Precession of the equinoxes

Earth's rotation axis is tilted relative to the plane of its orbit. Because Earth is slightly wider at the equator than at the poles, the gravitational attractions of the Sun and Moon exert a torque on this equatorial bulge. The axis consequently changes direction slowly, tracing a circle in the sky like the axis of a spinning top. This motion is called axial precession. Its projection onto the celestial sphere shifts the equinox points along the ecliptic, giving the phenomenon its traditional name, the precession of the equinoxes.

The full axial-precession cycle is approximately 25,772 years. As the direction of Earth's axis changes, the north celestial pole points toward different regions of the sky over millennia. Polaris is currently close to the north celestial pole; in roughly 12,000 years the pole will be nearer Vega, though not directly on it. The stars themselves are not responsible for the apparent shift of the equinox points: the changing orientation of Earth's observing platform produces it.

Axial precession changes the orientation of Earth's tilted axis relative to the locations of perihelion and aphelion, the closest and farthest points in its orbit around the Sun. As a result, which hemisphere's summer occurs near perihelion changes over long timescales, altering seasonal contrasts. In combination with the slow rotation of Earth's orbital ellipse, axial precession contributes to the climatic cycles studied in the Milankovitch framework. It is distinct from obliquity, the change in the angle of Earth's axial tilt, and from apsidal precession, the rotation of the orbital ellipse itself.

The ancient Greek astronomer Hipparchus recognized precession by comparing stellar positions in his observations with earlier records. The finding established that the reference direction associated with the equinox shifts over time. Modern explanations describe this as a consequence of gravitational torques acting on Earth's equatorial bulge, rather than a movement of the stars as a group.