Precession of the Equinoxes

The slow shifting of the equinoxes toward the west, the annual rate being 50 sec. This is caused by the earth's rotation on its axis, and the unequal action of the sun and moon on the equator. Precession of the equinoxes is so termed because the place of the equinox among the stars, at every subsequent moment, precedes, with reference to the diurnal motion, that which it occupied the moment before.