Precession

Precession is the gradual change in a rotating body's spin-axis orientation, usually caused by an external torque. It can trace a cone around a fixed direction.

Torque: A torque is the rotational effect of a force acting at a distance from an axis or pivot. An applied torque changes angular momentum, steering the spin axis rather than simply tipping the body over.
Foucault pendulum: A Foucault pendulum is a freely swinging pendulum whose oscillation plane appears to rotate as Earth turns. Its changing plane provides a visible demonstration of Earth-related rotational motion and precession.
Isaac Newton: Isaac Newton was an English mathematician and physicist who formulated the laws of motion and universal gravitation. Newtonian mechanics explains torque-driven precession and the gravitational forces that cause it.
Axial rotation: Axial rotation is the turning of a body around its own axis. Rotation spins the body about its axis; precession changes the axis's orientation.
Precession of the equinoxes: The precession of the equinoxes is the westward shift of the equinox points along the ecliptic caused chiefly by Earth's axial precession. It shifts the seasonal reference points against the stars over long periods.
Angular momentum: Angular momentum is the rotational momentum of a body, determined by its motion and mass distribution. A spinning body's angular momentum sets the direction that torque changes during precession.
Earth's axial precession: Earth's axial precession is the slow change in the direction of Earth's rotation axis, completing a cycle in about 26,000 years. It changes the celestial pole and the positions of equinoxes over millennia.
Hipparchus: Hipparchus was an ancient Greek astronomer who compared star observations and identified the precession of the equinoxes. His discovery established axial precession as a measurable change in the heavens.
Revolution: Revolution is the orbital motion of one body around another or around a common center of mass. An orbiting body's path around another object is distinct from its spin axis precessing.
Tropical year: A tropical year is the average interval between successive vernal equinoxes, about 365.2422 days. Equinox precession helps make the tropical year slightly shorter than Earth's orbital period relative to the stars.

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