Precession is the motion of the equinoxes along the ecliptic (the plane of the Earth's orbit) caused by the cyclic precession of the Earth's axis of rotation.
In compiling his famous star catalog (completed in 129 BC), the Greek astronomer Hipparchus noticed that the positions of the stars were shifted in a systematic way from earlier Babylonian (Chaldean) measures. This indicated that it was not the stars that were moving but rather the observing platform--the Earth. Such a motion is called precession and consists of a cyclic wobbling in the orientation of the Earth's axis of rotation with a period of almost 26,000 years. Precession was the third-discovered motion of the Earth, after the far more obvious daily rotation and annual revolution. Precession is caused by the gravitational influence of the Sun and the Moon acting on the Earth's equatorial bulge. To a much lesser extent, the planets exert influence as well.
The projection onto the sky of the Earth's axis of rotation results in two notable points at opposite directions: the north and south celestial poles. Because of precession, these points trace out circles on the sky. Today, the north celestial pole points to within just 1 of the arc of Polaris. It will point closest to Polaris in AD 2017. In 12,000 years the north celestial pole will point about 5 from Vega. Presently, the south celestial pole does not point in the vicinity of any bright star.
Excerpt from the Encyclopedia Britannica without permission.