Earth's rotation axis traces a circle in the sky over approximately 25,772 years. This means the equinox points drift westward against the fixed stars at about 50 arcseconds per year — roughly one full zodiac sign every 2,150 years. The phenomenon was discovered by Hipparchus in the 2nd century BCE. Precession is the reason the tropical zodiac (season-fixed) and the sidereal zodiac (star-fixed) no longer match.
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Astronomical · Glossary
Precession of the Equinoxes
The slow wobble of Earth's axis that shifts the equinox points westward by 50″/year.