens to a few hundred AUs. The stars in such systems are gravitationally bound to each other but otherwise do not interact, as do other close binaries, where one star may draw off material from the surface of the other. Less than 1,000 visual binary systems have been detected.
The apparent orbit of Alpha Centauri A and B.
The closest star system to us, Alpha Centauri, at 1.338 parsecs' distance, is a visual binary. α Cen A and α Cen B are separated by an average distance of about 23 AU - slightly greater than the distance between Uranus and the Sun. They orbit each other with a period of about 80 years. A third component, α Cen C, is also called Proxima Centauri because it is currently the closest star to us, at a distance of 4.22 light years (1.295 parsecs). For many years after its discovery in 1915, Proxima was thought to be a third member of the system at a much greater distance from the other two. Recent observations, however, suggest it may not be gravitationally bound to the system.
A small telescope reveals that Mizar or ζ UMa is also a visual binary star, with components ζ UMa A of magnitude 2.2 and ζ UMa B of magnitude 3.9, separated by 14.3".
## Spectroscopic Binaries
Binary stars which are too distant, or whose components are too close together to be resolved, will appear as one star visually. However, we can still detect the binary nature of the system by investigating its spectrum. The Doppler effect induced by the component stars' orbital motion will affect the stars' spectral lines. Such systems are called spectroscopic binaries. The majority of binary systems have been detected by Doppler shifts in their spectral lines.
Spectroscopic binaries are discovered by the doppler shift in their spectral linescaused by the component stars' orbital motion.
As the stars orbit each other, one star (A) may be moving towards us, while the other (B) may be moving away. The spectrum of A will be blue-shifted toward shorter wavelengths (higher frequencies) while B's spectrum will be red-shifted toward longer wavelengths (lower frequencies). As the stars continue orbiting, A will recede, and its spectral lines will move towards the red end of the spectrum, while B's will move toward the blue. If the stars are moving across our line of sight, then no Doppler shift occurs, so the lines stay in their mean positions.
In spectroscopic binaries, the component stars are often very close, and may in fact exchange material due to tidal interactions. Orbital periods range from a few hours to months, with separations of much less than an AU in many cases.
The first spectroscopic binary system discovered was Mizar or ζ Ursae Majoris in 1889. Mizar was already known as a visual binary, but spectroscopic analysis of the brighter component (ζ UMa A) showed that it was also a spectroscopic binary. Subsequent observations revealed that ζ UMa B was also a spectroscopic binary - thus the whole system was comprised of four stars.
## Eclipsing Binaries
Sometimes, in a binary star system, one star will pass in front of the other, producing what we call an eclipsing binary. Eclipsing binary systems are also variable stars, because the light of the combined system varies in brightness as one component eclipses the other.
There are a few thousand eclipsing binary systems known. Most are also spectroscopic binaries. A few are also visual binaries. As with spectroscopic binaries, the two stars in an eclipsing system are physically close and are often distorted by each other. Mass can be transferred from one star to the other.
A light curve must be obtained in order to classify a system as an eclipsing binary. This is simply a plot of apparent magnitude over time. Eclipsing binary light curves are characterised by periodic dips in brightness that occur whenever one of the components is eclipsed. Unless the two stars are identical in brightness, one of the eclipses (called the primary eclipse) will result in a greater drop in brightness than the other, secondary eclipse. One period of an eclipsing binary system therefore has two minima.
The light curve of the eclipsing binary star system Algol.From top to bottom: normal, secondary eclipse, normal, primary eclipse.
The first known and most famous eclipsing binary star system is Algol, or Beta Persei (β Per). Algol was given its name by the Arabs, to whom it was known as the Demon star, possibly due to its changing brightness. Algol varies from magnitude 2.2 to 3.5 over its 2.87-day orbital period.
## Astrometric Binaries
Sometimes one component of a binary star system is not observable, but can be detected as the other component moves across the sky. Since both stars are orbiting around their mutual center of mass, the visible component will show a perturbation or a "wobble" in its proper motion as it is observed repeatedly over time. Binary systems detected by such astrometric means are called astrometric binaries.
The motion of an astrometric binary.
Relatively few binaries have been detected astrometrically, primarily due to the need for long-term observations, and the uncertainty in position and proper motion measurements. The best known example of an astrometric binary is Sirius, or Alpha Canis Majoris (α CMa). In 1844, Friedrich Bessell pointed out that Sirius had a wobble in its proper motion. From this he inferred that the visible component (Sirius A) must have an dim, unseen companion (Sirius B). Sirius B was first observed telescopically by Alvan Clark in 1862, and is now known to be a dim white dwarf star. Procyon, or Alpha Canis Minoris (α CMi), was another star first detected as an astrometric binary. It too has a white dwarf companion that can now be observed telescopically.
## Binary Stars and Stellar Mass
Binary stars are of great importance to astronomers because they provide virtually the only means of directly determining the masses of stars other than our Sun. As a star's mass determines its life cycle and fate, being able to accurately determine stellar masses is vital to refining our models of stellar evolution.
To find the mass of a binary system we need to apply Kepler's Laws. Adapted for a binary system, they are:
1. The stars orbit each other in elliptical paths, with the center of mass (or barycenter) as one common focus.
2. A line between the stars sweeps out equal areas in equal periods of time.
3. The square of the orbital period, T, is directly proportional to the cube of the average distance from the centre of system mass, r: T2 = r3.
Determining the mass (M) of a binary star system whose component stars (A and B) are separated by a distance r.
# Galaxies
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Documentation for Galaxies
Galaxies
What do other galaxies look like? Galaxies come in many sizes and shapes, but can generally be classified into three types: elliptical (E), the spiral (S), and "none of the above": irregular (I). There are sub-classes for the elliptical and spiral galaxies. (For example, E0, E1, E2, and S0, S1, S2). This system of galactic classifications was first worked out by the astronomer Edwin Hubble.
Galaxy types according to Hubble's classification system.
Note that this chart is not an evolutionary track for galaxies; we do not know much about galactic evolution at the moment.
## Spiral Galaxies
Spiral galaxies contain both younger, bluish stars and older, reddish stars. As with our own Milky Way, the center of a typical spiral galaxy is a bulge called the nucleus, with a radius of a few tens of thousands of light years. The galaxy's spiral arms are contained in a flat disk. Usually the disk is only a few thousand light years thick, but up to several hundred thousand light years across.
Galactic nuclei are often the source of intense radio or X-ray emissions. We believe that tremendous black holes, with thousands of times the mass of our Sun, are the source of these emissions. Such black holes are thought to exist at the center of most spiral galaxies, including our own Milky Way.
The famous "edge-on" spiral galaxy NGC 4565 in Coma Berenices. (Jim Misti.)
The most prominent feature a spiral galaxy is the spiral arms. These consist of young, bluish stars, extending from the center to the edge of the disk. However, the laws of orbital mechanics should not allow the arms to exist at all, because the stars near the edge of the disk will orbit the nucleus much slower than the stars near the center. Hence the arms should "wrap around" each other be unable to survive.
The face-on spiral galaxy M 101 in Ursa Major (left), whose typical spiral structure is explained by the density wave theory (right). (Robert Gendler.)
The most widely-accepted theory to explain the existence of spiral arms is the density wave theory. This says that the stars do indeed orbit at different speeds, but that the density of the interstellar medium forms a wave. At the wave front, where the density is higher, star formation is triggered. That is why we find young stars along the spiral arms and old stars elsewhere.
Spiral galaxies are sub-divided into two classes: normal spirals and barred spirals. The most famous example of a "normal" spiral galaxy is the Andromeda Galaxy (M 31), about 2,000,000 ly away in the constellation Andromeda. M 31 is visible to naked eye on dark nights as an elongated, fuzzy patch. It is, in fact, the most distant object that can be seen with the naked eye.
Hubble Space Telescope image of barred spiral galaxy NGC 1300
The image above shows a barred spiral spiral galaxy, NGC 1300 in Eridanus. The core of a barred spiral is elongated, thus giving the name. There is evidence that our own Milky Way galaxy is also a barred spiral.
## Elliptical Galaxies
The galaxy in the photo is M87, a typical elliptical galaxy. Ellipticals usually consist of old stars. In general, they are fainter than spiral galaxies. Some dwarf ellipticals have as few as 10 million stars, making them not very different from a large globular cluster.
Elliptical galaxy M 87. (Jim Misti.)
Up to a few years ago, we believed ellipticals were the most common type of galaxies. But now, we believe the irregular galaxies are more common.
## Irregular Galaxies
These galaxies have an shape that cannot be classified into either of the categories above. The two satellite galaxies of our own Milky Way, the Large Magellanic Cloud and the Small Magellanic Cloud, are both irregular galaxies. They are clearly visible to the naked eye from the southern hemisphere.
The Large Magellanic Cloud, including the Tarantula Nebula (NGC 2070). (Robert Gendler).
Many irregular galaxies are thought to be formed by galactic collisions. When galaxies collide, the stars inside them never collide with each other. But the distribution of the stars is distorted by their mutual gravity. Streams of stars may be ejected\to form something like antennae, and/or the two galactic cores may merge. Some galaxies we see with multiple nuclei may be the end result of the merger of two galaxies a long time ago. Galaxy collisions may also trigger star births in their intersecting regions.
These colliding galaxies, NGC 4038 and NGC 4039 in Corvus, are known as the "Antennae" galaxies. (Jim Misti.)
Quasars
Another kind of interesting object is the quasar, which is an abbreviation of the term "quasi-stellar object". When observed in visible light, quasars are small and dim, appearing just like faint stars. However, in the infrared and radio parts of the spectrum, they are quite bright. Their spectra show large amount of redshift, indicating that they are very far away from us. Thus, we conclude that they must radiate enormous amounts of energy - even more than active galaxies nearby. A quasar is probably a super-massive black hole at the center of a young galaxy.
## Distances to Galaxies, Red Shift, and the Expanding Universe
Before 1920s, we did not know that galaxies were far away. We thought they were located inside our galaxy, and were called spiral nebulae.
Today, we mainly use two methods to measure the distances to other galaxies. The first method is the Cepheid variables. The second method is Type I supernovae. Recall that a Type I supernova is the a strong explosion of a nova that destroys a white dwarf. We have found that the luminosities of Type I supernovae are all about the same. Therefore, if we can see a Type I supernova in a galaxy, we can compute its distance by measuring its apparent brightness. Type I supernovae are much brighter than cepheid variables. As a result, the supernovae allow us to measure the distances to galaxies much further away. But they happens much less frequently. Each method has its advantages and disadvantages. Both are distance indicators.
The redshift of a galaxy (often referred to by astronomers as ‘z’) is the shift in the galaxy’s spectral lines due to the galaxy’s motion away from the Earth. A galaxy with a redshift of z = 0.01 will be receding at 3,000 kilometers per second, which is 1% of the speed of light. A few nearby galaxies actually have blue shifts (i.e. they are moving towards us), but these are just the small random motions of nearby objects. Galaxies at large distances in space uniformly have redshifts in their spectral lines, indicating motion away from us.
In the 1920s, Edwin Hubble discovered that galaxies at increasing distances had proportionately larger redshifts, and therefore higher radial velocities. The farther the galaxy, the faster is was moving away from us. The universe was expanding. The “Hubble Constant” is the numerical factor that relates a galaxy’s radial velocity to its distance:
radial velocity = Hubble Constant x distance
Hubble's Constant is currently estimated to be about 75 kilometers per second per megaparsec. This means that for each 75 kilometers/second of radial velocity, a galaxy will be 1 megaparsec away (3.26 million light years). If you know the radial velocity, you can find the distance. A galaxy receding at about 7,500 kilometers/second is at a distance of 100 megaparsecs (i.e. 7500 = 100 x 75). A galaxy receding at 15,000 km/second will be at a distance of about 200 megaparsecs (i.e. 15,000/75 = 200). The farthest known galaxy, Abell 1835 IR1916, has a redshift of about 10, and hence a distance of over 13 billion light years.
# How Stars Die
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How Stars Die
A star will spend about 90% of its life in the main sequence phase, in which hydrogen nuclei fuse into helium nuclei in its core. A common misconception is that a more massive star has more "fuel", and will have a longer lifetime. But in fact, because the core of a massive star is hotter and denser, it burns much faster and has a shorter life. The most massive stars will use up the fuel in their cores in about one million years. For less massive stars, their main-sequence lifespans could be up to tens of billions of years.
## The Death of Low Mass Stars
We are first going to discuss the fate of a star with a mass less than, or about equal to, that of our Sun.
When no hydrogen is left in the star's core, the fusion reaction stops. The helium core of the star starts to collapse. Its core keeps on contracting and heating until it is hot enough for a helium fusion reaction to occur. In this reaction, three helium nuclei fuse together to form a carbon nucleus.
The gravitational contraction will heat up the hydrogen envelope surrounding the core. Fusion therefore begins in the envelope, and the star's envelope expands. As the star becomes very large - up to 100 times its previous size or more - its surface temperature decreases, and the star's surface appears red. Although its surface is cool, the star's core temperature is still very high, and the star's total luminosity is also high because its surface area is so large. This kind of large, bright, cool, red star is called a red giant. All red giant stars were once on the main sequence.
The Sun, which formed about 4.6 billion years ago with the rest of our solar system, has a main-sequence lifetime of about 10 billion years. Thus, in about five billion years, our Sun will enter its red giant stage. The inner planets Mercury, Venus, Earth, and possibly Mars, will be swallowed into the expanding sun and will not survive its red giant stage.
Size comparison of the Sun as a main sequence and red giant star (left); internal structure of a red giant (right)
A red giant star's helium-rich core and hydrogen-burning shell do not produce energy in a stable and steady manner. All red giants are variable stars. Such stars pulsate, and eventually, will expel their outer envelope of material into interstellar space, creating a "planetary nebula" like the Ring Nebula (M 57) in Lyra.
Meanwhile, the star's helium-rich core collapses again. But now the core is not massive enough to heat up sufficiently to fuse helium into carbon. Gravity crushes the star's core until finally even the electrons making up a its atoms are smashed together. The star is supported by electron degenerate pressure. As it collapses, the star grows hotter but much smaller and fainter. It has become a white dwarf.
The Ring Nebula, M 57 (left), and Sirius and its white dwarf companion (right); both imaged by the Hubble Space Telescope.
A typical white dwarf is slightly smaller than the Earth, but with about the same mass as our Sun. Its density is about 300,000 times that of rock. A white dwarf has no source of energy, so its luminosity comes from residual heat in the core. After it has radiated away all of its residual energy, it becomes a black dwarf. However, the time required for this is much larger than the age of the universe. So, we believe there are as yet no black dwarfs in the cosmos.
The most famous of all of the white dwarfs is the companion of Sirius. It is visible in a modest-size telescope. The bright star Procyon also has a white dwarf companion.
If a star is less massive than about 0.4 solar mass, its life will be quite uneventful. It will quietly and steadily burn its hydrogen into helium and become a white dwarf without ever entering the helium-burning red giant stage.
## The Death of High Mass Stars
We will now discuss what happens to a main sequence star with a mass over about 5 solar masses. Because of the star's great mass, its core temperature and density are higher. The star's surface is hotter and bluer, so the star is of spectral type O, B or A. Like other main sequence stars, hydrogen nuclei fuse to form helium nuclei in its core. However, the massive star will have a shorter life span. A 15-solar-mass star will deplete its hydrogen after only about 10 million years.
But because of its large mass, the temperature and pressure at its core is high enough to trigger helium fusion into carbon. The helium will burn steadily, and its higher energy production rate will heat up the surface. The star will swell to a size even larger than a red giant, and we have a red supergiant.
A typical red supergiant can be about 100 times larger than a red giant. Its surface temperature is low while its total luminosity remains high - up to a million times that of our Sun. The stars Betelgeuse in Orion and Antares in Scorpius are both examples of red supergiants.
Evolutionary tracks of 1, 5, and 10 solar mass stars after the main sequence on the H-R diagram.
As with helium fusion, the strong gravitational forces at the star's core controls the carbon fusion. Light atoms fuse into heavier and heavier atoms. We believe all of the heavy elements found on Earth were made in a star somewhere a long time ago by this mechanism. Carbon (C) fuses to oxygen (O), nitrogen (N), and silicon (Si), until finally silicon is fused into iron (Fe). Iron is, in fact, the dead end of nuclear fusion. To fuse iron into even heavier elements, we have to supply more energy than the reaction generates. This is also why we can produce energy when we split an atom heavier than iron - like uranium - into several smaller ones.
The internal structure of a red supergiant on its last day.
Supernovae
After enough iron accumulates in a supergiant star's core, the pressure there decreases rapidly. In less than a second, the inner core collapses and heats up dramatically. All fuel, if not yet spent, will fuse to iron and nickel. The outer core collapses along with the inner core. The upper limit of nuclear density prevents the inner core from compressing too far, so the collapsing inner core bounces back outwards. The out-going inner core collides with the in-coming outer core. The collision sends off shock waves and creates heavy elements, like uranium. The outer layers of the star are thrown off into space. This is a supernova explosion.
A supernova is extremely violent. The brightness of the star will increase by up to 15 magnitudes, and it may outshine its entire galaxy for a few days or weeks. It is a spectacular astronomical event. The most recent supernova visible to human eyes was SN 1987A, located in a small nearby galaxy, the Large Magellanic Cloud. Another famous supernova was recorded in 1054 A.D. by Chinese astronomers in the Sung dynasty. They discovered a "guest star" in the constellation we now call Taurus. That star was visible in the day time and remained visible for two months. The remnant of that supernova, which contains the material ejected from the exploded star, became the Crab Nebula, or M 1. The Crab Nebula, visible through a small telescope today, is still expanding, and will eventually dissolve into the surrounding interstellar medium.
The Crab Nebula, M 1, imaged by the Hubble Space Telescope.
Neutron Stars and Pulsars
What happens to the remains of the star after the supernova explosion? It depends on the mass of the core that is left over. After the explosion, if remaining mass is less than 1.4 solar masses, a white dwarf will form. But if the remaining mass is more than 1.4 solar masses, then electron degenerate pressure is not strong enough to support the star against further collapse. The electrons are squeezed into the nuclei, and are combined with the protons to form neutrons. Then neutron degenerate pressure will stop the star from collapsing further. The star will contract to a size even smaller than a white dwarf. A neutron star is formed.
A neutron star is composed mostly of neutrons (about 95% - 99%), with trace amounts of electrons and protons. Its typical size is about 8 to 16 km in radius, which is roughly the size of New York City. The gravitational field on the surface of a neutron star is millions of times stronger than at the surface of the Earth.
Another very important property of a neutron star is its strong magnetic field. It ranges from 108 to 1015 times the magnetic field strength at the surface of the Earth. When electrons move in spirals around magnetic lines of force, they produce radio waves which radiate out along the two magnetic poles of the star. Usually, the magnetic poles do not align with its rotational axis, so the radio beams will sweep around like the beam of a lighthouse. What we observe on Earth is pulses of radio waves with very fast but very stable period. This is a pulsar - one can be found at the center of the Crab Nebula.
How a pulsar works (left). Image of Crab Nebula pulsar (right) by the Chandra X-Ray Observatory satellite.
As the mass of a supergiant collapses to form a neutron star, its rotation rate increases rapidly. This is caused by conservation of angular momentum, exactly the same reason as a twirling ice-skater with outstretched arms will spin faster when she brings her arms in. Many pulsars - spinning neutron stars - rotate as fast as 1000 times per second or more, and are among the most regularly-ticking "clocks" in the heavens. Due to their large masses, their rotational periods are very regular. We can specify the periods of some pulsars up to more than ten decimal places. When they were discovered in the 1960s, pulsar radio emissions were first thought to be signals from intelligent life. Astronomers now use pulsars to calibrate their measurements of the rotation of the Earth.
## Black Holes
For a star more massive than about 4 times the mass of the Sun, not even neutron degenerate pressure is strong enough to halt the star's final collapse. The star's core becomes a black hole - a mysterious object so dense that not even light can escape the intense gravitational pull at its surface. Because no light can escape a black hole, we cannot observe such an object directly. However, we can observe matter falling into the black hole, at speeds approaching the speed of light. Under these conditions, matter emits X-rays and other high-energy wavelengths. The most famous black hole in the sky, Cygnus X-1, was first the first black hole to be observed from its X-ray emissions.
# Motion Of The Planets
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Motion of the Planets
The ancients knew that in addition to the Sun and Moon, there were five other bright objects that slowly moved among the stars. Those other objects are the planets. In fact, the name "planet" comes from the Greek word for "wanderer". Over the course of weeks and months, the planets appear to move forward, then stop in their tracks, move backward, then resume course again. The strange, looping paths that the planets charted across the sky must have been a great source of curiosity and puzzlement to ancient observers.
The path of Mars through the constellations of the Zodiac over the course of a year.This is just one example year; the planets' paths are different every year.
The Geocentric Model
Contrary to popular belief, the ancient Greeks knew that the Earth was round. They knew this because of the way ships appeared mast-first over the horizon, and because of the round shape of the Earth's shadow on the Moon during a total lunar eclipse. The Greeks also believed that the Earth was at the center of the universe.
Around 400 B.C., Plato claimed that heaven is perfect, and that the circle is the most perfect form. Thus, the heavens are in uniform circular motion with the Earth at its center. This was the beginning of the geocentric model. This simple geocentric model cannot explain the retrograde motion of the planets, however. Around 140 A.D., Ptolemy proposed a refined geocentric model. (He proposed many refinements; we'll only discuss the simplest one.)
Ptolemy's geocentric universe.
In the Ptolemaic universe, a planet moves in a small circle called an epicycle, and the center of the epicycle moves along a larger circle around the Earth. Since Mercury and Venus never appear very far from the Sun, the centers of the epicycles of Mercury and Venus must lie on a line joining the Earth and the Sun. Stars are fixed to an outermost sphere.
This model gives predictions of the positions of the planets that are accurate to within a few degrees of their actual positions. The Ptolemaic model was generally accepted, and dominated the western world for about 1500 years.
## The Heliocentric Model
Nicolaus Copernicus (1473-1543) proposed the heliocentric model. In this model, the center of the universe is the Sun, not the Earth. The Earth is just another planet orbiting around the Sun, and it no longer has a special place in the universe.
Nicholas Copernicus (left); the heliocentric universe (right).
This model is simple and elegant, and it explains the retrograde motion of planets. Even though the accuracy of this model was as good as Ptolemy's, the Copernican model was not commonly accepted in its time.
The heliocentric model neatly explains the retrograde motion of Mars.
Galileo and the Telescope
Galileo Galilei (1564-1642) was a great defender of the heliocentric model. He did not actually invent the telescope; in fact, no one's sure who invented it. But Galileo was the first person to observe the sky with a telescope, in 1609. When he did so, Galileo made four major discoveries:
* The Moon's surface is covered with craters and mountainous terrain.
* There are dark spots on the Sun.
* Four satellites orbit around Jupiter.
* Venus goes through a full set of phases, just like the Moon.
The first two discoveries proved that the heavens were not perfect. The discovery of satellites orbiting Jupiter showed that there are other "centers" in the universe; in fact, those four satellites are now called the Galilean satellites. The phases of Venus proved that Venus must orbit the Sun, not the center of an epicycle.
Galileo's observation of the phases of Venus proved that Venus must orbit the Sun, not the center of an epicycle.
Galileo was condemned for "doing science." Galileo searched for the truth by observing and performing experiments - but the Vatican believed the truth could only be found in faith. The Inquisition sentenced Galileo to life imprisonment, and he was confined to his villa for the last ten years of his life.
But in the end, Galileo was right.
## Kepler's Three Laws
In the late 16th century, the wealthy Danish astronomer Tycho Brahe (1546-1601) undertook a program to measure the positions of the planets with unprecedented accuracy almost every night. The large amount of data he accumulated allowed his assistant and successor, Johannes Kepler (1571-1630), to discover the three laws of planetary motion.
Kepler's first law states that the orbits of the planets around the Sun ellipses (not circles) with the Sun at one focus. One way to draw an ellipse is to pin down the ends of a string, then use a pencil to stretch out the string. The positions of the two pins are the foci, and curve drawn around them is an ellipse.
Kepler's second law states that a line from a planet to the Sun sweeps over an equal area in an equal interval of time. This means that when the planet is closer to the Sun, it moves faster.
How to draw an ellipse (left); Kepler's second law (right).
Kepler's third law states that the square of a planet's orbital period (P) is proportional to the cube of its average distance from the Sun (a).
These three laws could explain Brahe's data to the limit of his observational accuracy, and finally proved that the heliocentric model was correct.
A demonstration of Kepler's third law (left); Kepler and Brahe (right).
Newton and Universal Gravitation
Kepler did not know the reasons behind his three laws; he had just deduced them from observational data. He knew that they were true, but he did not know why they were true. Sir Issac Newton (1642-1727) provided the theoretical basis for Kepler's three laws - and much more - from even more basic assumptions.
Newton realized that four basic principles could explain the motion of (essentially) anything in the universe. The first three principles are known as Newton's Three Laws of Motion. The fourth is the Law of Universal Gravitation. Newton's laws are as follows:
1 - Every object stays at rest, or in uniform motion in a straight line, unless another force acts on it. This is inertia.
2 - If a force acts on an object, the objects accelerates in the direction the force was applied, at a rate that is directly proportional to the force, and inversely proportional to the object's mass. In other words, F = M a.
3 - Whenever one object exerts a force on a second object, the second exerts an equal and opposite force on the first.
4 - There is an attractive force between any two objects, equal to a constant (G) times the product of the objects' masses (M1 and M2), divided by the square of the distance between them: F = G M1 M2 / r2
An apple, for example, falls to the ground because the apple and the Earth attract each other. Now, it is not difficult to understand why the Moon orbits around the Earth. If the Earth were not there, the Moon would fly away in a straight line (inertia). The Earth and the Moon attract each other (gravity), so the Moon falls toward the Earth, just like an apple. This falling keeps the Moon in its orbit. Similarly, the Earth and other planets are constantly "falling" around the Sun.
Inertia and Newton's Law of Universal Gravitation (left) explain why the Moon orbits the Earth (right).
Newton's gravitational theory also predicts that, in general, an object's orbit can be any one of the four conic sections: a circle, an ellipse, a parabola, or a hyperbola. Conic sections get their names because they take the shapes of the cross sections of a cone. We have found some comets in parabolic or hyperbolic orbits.
Newton's laws predicted that all orbits must take the shape of a conic section.
The story that an apple hitting Newton's head was the inspiration for his law of gravitation might not be true. But Newton did make the realization that the same forces which affect objects on Earth also explain their motion in the heavens. Newton was the founder of modern physics.
# Nebulae
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Nebulae
Nebulae are clouds of gas and dust, illuminated by starlight.
In most regions of interstellar space, there is about 1 atom per cubic centimeter. However, in some regions, the density is over 1000 atoms per cubic centimeter. (This still about the density of the best vacuum made by humans!) These regions are called nebulae.
## Bright Nebulae
A nebula by itself cannot produce any light energy. But if there are some bright stars inside the nebula, the gases in the nebula may be ionized by energy from those stars and emit light. This is exactly how the gas in a neon tube emits light, when ionized by electricity. Thus, this kind of nebula is known as an emission nebula. The most common gas in interstellar space is hydrogen, which emits red light when ionized. So, emission nebulae usually appear reddish in color.
A second kind of bright nebula reflects light from nearby stars in front of it, rather than emitting it from ionized gas. This kind of nebula is known as a reflection nebula, and is typically bluish in color. Emission and reflection nebulae are often found near each other - a classic example is the Trifid Nebula (M 20) in Sagittarius.
The Trifid Nebula, M 20. (Jim Misti.)
Nebulae are the birthplaces of the stars, and are often associated with the star clusters that are forming from it. The typical size of a nebula is about a hundred light years. Most nebulae are located in our Milky Way galaxy, and so (like open clusters) are concentrated along the plane of the Milky Way in our sky.
# Orbital Mechanics
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Orbital Mechanics
Today, astronomers still use the elliptical orbits described by Kepler, and predicted by Newtonian physics, to describe the motions of the planets, comets, asteroids, and nearly every other object in the solar system.
## Orbital Elements
A Keplerian orbit can be defined by six mathematical quantities, called the orbital elements. The first two elements define the shape and size of the orbit; the remaining elements define the orbit's orientation in space, and the object's position in its orbit.
The eccentricity (left) defines the shape of the orbit; the other elements (right) define its size and orientation.
* The Eccentricity (e) defines the shape of the orbit. It is the ratio of the distance between the orbit's foci to the length of its major axis, and it defines the shape of the orbit. For elliptical orbits, the eccentricity is less than 1; for a circular orbit, it is zero; for parabolic orbits, the eccentricity is exactly 1; and for hyperbolic orbits, the eccentricity is greater than 1.
* The semimajor axis (a) is the size of the orbit. The semimajor axis is defined as half the length of the long axis of the orbit. The periapsis distance (q) is sometimes used instead of the semimajor axis. The periapsis is the point where the object is closest to the body it orbits.
* The inclination (i) is the angle between the object's orbital plane and the ecliptic plane.
* The longitude of ascending node (Ω) is the angle in the Ecliptic plane from the vernal equinox to the ascending node. The ascending node is where the object's orbit crosses the ecliptic plane from south to north.
* The argument of periapsis (ω) is the angle in the orbital plane from the ascending node to the periapsis. The longitude of periapsis (π) is sometimes used instead of the argument of periapsis. If so, you can find the argument of periapsis by subtracting the longitude of the ascending node (Ω): in other words, ω = π - Ω.
* The mean anomaly (M) describes the object's position in its orbit at a particular point in time. When the object is at periapse, the mean anomaly is zero. Sometimes a different quantity, the mean longitude, is used instead of the mean anomaly. If so, you can find the mean anomaly by subtracting the longitude of perihelion (π). In other words, L = M + π = M + ω + Ω.
The orbital elements of the planets, comets, asteroids, and even other objects such as artificial earth satellites are usually expressed in this format. Using those elements, the object's position can be computed using Kepler and Newton's laws for years in advance to a very high degree of accuracy.
Perturbations and Osculating Elements
If an object and the object it orbits (called its primary) were the only two objects in the universe, then their trajectory would be a perfect Keplerian orbit such as described above. Hence, this kind of orbit is sometimes called a "two-body" orbit. But in reality, there are far more than two objects in the universe. Because of the gravitational perturbations of the other planets, the orbit of any object in the solar system is always changing. For earth-orbiting satellites, other forces - such as atmospheric drag - also apply, so a single set of orbital elements degrades much faster (weeks instead of years).
A single set of orbital elements will gradually become less accurate over time. An object's orbital elements at a particular moment in time (or epoch) are referred to as its osculating elements. At that one moment in time, the path predicted by those elements will exactly match, or "kiss", the object's true path through space (the word "osculating" derives from the Latin verb for "to kiss".)
Using only a single set of osculating orbital elements, the error in the predictedposition of as object (in this case the asteroid Ceres) grows over time.
In fact, modern science can only predict the positions of the planets to "trustworthy" accuracy within about 10,000 years of the present. The fundamental problem is that the motions of the planets are chaotic - that is, essentially unpredictable - in the long term.
We do not know the current positions, masses, and velocities of all planets and moons in the solar system to calculate their positions to arcsecond precision more than a few thousand years from the present. For instance, the positions of Saturn’s moons is affected by the gravity of Saturn’s rings, which in turn depends on the mass of Saturn’s rings - and that is an all-but-unknown quantity. While we do know that 500,000 years ago, the Earth was in an orbit roughly the same size and shape as it is today, we simply can’t predict which side of the Sun it was on.
# Phases And Tides Of The Moon
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Phases and Tides of the Moon
Other than the Sun, the second-most obvious object in the sky is of course the Moon.
The time between successive new Moons, called the synodic period, is about 29.5 days. Interestingly, the rotational period of the Moon relative to the Sun is exactly the same as the synodic period. Thus, from the Earth, we can only see one side - by definition the near side - of the Moon. This is called synchronous rotation. It is not a coincidence. The mutual gravity of the Earth and Moon locks Moon's the revolution and rotational rates.
## Phases of the Moon
Since the Moon only shines by reflecting sunlight, the amount of its surface that is illuminated - its phase - varies at different times of the month. The cycle of phases starts from new moon to waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, waning crescent, then to new moon again.
The phases of the Moon.
The moon rotates at a uniform rate, which is the same as its average orbital period around the Earth. But the moon's orbit is not perfectly circular, so sometimes the moon is slightly ahead of its average position in its orbit. The moon's orbit is also slightly inclined (tilted) to its rotational axis. Because of these facts, we can sometimes see a little bit more of one side of the Moon than the other. This phenomenon, called libration, lets us see just over 60% of the Moon's surface, in total.
## The Tides
The tides are probably the most well-known phenomenon we experience on the Earth that is due to the influence of the Moon - werewolves notwithstanding! The Moon's gravity exerts a stronger attractive force on the near side of the Earth, less at the center of the Earth, and even less on the far side. Relative to the center of the Earth, the effective forces pull water on the Earth both toward and away from the Moon. They lead to two bulges of water on the Earth's surface. Therefore, we have two high tides per day.
Why there are two tides per day.
In addition to the Moon, the Sun also attracts water on Earth. When the Moon is new or full, the tidal effects from the Sun and Moon reinforce each other. The high tides at these times - called spring tides - will be higher than the high tides at other times. When the Moon is at first or last quarter, the tidal effects tend to cancel each other out, so the high tides are lowest then. These are called neap tides.
Spring Tides (top) and Neap Tides (bottom).
Tidal Evolution of the Earth and Moon
Because the Earth is rotating, the tidal bulge does not point exactly toward the Moon. The Earth turns faster than the Moon orbits, so the Earth's tidal bulge actually points slightly ahead of the Moon (by about 10°). The Moon's gravity tugs back on the bulge, which very slowly causes the Earth's rotation rate to slow down. Similarly, the gravity of the bulge tugs the Moon forward, which very slowly causes the Moon to speed up in its orbit. Due to this tidal interaction, the length of both the day and the month is slowly increasing, and the Moon is slowly receding from the Earth. Hundreds of millions of years from now, the Earth and Moon will both be "tide-locked" to each other: the Earth will rotate at exactly the same rate that the Moon orbits. The Earth will always keep the same face toward the Moon, just as the Moon, today, always keeps the same face toward the Earth.
Tidal interactions slow the Earth's rotation and lengthen the Moon's orbital period.
Just as the Moon raises tides on the Earth, the Earth's gravity raises tides in the solid structure of the Moon. But because the Earth is much more massive than the Moon, the Earth tide-locked the Moon's rotation billions of years ago. This is why the Moon always keeps the same face pointed toward the Earth.
We encounter many other examples of such tidal locking in other pairs of objects in the Solar System.
# Precession Of The Equinoxes
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Precession of the Equinoxes
The line of equinoxes is the line where the plane of the Earth’s orbit intersects the plane of the Earth’s equator. The vernal equinox is the point where the Sun, traveling around the Ecliptic, crosses the celestial equator from south to north, around March 21st every year. The vernal equinox defines the zero point of both right ascension (for equatorial coordinates) and longitude (for ecliptic coordinates).
Unfortunately, the Earth’s polar axis shifts very slowly, in a roughly-circular motion around the north ecliptic pole, over a period of about 26,000 years. This motion is called the precession of the equinoxes or just precession. As a result of precession, the vernal equinox slowly creeps around the Ecliptic over the same 26,000-year period. Thus the equatorial and ecliptic coordinates of the stars slowly shift over time, and different stars are seen near the celestial poles.
## Hipparchus Discovers Precession
In the second century B. C., the ancient Greek astronomer Hipparchus found that the distances of certain stars from the vernal equinox had changed. When he compared his observations of Spica with those made by earlier astronomers, he saw that its distance measured eastward from the vernal equinox had increased by about two degrees over 150 years, or about one minute of arc per year.
Other stars gave similar results. Hipparchus concluded that the change was not a motion of individual stars, but as a change in the positions of the equinoxes. In the 15th century, Copernicus argued that the westward drift of the equinoxes resulted from a wobbling motion of the earth like that of a spinning top. The earth’s precessional motion causes the equinoxes to move westward along the ecliptic, completing a cycle in about 26,000 years.
The wobble of the Earth's axis causes precession of the equinoxes.
If you were to extend the Earth’s axis into the sky, over thousands of years it would trace out a circle among the stars. The center of the circle is called the ecliptic north pole. The radius of the circle is 23 1/2 degrees; this is the inclination of the earth’s axis from the plane to the ecliptic.
## Precession and Coordinates
As a result of precession, the vernal equinox slowly creeps around the Ecliptic over the same 26,000-year period. And since the equinox defines the zero point of right ascension and ecliptic longitude, precession means that those coordinate systems are (slowly) moving as well. So to unambiguously describe an object's position in equatorial or ecliptic coordinates, you also need to specify the year (usually called the epoch in this context) for the coordinates system as well. Equatorial coordinates are usually given for "standard epochs" such as 1950, 2000, etc. - but are sometimes also given for intermediate dates (e.g. 1984.5) or the "current epoch" (which is roughly 2007.9 at the time of this writing).
How precession affects equatorial and ecliptic coordinates.
Pole Stars of the Past and Future
As a result of precession, the "Pole Star" Polaris, which currently lies above the earth’s north pole, was not the pole star many years ago. At the time of Copernicus and Columbus, the earth’s axis pointed nearly 4 degrees away from Polaris. Presently Polaris is about 3/4 of a degree from the pole, and it is getting closer each year; in 2110, Polaris will only be 1/2 degree from the pole. In the following centuries, the Earth’s axis will no longer point toward Polaris, and other stars will take their turns at being the "North Star". The brilliant star Vega will be about 5 degrees from the pole in 14,000 A.D.
The earliest pole star for which we have historical records is Thuban (Alpha Draconis), in the constellation Draco. When the Great Pyramid of Gizeh was built in Egypt, around 3500 BC, Thuban was about 3 1/2 degrees from the pole. The pyramid has an inclined gallery directed toward a point about 3 1/2 degrees below the pole. An observer at the bottom of this narrow passage could see Thuban as it crossed the meridian below the pole each night. Thuban was closest to the pole around 2800 B.C.; 15,000 years ago the pole star was Deneb (Alpha Cygni).
# Properties Of The Stars
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Properties of the Stars
The stars in the sky vary greatly in brightness, color, and distance. These are the basic properties of the stars, and measuring them is fundamental to understanding their lives and evolution.
## Brightness and Magnitude
The most obvious thing about the stars is that they differ in brightness. The brightness of the stars was first estimated by the Greek astronomer Hipparchus in the second century B.C.. Hipparchus’ system divided the stars into five magnitude classes. The brightest stars in the sky were assigned first magnitude and the faintest stars were classified as fifth magnitude, with all others falling somewhere in between. In general, the brighter an object is, the smaller its magnitude is. The brightest objects, like the Sun or the Moon, actually have negative magnitudes.
The Magnitude Scale
This scale has been formalized in modern times so that two stars which differ by 5 magnitudes have a ratio of brightness of exactly 100. A first magnitude star is exactly 100 times brighter than a sixth magnitude star. A difference of one magnitude corresponds to a brightness ratio of 2.512 (the fifth root of 100), and the ratio for a difference of two magnitudes is 6.31 (2.512 times 2.512). The magnitude scale is not linear!
Using this system, the brightest star in the night sky is Sirius, and has a magnitude of -1.46. The planet Venus is even brighter at magnitude -4.6, and the daytime Sun has magnitude -27. The faintest stars visible on a clear night are about 6th magnitude. A small telescope allows one to easily see stars to 10th magnitude and fainter. The world’s largest telescopes are capable of detecting objects of magnitude 25, and the Hubble Space Telescope reaches magnitude 28 and fainter.
## Star Color and Temperature
It is apparent to any observer that the stars vary in color as well as brightness. Stars differ in color because they have different temperatures. By accurately measuring the colors of the stars, astronomers can calculate the temperature on the surface of the star.
When a piece of metal is heated, it changes colors as the temperature changes. As it starts to glow, it first changes to red. As the metal gets hotter, it turns orange. Next it turns yellow-white, then blue-white. This color pattern also applies to the stars.
The reddish stars Betelgeuse and Antares are cool stars, though still about 2,500 degrees Kelvin. Stars like Arcturus are orange at about 5,000 degrees Kelvin. Sun-like stars are yellow at 6,000 degrees Kelvin. White stars like Vega are hotter still at 10,000 degrees Kelvin. The Blue giants like Rigel and Deneb have surface temperatures of about 20,000 degrees Kelvin.
Measuring the apparent color of a star requires observing its magnitude at different wavelengths. This involves several factors: the wavelengths considered (some objects are brighter at infrared or ultraviolet wavelengths than in visible light), and the sensitivity of the detector (the eye is less sensitive to blue light than standard photographic film). A star's visual or "V" magnitude represents its brightness at the wavelength to which the human eye is most sensitive. A star's blue or "B" magnitude is its brightness at the wavelength to which photographic film is most sensitive. The difference in these magnitudes, or "B-V", is called the star's Color Index, and it represents an accurate measurement of the star's color.
Star surface temperature vs. B-V color index.
The coolest, reddest stars, such as Betelgeuse and Antares, have a B-V color index of about +2.0. The hottest, bluest stars, like Rigel in Orion, have a color index of about -0.25. Stars with a color index of 0.0, such as Vega, appear white. The Sun has a color index of about +0.62.
## Parallax and Distance
The first measurement of the distance to a star was published in 1838 by the German astronomer Friedrich Bessel. The star was 61 Cygni, an inconspicuous 5th magnitude star in the constellation Cygnus.
To find the distance to 61 Cygni, Bessel measured the star's parallax. Parallax is the apparent shift in the position of an object caused by a shift in the observer's position. To see how this works, hold your thumb in front of your face, and view it with your left eye. It will be seen against the background objects in the room. If you now close the left eye and use your right eye, your thumb will appear to move in comparison with the other eye. Your thumb seems to shift back and forth as you alternate viewing with your right and left eye. This is parallax.
In a similar way, we can observe the position of a star at a particular time of year against the more distant background stars. Then, six months later, when the Earth is on the opposite side of its orbit, a second observation will show a small shift in the star's position in relation to the distant stars. This angular shift is the parallax of the star, measured by observing from opposite sides of the Earth's orbit. Since the radius of the Earth's orbit is exactly 1 AU, or about 150 million kilometers (93 million miles), the distance to the star can be calculated.
@ Diagrams/Parallax @
\| The Parallax of a star (p) lets us calculate its distance (d). |
A nearby stars will have a large parallax, and a more distant star will have a small parallax. Unfortunately, even the nearest stars have a parallax angle smaller than 1 arc second or 1/3600 of a degree. This is about the size of a postage stamp when seen from a distance of 5 miles. 61 Cygni is in fact one of the nearest stars known, with a parallax of 0.287 arc seconds. The nearest star, Proxima Centauri, has a parallax of 3/4 arc second, and it is over 4.2 light-years from our solar system.
The parallax technique is still the most reliable method of finding stellar distances. The European Space Agency's Hipparcos satellite, launched in 1991, measured the parallaxes of over 120,000 stars with unprecedented accuracy. The Hipparcos Catalog, resulting from that mission, remains the most accurate compendium of stellar distances today.
## Light-Years and Parsecs
To express the very large distances to the stars, astronomers need a special unit of distance. Miles or kilometer are much too small to be practical. The method of parallax leads to an astronomical yardstick called the "parsec". One parsec is simply the distance at which a star would have a parallax of 1 arc second. One parsec is an immense distance - over 30 trillion kilometers, or 19 trillion miles.
Astronomers also use a unit of distance called the light-year. One light-year is the distance that light travels in one year. Since the velocity of light is 300,000 kilometers per second or 186,000 miles per second, light travels very far in one year - about 10 trillion kilometers (6 trillion miles). One parsec is about 3.26 light-years. Light-years and parsecs are analogous to feet and meters.
When discussing distant clusters and nebulae, distances are often measured in "kiloparsecs". A kiloparsecs is 1000 parsecs, just as a kilometer is 1000 meters. When discussing the distances to galaxies, the term "Megaparsec" is commonly used. A Megaparsec is 1 million parsecs or one thousand kiloparsecs.
## Absolute Magnitude and Luminosity
A star can be bright because it is relatively close to our solar system, or because it is far away, but very luminous. Two stars of the same apparent magnitude will differ in their true brightness, or luminosity, if one star is at a greater distance.
To determine the true brightness of a star, its magnitude must be adjusted to indicate how bright the star would appear if it were seen at some standard distance. The standard distance used to compare the brightness of stars is 10 parsecs. The magnitude that a star would have if it were at a distance of 10 parsecs is called its absolute magnitude. The absolute magnitude measures the intrinsic brightness or luminosity of the star; its apparent magnitude measures how bright the star appears.
The Sun has an apparent magnitude of -26.7, but its absolute magnitude is a modest +4.85. From 10 parsecs, the Sun would be a faint 5th magnitude star.
The brightest star in the sky is Sirius has an apparent magnitude of -1.46, and is about 2.64 parsecs (or 8.6 light-years) away. With an absolute magnitude of +1.45, Sirius is 23 times more luminous than our Sun. But its energy output is modest when compared with some other giant stars. We see Sirius as a bright star because it is very close to our Sun.
The second brightest star in our night sky is Canopus (magnitude -0.62), which is almost as bright as Sirius. But Canopus is nearly 96 parsecs away - 36 times farther away than Sirius. Canopus must be a very luminous star to appear so bright from such a great distance. It has an absolute magnitude of -5.53.
The bluish-white star Deneb in Cygnus is among the brightest stars in the summer sky (magnitude +1.33), and yet it is nearly 1000 parsecs (3200 light-years) away. Deneb is bright because it is a very luminous star. It emits over 250,000 times as much light energy as the Sun. At a distance of only 10 parsecs, Deneb would shine at magnitude -8.65. It would rival the first quarter moon in brightness, and it could be easily seen in the daytime sky.
## Star Spectral Types
When directed through a prism, the light of a star divides into a rainbow or spectrum of colors that can be observed and recorded on photographic film. Under careful observation, the spectrum of a star usually has a sequence of dark lines, marking wavelengths of light that are absorbed by the chemical elements in the star's atmosphere. The temperatures and chemical compositions of the stars vary greatly, which affects the strengths of their spectral lines.
Early 20th-century astronomers divided stellar spectra in a sequence of classes designated by letters. This system has evolved into the present-day set of spectral classes: O, B, A, F, G, K, and M. A few stars do not fit into this scheme, and they have been given their own special classes: R, N, S, W. Modern instruments are able to resolve a star's spectra into tenths of a class. For example, you will see stars of spectral type B9, G4, and K3.
Dark lines in the spectrum of star reveal the chemical elements in its atmosphere.
The O and B stars are hot and blue. The bright star Sirius is of spectral type A. The Sun is a yellow G type star. The K and M stars are red in color and comparatively cool. The brightest of these are the red giants such as the stars Antares and Betelgeuse.
The main stellar spectral types: O, B, A, F, G, K, M.
Within a given spectral class, stars are divided into luminosity classes. These classes permit differentiating, for example, between a red giant and red dwarf. The luminosity classes are:
I - Supergiant
II - Bright Giant
III - Giant
IV - Subgiant
V - Main Sequence
VI - Subdwarf
VII - Dwarf
Doppler Shift and Radial Velocity
A star's spectrum tells us something else about the star: how fast it is moving toward or away from us. We can determine this because of the Doppler Effect, named after Johann Christian Doppler (1803-1853) who first proposed the effect in 1842.
You are already familiar with the Doppler effect - you hear it every time a train or an ambulance comes towards you, then goes past you. The pitch of the sound made by the train rises as it approaches you, then drops as it goes by - this is the Doppler effect for sound waves. As the train moves towards you, its sound waves are compressed, so their wavelength is shortened. We hear the decrease in wavelength as a rise in pitch. As the train moves away from you, the sound waves stretch out. We hear the increase in wavelength as a drop in pitch.
For light waves, the same thing happens when the light comes from a source that is moving toward or away from us. But we see the change in wavelength as a change in color, not pitch. So if a star is moving towards us, the light waves it emits will have their wavelength shortened, and we will see a shift toward the short-wavelength end of the spectrum - a blue shift. If the star is moving away from us, its light will have its wavelength increased, and we will see a shift toward the long-wavelength end of the spectrum - a red shift. We can precisely measure the blue or red shift by observing the positions of the star's spectral lines, compared to the corresponding spectral lines from a stationary light source. This is how the Doppler effect is used by astronomers to work out the velocity of a star toward or away from the Earth.
The Doppler Effect for sound (top left), for light (top right) and its effect on a star's spectral lines.
Radial velocity is the term which astronomers use to mean the velocity that an object is moving toward or away from us. As a convention, radial velocity is positive if the source is moving away from us, and negative if the source is moving towards us.
For example, the spectral lines emitted by hydrogen gas in distant galaxies is often observed to be considerably redshifted. The spectral line emission, normally found at a wavelength of 21 centimeters on Earth, might be observed at 21.1 centimeters instead. This 0.1 centimeter redshift would indicate that the galaxy is moving away from Earth at over 1,400 kilometers per second (over 880 miles per second).
Modern astronomical spectrographs are capable of extremely precise measurements of radial velocity. They can detect doppler shifts which correspond to radial velocity changes of only a few meters per second. With this level of precision, astronomers can detect the very slight changes in a star's radial velocity caused by a nearby planet's gravitational pull tugging the star back and forth as the planet orbits the star. In fact, this is exactly how the first confirmed planet orbiting another star, 51 Pegasi B, was discovered in 1995.
# Solar And Lunar Eclipses
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Solar and Lunar Eclipses
The fact that the Moon always keeps the same face turned toward the Earth is not a coincidence. What is a coincidence is the fact that the Sun and Moon appear almost the same size in our sky. This coincidence of nature produces the phenomenon of solar eclipses, which are among the most spectacular astronomical events one can witness. The moon also experiences lunar eclipses, which you can think of as solar eclipses seen from the Moon.
## Lunar Eclipses
Lunar eclipses occur whenever the Moon passes into the shadow of the Earth. The shadow of the Earth can be divided into two parts, the umbra and the penumbra. In the umbra, the Sun is totally blocked by the Earth. In the penumbra, the Sun is only partially blocked. Therefore, if the Moon is completely inside the umbra, there will be total lunar eclipse. If only part of the Moon passes inside the umbra, there will only be a partial lunar eclipse. If the Moon only passes through the penumbra, we say that there is a penumbral lunar eclipse. If you were on the Moon at that time, you would see a partial solar eclipse.
A lunar eclipse.
For a lunar eclipses to occur, the Moon must be "behind" the Earth as seen from the Sun. If so, then why we do not have a lunar eclipse every month? The reason is that the plane of the Moon's orbit does not coincide with the plane of the Earth's orbit. So, during most full moons, the Moon is either south or north of the orbital plane of the Earth. For the same reason, we do not see a solar eclipse during every new moon.
Why we don't see an eclipse every month.
Solar Eclipses
If you reverse the roles of the Earth and Moon in the above paragraphs, you will get a solar eclipse instead of a lunar eclipses. The most common kind of solar eclipse is the partial solar eclipse. If you are at the right position at the right time, you might see a total solar eclipse. However, because the path of a total eclipse is very narrow, you will probably not see even one total solar eclipse in your lifetime if you stay in one place on the Earth and wait.
A total solar eclipse.
A total solar eclipse is a very stunning experience, and people will travel around the globe to watch it. During a total eclipse, the sky is dark enough to see bright stars and planets. The main disk of the Sun is blocked by the Moon. The Sun's much dimmer corona becomes visible. A total eclipse typically lasts about two minutes. Then, you see the Sun again, and the total eclipse is over.
There is another kind of solar eclipse. Since the distance between the Moon and the Earth is not constant, the angular size of the Moon can have a small variation. It could happen that the Moon is at the right position, but its angular size is too small, to cover the whole Sun. This is called an annular eclipse. Viewers in the path of an annular eclipse will see a ring of Sun at mid-eclipse.
An annular solar eclipse.
Because of their tidal interactions, the Moon is slowly receding from the Earth. Millions of years from now, there will be no more total solar eclipses, because the Moon will be too far away to cover the Sun in the Earth's sky.
# Star Clusters
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Star Clusters
A star cluster is a group of stars held together by gravity. Star clusters consist of many more stars than individual star systems, from tens to millions of stars. Stars in a cluster all formed at the about the same time; hence have roughly the same age. They are also at approximately the same distance from the Earth. This greatly simplifies analysis of the stars in the cluster. For example, their relative apparent magnitudes are equal to their relative absolute magnitudes. Thus, studies of star clusters have provided astronomers with valuable insights into stellar evolution.
## Open Clusters
Open clusters are groups of up to several-hundred stars, loosely held together by gravity. They are called open because their stars are relatively far apart and their shapes are irregular. The diameter of an open cluster is typically less than 100 light years.
The stars in an open cluster are mainly younger stars, and the cluster is often associated with nebulosity - a cloud of gas and dust - from which the stars formed. Over time, the stars in an open cluster will disperse as the gravity of the Milky Way pulls the stars in different directions. (Our Sun was formed in, and was once a member of, an open cluster.)
The Pleiades (M 45) open cluster, imaged by Robert Gendler.
Most of the open clusters we see are located in our Milky Way galaxy; hence we see them concentrated along the plane of the Milky Way in the sky. Well-known examples include the Pleiades or "Seven Sisters" (M 45) in the constellation Taurus, and the Jewel Box cluster (NGC 4755) near the Southern Cross. The brightest open clusters, like the PIeiades, are easily visible to the naked eye.
## Globular Clusters
A globular cluster consists of up to several million stars tightly bound by gravity into a region only a few tens of light years across. The stars are extremely concentrated - if our Sun were located at the center of a globular cluster, the night sky would be filled with thousands of stars rivalling the full moon in brightness.
The stars in a globular cluster are typically much older than those in an open cluster. Globular clusters are dominated by red giants, white dwarves, and other examples of late stellar evolution. Most globulars orbit our Milky Way galaxy at tens of thousands of light years distance, and are (roughly speaking) evenly distributed around the center of the Milky Way. Since the center of the Milky Way lies in the direction of the constellation Sagittarius, we see the distribution of globulars concentrated toward Sagittarius in the sky.
Globular cluster M 13 in Hercules. (Jim Misti.)
The best-known examples of globular clusters are probably the Great Hercules Cluster (M 13), and Omega Centauri (NGC 5139) in the southern hemisphere. A few of the brightest globular clusters (like M 13) are barely visible to the naked eye on dark nights; binoculars will show them as a faint, fuzzy glow. Backyard telescopes can resolve globulars into their thousands of individual stars, and with large observatory telescopes, astronomers can observe globular clusters orbiting other galaxies such as the Andromeda Galaxy (M 31) as well.
# Star Formation And Evolution
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Star Formation and Evolution
There is wide variation in the luminosities, spectral types, and temperatures of the stars. One of the great astronomical discoveries of the 20th century was understanding the connection between the luminosity and temperature of the stars. Over a star's lifetime, these properties change as the star evolves to a final fate, determined by its initial mass.
## The Hertzsprung-Russell (HR) Diagram
In the first decade of the 20th century, the Danish astronomer Ejnar Hertzsprung and American astronomer Henry Russell made the first study comparing the luminosity (absolute magnitude) with the spectral types of the stars. In 1914 they published what has come to be known as the Hertzsprung-Russell (HR) diagram. For nearly a century, the HR diagram has been a powerful visual tool for understanding the properties and evolution of stars.
The HR diagram is a graph with the absolute magnitude on the vertical axis and the spectral type on the horizontal axis. The spectral sequence is equivalent to a temperature sequence, with the hot O and B stars on the left, and the cool K and M stars on the right.
For most stars, there is direct relationship between spectral type and luminosity. The HR diagram features a broad diagonal band known as the main sequence. It shows a direct connection between the luminosity and the spectral type or temperature. The most luminous stars are very hot with spectral types of O, B and A; examples are Rigel in Orion and Murzim in Canis Major. These stars are in the upper left of the HR diagram.
Average stars like the Sun are of spectral type G in the middle of the graph. The dim K and M stars, such as Barnard's Star and Proxima Centauri, are cool and red at the bottom right of the main sequence. In general, stars have increasing luminosity with increasing temperature.
The Hertzprung-Russell Diagram.
There are many stars not on the main sequence. In the upper right corner of the H-R diagram is a group known as red giants. Some of the more famous stars in the sky are red giants: Betelgeuse in Orion, Antares in Scorpius, Delta Cephei, and Aldebaran in Taurus.
In the lower left corner of the HR diagram is a unique group of low luminosity stars known as white dwarfs. This is the "stellar graveyard". A relatively low mass star like the Sun will eventually become a white dwarf.
## Stellar Mass and Evolution
Modern astronomy has revealed that the initial mass of a star is the key to understanding a star's evolution. A star's mass is its fuel reservoir. The most massive stars burn hydrogen at a prodigious rate. They are brilliant, but their lives are short - no more than a few million years. Lower mass stars like the Sun are far less luminous, but they can shine for billions of years.
The masses of stars are found from binary stars with well-known parallaxes like Alpha Centauri. Using the parallax, i.e. the distance, to a binary star system, the true size of its orbit in space can be calculated from the size of its apparent orbit in the sky. Then, given the orbital period, the masses of the individual component stars can be calculated. The known range of stellar masses is less than 0.01 to over 1000 times that of the Sun.
We can represent the formation and evolution of a star by an evolutionary track on the H-R diagram. Note that a star's position on the H-R diagram shows its physical properties, not its position in the sky. A track on the H-R diagram represents the changes in a star's luminosity and temperature throughout its lifetime, not its motion through space!
## Star Birth and the Main Sequence
Stars form inside clouds of gas and dust in interstellar space. If the cloud is visible, we call it a nebula. Interstellar clouds can be extremely large and massive, up to thousand of light years in diameter, and contain from 10 to 1000 times the mass of the sun. But the density of a nebula is very low. And it contains mostly hydrogen.
Star-forming region at the heart of the Eagle Nebula, M16, seen by the Hubble Space Telescope.
If undisturbed, the interstellar cloud will not change. However, disturbances do occur. Such disturbances may be caused by the collisions of galaxies, a density wave in the spiral arms of the galaxy, the shock wave of a supernova, or even the birth of a new star nearby. A slight change in density will trigger a contraction of the cloud due to its own gravity. A sphere known as protostar is then formed. Models of protostars show that they will have accretion disks and jets. The jets are not long-lived, and last only about 100,000 years. If the gas and dust still cover the young star, we may not be able to see the star but we might see the two clouds produced by the jets.
Upon contraction under its own gravity, the protostar heats up. Since there are as yet no nuclear reactions inside a protostar, a protostar is not a star yet. If it is massive enough (the lower mass limit is thought to be about 0.1 solar mass), the gas in the protostar continues to heat up until the central portion becomes hot and dense enough for the hydrogen atoms to overcome their mutual electrical repulsion. Nuclear fusion then takes place, and a star is finally born. The light and heat generated by the star will push out the surrounding gas and dust. The accretion disk remains and becomes the protoplanetary disk, where the planets are formed later on. The first direct observation of a protoplanetary disk around another star (Beta Pictoris) was made in 1984.
Diagram of a protostar (left); image of the accretion disk around Beta Pictoris (right) in infrared light.
It takes anywhere from 10,000 to 100 million years for the cloud to become a star. A protostar with a mass of 10 to 30 solar masses contracts to approximately the size of our solar system in only 10,000 years or so, and becomes an O- or B-type star. Less massive protostars will eventually become stars of spectral types G, K, or M. If the protostar is not massive enough to burn its nuclear fuel, it becomes a brown dwarf, which is very dim and hence very difficult to find.
Before the protostar contracts, it is very cold and dim. Thus, it is represented by a point on the lower right in the H-R diagram. Upon contraction under its own gravity, the protostar heats up, and thus moves to the left in the diagram. Its luminosity per surface area increases because the temperature increases. But if a star is not very massive, its luminosity will drop because the size of the protostar decreases much faster.
The evolutionary tracks of protostars of 1, 5, and 9 solar masses.
After a star enters the main sequence stage, its energy comes from nuclear fusion, the combination of several hydrogen atoms into a helium atom. The main sequence is the region of stability on the H-R diagram. Stars live out the majority of their lives with a balance between gravitation and the radiation pressure from the nuclear reactions in their interior. Main sequence stars have stable luminosities and sizes. They are quite "boring" compared to other stars.
# Star Names And Catalogs
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Star Names and Catalogs
The brightest stars have proper names. For example, the brightest star in the sky is Sirius. Many of the stars with you are familiar with have have names like Vega, Deneb, Altair, Betelgeuse, Rigel, Regulus, Polaris, etc. Those names have mainly been handed down to us from the ancient Greeks or Arabs, from many hundreds or thousands of years ago.
There are about 10,000 stars bright enough to be seen with the naked eye. With telescopes, we can see hundreds of thousands, if not millions, more. That is far too many star to require proper names for each one! Hundreds of years ago, astronomers devised different systems for naming stars; those systems are still commonly used today.
## Bayer (Greek) Letters
While only the brightest stars have proper names, a larger set of stars have Bayer or Greek letters. Around the year 1600 Johannes Bayer, in what is now Germany, applied lower case Greek letters to the stars in each constellation, more or less in order of brightness. The Latin possessive form of the constellation is appended to the Greek letter. For instance, the brightest star in Canis Major (Sirius) is Alpha Canis Majoris; the second-brightest star in Orion (Rigel) is Beta Orionis, etc. Constellation possessives are often written with a three-letter abbreviation, e.g. Alpha Canis Majoris is abbreviated α CMa, and Beta Orionis is β Ori. All stars with proper names generally also have Bayer letters.
For reference, here is a table of the letters in the Greek alphabet:
α - Alpha
β - Beta
γ - Gamma
δ - Delta
ε - Epsilon
ζ - Zeta
η - Eta
θ - Theta
ι - Iota
κ - Kappa
λ - Lambda
μ - Mu
ν - Nu
ξ - Xi
ο - Omicron
π - Pi
ρ - Rho
σ - Sigma
τ - Tau
υ - Upsilon
φ - Phi
χ - Chi
ψ - Psi
ω - Omega
Not every constellation uses every letter in the Greek alphabet. And with only 24 letters, Bayer quickly ran out of letters in larger constellations. He increased the possible number of names by applying superscripts to stars that fall near one another (a string of stars in Orion became π1 Ori, π2 Ori, π3 Ori, and so on). But the system was still limited. Bayer therefore followed the lower-case Greek alphabet with lower-case Roman letters, then with upper case Roman letters. These are rarely used nowadays, however, and only a few vestiges like "h Persei" (actually a star cluster in Perseus) and G Scorpii survive.
Flamsteed Numbers
To organize more of the naked eye stars, the eighteenth century John Flamsteed numbered the stars in each constellation by right ascension, from west to east. For example, 1 Lyrae would be the westernmost star in Lyra, 2 Lyrae the next, and so on. 3 Lyrae also happens to be Vega, or α Lyrae. All of the stars with a Bayer letter also have a Flamsteed number. However, if a star has both, it's more common to use the Bayer letter.
## The Yale Bright Star (HR) Catalog
The first of the 20th century star catalogs was the "Yale Bright Star Catalogue," which serially numbers 9096 naked-eye stars from west to east beginning at right ascension zero hours. Though now published at Yale, it derives from a catalogue produced at Harvard in 1908, so the star catalog numbers take on the name "HR" for "Harvard Revised." Vega = α Lyrae = 3 Lyrae = HR 7001. For naked eye stars, HR numbers are still in common use today.
## The Durchmusterung (BD, CD, CP) Catalogs
With the advent of astro-photography in the 1850s, stars below naked-eye visibility needed catalogue numbers, too. The most famous general catalogue for fainter stars, the "Bonner Durchmusterung" (the Bonn Survey), was compiled in Germany in the nineteenth century, and lists stars through around tenth magnitude. That is about 50 times fainter than the naked eye can see alone. It divides the sky into declination bands one degree wide, and then serially numbers the stars from west to east according to their right ascension (for the year 1855). The catalogue name incorporates the declination. Vega, for example, is BD+38°3238, which means "the 3238th star in the declination strip between 38 and 39 degrees north".
The BD covers stars from the north celestial pole to 2 degrees south of the celestial equator. The rest of the southern hemisphere is covered by the "Cordoba Durchmusterung" (the Cordoba, Argentina, Survey), or CD; and the "Cape Photographic Durchmusterung", or CP. Canopus (HR 2326) is CD-52°914, or the 914th listed star between declination 52 and 53 degrees south. BD, CD, and CP are sometimes combined as "DM" for "Durchmusterung."
Precession has now moved many stars out of their original declination strips.
## The Henry Draper (HD) Catalog
The most commonly used catalog for fainter stars is the Henry Draper (HD) Catalogue. This catalog was first published in 1924, and was originally created to list the spectral classes of over 300,000 stars. It has been updated and extended over the subsequent years. The current HD catalog (1985) includes position, magnitude, and spectral type data for 272150 stars. The HD catalog serially numbers stars through roughly tenth magnitude according to their right ascension in the year 1900. Vega is HD 172167; Canopus is HD 45348.
## The Smithsonian Astrophysical Observatory (SAO) Catalog
In the 1960s, over ten positional catalogues were combined into the Smithsonian Astrophysical Observatory (SAO) star catalogue. It serially numbers over 259,000 stars (according to right ascensions for the year 1900) to ninth magnitude, in 10 degree declination strips from north to south. Vega is SAO 067174; Canopus is SAO 234480. Though it enjoyed some popularity, the SAO catalog is no longer in common use.
## The Hipparcos (HIP) and Tycho (TYC) Catalogs
The Hipparcos satellite was launched in 1989 to measure the distances, brightness, and motions of the stars with unprecedented accuracy. The resulting Hipparcos Catalogue lists data for 118,218 stars, most of which are brighter than magnitude 8 (although several thousand fainter stars of astrophysical interest were also included). The Hipparcos catalog, which emerged from this mission, was the most accurate source of basic star data available for a long time. (In 2016, the data from the Gaia mission were published, with over one billion stellar positions, and distances to more than two million stars.) Vega is HIP 91262, Canopus is HIP 30438.
The Hipparcos satellite also measured the positions of many fainter stars to a lesser degree of accuracy. This secondary set of measurements formed the Tycho catalog. The Tycho catalog includes 1,058,331 stars, most brighter than 11th magnitude.
An updated version of the Tycho catalog (Tycho-2) was released in 2000. This new version combined Hipparcos mission data with measurements from the 19th-century photographic catalogs. Separated by more than 100 years in time, these observations yielded (after Hipparcos) the most accurate source of stellar proper motion data available. Tycho-2 contains over 2.5 million stars with precise positions and proper motions, and magnitude information at multiple wavelengths.
The Tycho-1 and Tycho-2 catalogs use the same star-numbering scheme as the original Hubble Guide Star Catalog (see below).
## The Hubble Guide Star Catalog (GSC)
The Hubble Guide Star Catalog was created in the late 1980s to support the Hubble Space Telescope mission. It was by far the largest star catalog created up to its time. It listed positions for nearly 19 million objects, of which about 15 million were stars. The rest were faint galaxies, photographic plate defects, artifacts of the image processing system, etc. The GSC divides the sky into 9357 zones, and numbers the objects sequentially within each zone. For example, GSC 3118-1106 is a 13th-magnitude star near Vega in Lyra.
In 2001-2002, the original photographic material generating the GSC was reprocessed and combined with Tycho-2 data. The resulting Guide Star Catalog 2 contains data for an even larger number of objects - almost 1 billion! - and has much more accurate position and brightness measurements than the original GSC. The GSC2 uses a completely different numbering scheme, however. In the GSC2, the 13th magnitude star 3118-1006 has the number N0220212-290.
# The Milky Way
Source: https://userguide.skysafariastronomy.com/general/basic-concepts/the-milky-way
Documentation for The Milky Way
## Overview
The Milky Way
Galaxies are systems of billions of stars, clusters, and nebulae, and are located many millions or billions of light-years from Earth. All the stars visible to the naked eye are located inside our own galaxy, the Milky Way.
The Milky Way
When viewed from the Earth, our own Milky Way galaxy appears to be a band of light across the sky. The stars in the Milky Way are so numerous and so dense that the ancients saw it as as a kind of "river" through the heavens. The first person to understand the true nature of the Milky Way was Galileo. When he turned his small telescope skyward in 1601, he was able to resolve the Milky Way into its millions of individual stars.
The center of the Milky Way is located in the direction of Sagittarius, which is a summer constellation in the northern hemisphere. Thus, the Milky Way is more spectacular in the summer. To see the Milky Way in its full glory, very dark skies are required, far away from city lights. Anyone observing the Milky Way will also notice dark "dust lanes", which block the light of stars further behind them. Observers in the southern hemisphere will also see two smaller patches of light, the Magellanic Clouds (named after Ferdinand Magellan, who was the first European to see them). The Magellanic Clouds are actually two small, nearby satellite galaxies which orbit the Milky Way.
Panorama of the Milky Way centered on Sagittarius. Note the Magellanic Clouds to the lower right. (Axel Mellinger.)
The discovery of true size and shape of the Milky Way was one of the great advances of 20th-century astronomy. We now know that the Milky Way contains about 200 billion stars - more than the total number of humans who have ever lived!
At the center of the Milky Way is the galactic nucleus, with a radius of about 10,000 light years. The nucleus consists of mainly older stars, and is very densely packed. Orbiting around the nucleus, the remainder of the Milky Way's stars, gas, and dust are spread out into a flat disk. The diameter of the disk is about 100,000 light years, but its thickness is only a few thousand light years. Thus, the disk is very thin compared to its diameter.
It is difficult to detect the spiral arms in our galaxy because: 1) we are inside the galaxy, and 2) the dark dust lanes block our view. Radio astronomers have traced out the spiral structure of the arms using interstellar hydrogen, which generates a characteristic radio emission with a wavelength of 21 cm. Most of the sources of this 21-cm emission are hydrogen gas clouds, which are abundant in the spiral arms. Our Sun is located in the Orion spiral arm, about two-thirds of the way out from the center.
The structure of the Milky Way galaxy.
The nucleus and disk are surrounded by the galactic halo, which is spherical and even larger than the galactic disk. The halo consists of old stars (some in globular clusters) and interstellar matter.
# The Seasons
Source: https://userguide.skysafariastronomy.com/general/basic-concepts/the-seasons
Documentation for The Seasons
## Overview
The Seasons
The Earth has seasons because its axis of rotation is tilted with respect to the plane of its orbit. During the summer months in the Northern Hemisphere, the axis leans towards the Sun resulting in long days and short nights. The first day of summer occurs about June 21 each year; this is the summer solstice. At the same time in the Southern Hemisphere, the axis leans away from the Sun. The winter days are short and the nights are long. June 21 is the winter solstice in the Southern Hemisphere.
In six months the Earth is on the opposite side of its orbit. The axis has remained fixed in space, but from this position in the orbit, the northern tip of the axis now leans away from the Sun. This is the winter solstice. At this same time in the Southern Hemisphere, the axis tilts toward the Sun. Summer begins and the days grow long.
About March 21 the Earth is midway between winter and summer. The Sun lies on the celestial equator, and day and night are of equal length all over the Earth. This is the spring equinox (meaning "equal night") when spring begins in the north and fall begins in the south. Around September 21 is the fall equinox when spring begins in the south and fall begins in the north.
The Seasons.
Throughout the year, the Sun appears alternately above and below the celestial equator for six month intervals. When the Sun is above the celestial equator and high in the sky in the Northern Hemisphere, it is low in the sky in the Southern Hemisphere. Seasons alternate between the hemispheres every six months. Summer in Australia and South America is winter in North America and Europe.
# Variable Stars Novae And Supernovae
Source: https://userguide.skysafariastronomy.com/general/basic-concepts/variable-stars-novae-and-supernovae
Documentation for Variable Stars Novae And Supernovae
Variable Stars, Novae, and Supernovae
Some stars appear to change in brightness over time. Some of these variable stars exhibit periodic behaviour, changing their brightness in a repeating pattern. Other stars vary in an irregular, unpredictable fashion. Still others exhibit a one-time dramatic change in brightness by orders of magnitude before fading away to obscurity.
Astronomers classify variable stars according to their observable properties. The first criteria for classification is whether a variable star in an intrinsic or an extrinsic variable. Intrinsic variables are those whose change in brightness is due to some physical change in the star itself. Extrinsic variables are those in which the light output changes due to some process external to the star itself.
The main types of variable stars.
A graph showing the variation of brightness with respect to time is called a light curve. Light curves are used to distinguish and classify variable stars.
## Eclipsing Binaries
These variables are binary star systems. When one star goes behind the other, there is a reduction of their combined luminosity. Since the orbital motion of the stars is periodic, the light curve is also periodic. However, not all binary systems are eclipsing - it depends on the inclination angle of the orbital plane of the system to our line of sight.
## Rotating Variables
Our Sun has sunspots on its visible surface, which are cooler regions that appear darker than the surrounding areas. A side of the Sun with a lot of sunspots would have a fractionally lower light output than a side with fewer spots. This principle applies to other stars, some of which are have much more "starspot" activity. Starspots can be either dimmer or brighter than surrounding regions. As a star with starspots rotates, its brightness changes slightly. Stars exhibiting such behaviour are called rotating variables.
## Pulsating variables
Pulsating variables periodically expand and contract their surface layers. In the process they change their size, effective temperature and spectral properties. Unlike the eclipsing and rotating variables whose brightness changes are due to geometry, these stars are variables because of their intrinsic structure.
Cepheid variables are named after Delta Cephei (δ Cep), whose variability was discovered by John Goodricke in 1784. Cepheids have light curves similar to the one shown above. Their periods have a range of a few days to a few months, and show a definite relation to their luminosities. The longer the period, the brighter the star is. Thus, by measuring the period of a Cepheid variable, we know its absolute magnitude; from that, we can tell how far the star is by comparing its apparent magnitude. Cepheid variables, therefore, are a vital tool in galactic and extragalactic distance determination.
Light curve of Delta Cephei, the prototypical Cepheid variable.Delta Cephei has a period of 5.37 days and a magnitude rang of just under 1.
Long Period Variables have periods of months to years. They are further classified according to whether they exhibit regular periodicity, or more irregular behavior. They are cool red giants or supergiants whose luminosities can range from 10 to 10,000 times the Sun's.
The first long-period variable discovered was Mira or Omicron Ceti (ο Cet), established as a variable star in 1638. Mira has a period of 331 days, and varies in brightness by almost 6 magnitudes over the course of one cycle. Its radius varies by 20 percent, peaking at 330 times that of our Sun. As a red giant, its surface temperature ranges from 1900 K to 2600 K.
Semiregular Variables show some periodicity, but also exhibit irregularities where they appear to be stable. They are giant and supergiant stars, with periods ranging from a few days to several years. Their change in brightness is typically less than two magnitudes. The light curves of semiregulars have a variety of shapes. Prominent examples of this type include Antares or Alpha Scorpii (α Sco), and Betelgeuse or Alpha Orionis (α Ori).
## Eruptive Variables
Pulsating variables vary periodically, but some stars exhibit sudden changes in their brightness due to violent outbursts caused by processes within the star. Such kinds of stars are called eruptive variables. They include novae and supernovae.
A nova is characterised by a rapid and unpredictable rise in brightness of 7 - 16 magnitudes over a few days. The eruptive event is followed by a steady decline back to the pre-nova magnitude over a few months. This suggests that the event causing the nova does not destroy the original star. Novae are usually close binary stars where one component is a white dwarf that draws material off its companion. Material is transferred from one component to another until there is sufficient matter accumulated to trigger a thermonuclear reaction that then blasts the shell of material off into space.
A nova occurrs in a binary system where one component is a white dwarf which draws material from the other.
A supernova is a cataclysmic event characterised by a sudden and dramatic rise in brightness. In a supernova, a star become brighter by up to 20 magnitudes, to an absolute magnitude of about -15. This means that a typical supernova may outshine its entire galaxy for a few days or weeks.
Supernovae are caused by one of two main mechanisms:
Type I supernovae take place when accreting material falling onto a white dwarf in a binary system takes its mass over the Chandrasekhar limit. The resulting instability triggers a runaway thermonuclear explosion that destroys the star and releases large amounts of radioactive and heavy elements into space.
Type II supernovae occur in very massive stars once all the material in their core has been fused into iron. Since fusion in elements heavier than iron consumes more energy than it produces, gravitation overwhelms the core, which rapidly implodes. The core material gets crushed to form degenerate neutron-density material, while the extreme temperature and pressure in the surrounding layers cause rapid nuclear reactions that synthesise the heaviest elements, ripping the star apart. Such core collapse supernovae result in neutron stars and/or black holes.
Supernova 1987A (right) and its progentor star, Sanduleak -69 202 (arrow, left) in the Large Magellanic Cloud.Images copyrighted by the Anglo-American Observatory.
Although we expect two or three supernovae in our galaxy each century, they may not all be visible due to galactic dust. The most recent supernova visible to the naked eye was SN1987A. This was a core-collapse (Type II) event that took place in the Large Magellanic Cloud, a satellite galaxy of our own about 50,000 parsecs distant.
# Clear Sky Chart
Source: https://userguide.skysafariastronomy.com/general/clear-sky-chart
Documentation for Clear Sky Chart
## Clear Sky Chart
[Open Clear Sky Chart](https://www.cleardarksky.com/csk/)
# Data Migration Help
Source: https://userguide.skysafariastronomy.com/general/data-migration-help
Documentation for Data Migration Help
Data Migration Help
## Migrating from SkySafari 7 to SkySafari 8
Login to SkySafari 8 (Settings - LiveSky) with your existing SkySafari 7 [LiveSky.com](https://livesky.com) account to automatically import and sync your data.
**Note:** If you never created a LiveSky account in SkySafari 7, the app may have assigned you an automatic, anonymous account for backup and sync. In that case, open SkySafari 7, go to Settings → Account, and sign in (or sign up) so the app links your data to a real LiveSky account. Then log in with that same account in SkySafari 8.
**Important:** Please note that your sky chart settings are not synced via LiveSky. However, you can save your current sky chart settings in SkySafari 7 by creating a Saved Setting file under "Settings - Saved Settings - Create Settings". Then go back to SkySafari 8. If you are logged in with the same LiveSky account, the file your just created and saved in SkySafari 7 will be listed in under "Settings - Saved Settings"". Tap the file and select "Apply Settings".
You will have to manually import custom horizon images. To do this, open the Files app on iOS and place your custom horizon image under the root of the "SkySafari \[app name]" folder.
# Links
Source: https://userguide.skysafariastronomy.com/general/links
Useful astronomy links
# Links
* SkySafari Astronomy
* Sky and Telescope
* BBC Sky At Night Magazine
* Astronomy Magazine
* Night Sky Network
* International Dark Sky Association
* Astronomers Without Borders
* Astronomy Technology Today
* Astronomy Picture of the Day
* Universe Today
* Bad Astronomy
* SPACE.com
* Celestron International
* Meade Instruments
* Orion Telescopes
# Introduction
Source: https://userguide.skysafariastronomy.com/introduction
Documentation for Introduction
## Introduction
SkySafari is a powerful planetarium that fits in your pocket. It puts thousands of stars, planets, and other objects at your fingertips. With SkySafari Plus and Pro, you can fly into orbit around other planets and stars, and control computerized telescopes wirelessly.
SkySafari's basic version includes everything you need to get started learning the night sky: 120,000 stars; over 200 star clusters, nebulae, and galaxies; all major planets and moons, and dozens of asteroids, comets, and satellites - including the ISS. Here's what you can do with SkySafari:
* Simulate the night sky from anywhere on planet Earth, up to 100 years in the past or future.
* Search for celestial objects, and locate them in the sky using your iDevice's built-in motion hardware.
* Animate transits, conjunctions, eclipses, and other events with SkySafari's Time controls.
* Explore the sky with Night Vision turned on, and preserve your eyesight after dark.
* Learn the history, mythology, and science of the heavens from SkySafari's hundreds of object descriptions.
* Browse hundreds of astronomical photographs and NASA spacecraft images. Stay up-to-date with SkyWeek, Sky & Telescope's mobile portal to all major sky events for every day of the week. Miss nothing!
* OneSky connects you with other users around the world in real-time. This feature highlights objects in the sky chart being observed and displays the number of users observing the object.
* And now, the Cosmos Collection in-app purchase allows you to travel to other planets and stars, visualize the 3-D location of deepsky objects in our galaxy, and listen to hours of audio guided tours of the heavens.
All this is built into SkySafari - you don't need an internet connection to use it. SkySafari Plus and Pro add even more - but the basic version is everything the casual stargazer will ever want. (Please note: there is no discount upgrade path to SkySafari Plus or Pro.)
## SkySafari Plus
SkySafari Plus is everything in SkySafari, plus more. Plus adds a hugely expanded database, wired or wireless telescope control, observing tools, and spaceflight to SkySafari.
The Plus version includes 2.5 million stars, 31,000 deep sky objects, the Solar System's major planets and moons, and over 4,000 updatable asteroids, comets, and satellites. Simulate the sky from any place in the Solar System, up to 10,000 years in the past or future. Expand your astronomical knowledge with over 1500 encyclopedic descriptions of the constellations, stars, and planets. Enjoy more than 800 images from NASA space missions, the Hubble Space Telescope, and the world's foremost astro-photographers.
SkySafari Plus adds wired or wireless control for Celestron, Meade, Orion, SkyWatcher, and many other computerized telescopes, including support for ASCOM Alpaca and INDI. It can point your GoTo or "Push-To" telescope anywhere in the sky, using your iPhone/iPad/iPod's built-in WiFi, and our SkyFi or SkyWire serial accessories (sold separately). Plan observing sessions with Observing Lists, log your observations into them, and share them with friends. More astronomers use SkySafari for telescope control than any other app!
## SkySafari Pro
SkySafari Pro has the largest database of any astronomy app, period. It contains everything in SkySafari Plus, and adds over 1.2 GB of data, including 25 million stars from both UCAC5 Star catalog, over 740,000 galaxies down to 18th magnitude, and over 630,000 solar system objects - including every comet and asteroid ever discovered. It simulates the view from anywhere in the solar system - or beyond it - at up to a million years in the past or future.
SkySafari Pro includes NASA's latest Moon and Mars maps, with 8x the resolution of any other SkySafari version. It's an astronomical encyclopedia, with over 1500 encyclopedic descriptions of the constellations, stars, and planets written by professionals. And it includes 800 images from NASA space missions, the Hubble Space Telescope, and the world's foremost astro-photographers - now in breathtaking high definition on your iPad!
SkySafari Pro will revolutionize your astronomical viewing experience, and replace desktop astronomy software costing ten times more. If you're serious about astronomy, it's a deal you can't afford to miss.
# Main Menu
Source: https://userguide.skysafariastronomy.com/main-menu
SkySafari Pro Help main menu
# SkySafari Pro Help
## General
1. Introduction
2. Quick Start
3. Basic Concepts
4. Links
5. Clear Sky Chart
## App Specific
1. Sky Chart Help
2. Search Help
3. Selection Help
4. Settings Help
5. Observe Help
6. Time Flow Help
7. Scope Control Help
8. Image Gallery
9. SmartEye & Camera Configuration
10. Tonight at a Glance Help
11. Compass & AR Help
12. Night Vision Help
13. Calendar Help
14. Social Stargazing Help
# Quick Start
Source: https://userguide.skysafariastronomy.com/quick-start
SkySafari Pro Help quick start
# Quick Start