precession

Precession is a gradual change in the orientation of a rotating object's axis or of an orbiting body's orbital plane. It describes a slow, conical motion of the spin axis or a slow reorientation of the orbit in space, occurring over long timescales.

There are two broad categories. Spin or axial precession refers to the slow rotation of a body's rotation axis about a fixed direction in space. Orbital precession refers to changes in the orientation of an orbital plane or of the orbit itself, such as the gradual rotation of the orbit’s ascending node or of the ellipse within its plane.

Causes vary by context. In celestial mechanics, precession is often driven by torques exerted by gravitating bodies on a non-spherical primary (for example, the Sun and Moon on Earth’s equatorial bulge). In general relativity, relativistic effects cause precession of planetary perihelia, notably Mercury’s, at a measurable rate. In laboratory or engineering contexts, spinning gyroscopes exhibit precession when subjected to external torques, with the precession rate proportional to the applied torque and inversely proportional to angular momentum.

Earth provides classic examples: the axis undergoes axial precession with a cycle of about 26,000 years, causing the positions of stars to drift slowly relative to the calendar. The precession of the equinoxes shifts the timing of seasons over millennia, and Milankovitch cycles describe how these precessional effects influence climate. In planetary systems, nodal and apsidal precession alter the orientation of orbits over time.