Axial precession

Axial precession is the gradual change in a rotating body's spin-axis direction, often as the axis traces a cone. Earth's axis completes one cycle in roughly 26,000 years.

Torque: Torque is the rotational effect of a force, determined by its magnitude and lever arm. The Sun's and Moon's gravitational forces exert torque on Earth's equatorial bulge.
Precession of the equinoxes: Precession of the equinoxes is the westward drift of equinox points along the ecliptic caused mainly by Earth's axial precession. It is the sky-coordinate expression of Earth's changing rotational-axis direction.
Hipparchus: Hipparchus was an ancient Greek astronomer who compared star positions and identified precession around 130 BCE. His comparison of stellar observations revealed that the equinox points had shifted.
Axial tilt variation: Axial tilt variation is the long-term change in the angle between a planet's spin axis and its orbital plane. Unlike precession, it changes the tilt angle rather than mainly redirecting the axis.
Angular momentum: Angular momentum is the quantity of rotational motion, determined by mass distribution and angular velocity. Earth's large spin angular momentum makes its axis respond gradually to torque.
Equatorial coordinate system: The equatorial coordinate system locates celestial objects using right ascension and declination relative to Earth's equator. As Earth's axis precesses, the celestial equator and coordinate reference points shift.
Ptolemy: Ptolemy was a Greco-Roman astronomer whose Almagest preserved and developed ancient mathematical astronomy. His account transmitted the classical explanation and observations of precession.
Polar motion: Polar motion is the movement of Earth's rotation axis relative to its crust over time. It is a shorter-term wobble within Earth, not the long-term motion of the axis in space.
Gyroscope: A gyroscope is a spinning rotor whose orientation resists changes and responds characteristically to applied torque. A gyroscope demonstrates the torque-driven axis motion that also governs Earth's precession.
Polaris: Polaris is a star in Ursa Minor currently located close to the north celestial pole. Earth's precession changes which bright star lies nearest the north celestial pole.

Linked from 23 pages