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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.

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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.

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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).