Precession of the equinoxes
The slow 26,000-year conical sweep of Earth's rotation axis, which makes the equinoxes drift through the zodiac.
Definition
Earth is an oblate gyroscope: it spins, it has an equatorial bulge, and the Sun and Moon pull on that bulge from a direction tilted 23.5° from the spin axis. The net gravitational torque makes the axis precess — it traces a slow cone about the ecliptic pole with a period of roughly 25,770 years. As the axis swings, the intersection points of the celestial equator with the ecliptic — the equinoxes — drift westward through the zodiac at about 50 arcseconds per year.
Hipparchus noticed the drift around 127 BCE by comparing his own stellar coordinates to those Timocharis had catalogued 150 years earlier. The pole star changes with the cycle: Thuban in 3000 BCE, Polaris now, Vega around 14,000 CE. Newton gave the first physical explanation in the Principia (1687); d'Alembert (1749) and Laplace (1777) made the calculation rigorous.
History
Discovered by Hipparchus c. 127 BCE. First dynamically explained by Newton (1687). Full analytical treatment by d'Alembert (1749) and Laplace (1777).