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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. The Celestial Sphere 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.

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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. Azimuth Coordinates 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.

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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. Equatorial Coordinates 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’.

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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. The Ecliptic 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.