precession of the equinoxes

noun
1.
the earlier occurrence of the equinoxes in each successive sidereal year because of the slow retrograde motion of the equinoctial points along the ecliptic, caused by the precession of the earth's axis of rotation; a complete precession of the equinoxes requires about 25,800 years.
2.
the resulting drift of celestial coordinates with respect to the positions of celestial objects.
Origin
1615-25
British Dictionary definitions for precession of the equinoxes

precession of the equinoxes

noun
1.
the slightly earlier occurrence of the equinoxes each year due to the slow continuous westward shift of the equinoctial points along the ecliptic by 50 seconds of arc per year. It is caused by the precession of the earth's axis around the ecliptic pole, with a period of 25 800 years