axial precession

Axial precession is the slow rotation of the Earth's spin axis around the pole of the ecliptic, tracing a cone of half-angle 23.4° with a period of about 25,800 years. It is driven by the combined gravitational torque that the Moon and Sun exert on the Earth's equatorial bulge — an approximately 21-km deviation of the Earth's shape from a perfect sphere, caused by centrifugal deformation of its rotation. The torque is perpendicular to the Earth's angular-momentum vector and so rotates it in space rather than tipping the axis over.

Hipparchus of Nicaea discovered the precession around 130 BCE by comparing celestial longitudes of stars in his own observations with those catalogued by Timocharis 150 years earlier. Newton gave the first physical explanation in the Principia (1687), attributing it to the Moon's and Sun's torques on the bulge. Modern theory accounts for the Moon at about 2/3 of the total precessional rate and the Sun at 1/3, matching the observed 50.3 arcseconds per year (period 25,800 years) to high precision.

Axial precession has cosmological consequences: the identity of the "pole star" changes over millennia. In ancient Egypt, Thuban (α Draconis) was the pole star. Today it is Polaris. In about 13,000 years, Vega will be within 5° of the celestial north pole. Axial precession also shifts the seasonal dates of the solstices and equinoxes against the backdrop of the stars, which is one of the phenomena astronomers try to separate from the Earth's orbital eccentricity variations (Milankovitch cycles) when modelling long-term climate.