Precession of the equinoxes
Precession of the equinoxes, westward motion of the equinoxes along the ecliptic . This motion was first noted by Hipparchus c.120 B.C. The precession is due to the gravitational attraction of the moon and sun on the equatorial bulge of the earth, which causes the earth's axis to describe a cone in somewhat the same fashion as a spinning top. As a result, the celestial equator (see equatorial coordinate system ), which lies in the plane of the earth's equator, moves on the celestial sphere, while the ecliptic, which lies in the plane of the earth's orbit around the sun, is not affected by this motion. The equinoxes, which lie at the intersections of the celestial equator and the ecliptic, thus move on the celestial sphere . Similarly, the celestial poles move in circles on the celestial sphere, so that there is a continual change in the star at or near one of these poles (see Polaris ) . After a period of about 26,000 years the equinoxes and poles lie once again at nearly t...