1. A slow change in the direction of the tilt of earth's axis, which results in an apparent change in the position of the stars.
2. Slow wobble of the earth on its axis, like that of a spinning top.

The gravitational pulls of the sun and moon on the earth's equatorial bulge cause the earth's axis to trace out a circle on the sky every 25,800 years.

The position of the celestial poles is constantly changing because of procession, as are the positions of the equinoxes (the points at which the celestial equator intersects the sun's path around the sky).

The precession of the equinoxes means that there is a gradual westward drift in the ecliptic or the path that the sun appears to follow and in the coordinates of objects on the celestial sphere.