precessional

Precessional refers to phenomena involving precession, the slow change in the orientation of a spinning object's axis or angular momentum due to applied torques. Precessional motion occurs in astronomy, classical mechanics, and modern physics, and can arise from gravity, electromagnetic forces, or mechanical constraints.

In astronomy, axial precession is the gradual wobble of a planet’s rotation axis. Earth’s axis precesses because the planet’s equatorial bulge experiences gravitational torques from the Moon and the Sun. The result is a roughly 26,000-year cycle that slowly turns the orientation of the axis, shifting the location of the celestial poles and equinoxes and influencing long-term climate cycles known as Milankovitch cycles. Planets and their satellites can exhibit analogous precessional motions in their rotational or orbital states.

In mechanics, a spinning object under torque tends to precess rather than tip immediately. A gyroscope or a spinning top will have its rotation axis describe a conical path about the direction of the applied torque. The precession rate is related to the applied torque and the angular momentum; in many textbook cases, a simple relation links the precession rate to the torque divided by the spin angular momentum. The Foucault pendulum illustrates a related idea: its plane of swing appears to precess because of Earth’s rotation, with a rate that depends on latitude.

In physics and chemistry, magnetic moments in external magnetic fields exhibit Larmor precession, a rapid precessional motion around the field direction, fundamental to techniques such as NMR and MRI.

Precessional motion is distinct from nutation, a smaller, periodic wobble superimposed on precession.