precesses

Precession is the gradual change in the orientation of the rotation axis of a spinning object caused by external torques. The verb form “to precess” describes the act of undergoing such a change; “precesses” can be used as the present-tense form for a subject that precesses. In physics and astronomy, precession describes how angular momentum vectors or orbital axes rotate slowly over time.

A common example is gyroscopic precession. For a spinning top or gyroscope, gravity exerts a torque τ = r × F that tends to tilt the spin axis. Because angular momentum L resists change, the axis precesses with a rate approximately given by Ω_p ≈ τ / L for small tilts. The precession direction is perpendicular to the plane formed by the spin axis and the torque, as dictated by the right-hand rule.

In astronomy, axial precession refers to the slow reorientation of the Earth’s rotation axis relative to the background stars, completing a cycle roughly every 26,000 years. This precession, known as the precession of the equinoxes, slowly shifts celestial coordinates and the timing of equinoxes and solstices.

Orbital precession describes the gradual rotation of a body's orbital ellipse within space, caused by perturbations from other bodies and relativistic effects. Mercury’s perihelion precession is a famous case that requires general relativity for full explanation; relativistic contributions include geodetic (de Sitter) precession and, for rotating masses, Lense–Thirring (frame-dragging) precession.

Precession also appears in practical demonstrations, such as the Foucault pendulum, which shows Earth’s rotation through the precession of the pendulum’s swing plane. In astrophysics, precession can occur in jets or accretion disks due to torques from companions or gravitational effects.