A slow westward shift of the equinoxes along the plane of the ecliptic, resulting from precession of the earth's axis of rotation, and causing the equinoxes to occur earlier each sidereal year. The precession of the equinoxes occurs at a rate of 50.27 seconds of arc a year; a complete precession requires 25,800 years.
The occurrence of the equinoxes earlier in each successive sidereal year because of a slow wobble in the earth's axial spin which shifts the equinoctial points slightly westward along the ecliptic: the wobble is caused by the pull of the sun and moon on the earth's equatorial bulges and makes the poles move around a center point (axis of the ecliptic), taking about 25,800 years to return to the same orientation with the stars.
Owing to the precession of the equinoxes it is longer than a tropical or sidereal year by 25 minutes and 2.3 seconds.
trepidare, to tremble), a term meaning, in general, fear or trembling, but used technically in astronomy for an imagined slow oscillation of the ecliptic, having a period of 7000 years, introduced by the Arabian astronomers to explain a supposed variation in the precession of the equinoxes.
These two motions are defined with greater detail in the articles Precession Of The Equinoxes and Nutation.
The precession of the equinoxes is due to the fact that the earth performs a motion of this kind about its centre, and the whole class of such motions has therefore been termed precessional.
The laws of motion of the ecliptic and equator are stated in the article Precession Of The Equinoxes.