Astron. 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