Knowra Precession of the equinoxes Precession of the equinoxes The gradual westward shift of the equinox points along the ecliptic, caused mainly by the changing orientation of Earth’s rotational axis under the gravitational pull of the Sun and Moon.
Axial precession : The slow change in the direction of a rotating body's spin axis, without a comparable change in its tilt. Earth’s axial precession is the main motion that carries the equinox points westward.
Tropical year : The interval between successive passages of the Sun through the vernal equinox, about 365.2422 days. Its reference point moves with precession, making it shorter than the sidereal year.
Hipparchus : A Greek astronomer of the second century BCE who compiled precise star positions and identified precession. Comparing stellar positions across generations, he recognized that the equinox had shifted.
Ecliptic : The apparent annual path of the Sun across the celestial sphere, corresponding closely to Earth’s orbital plane. The equinox points move along this great circle as precession proceeds.
Axial tilt variation : Long-term changes in the angle between a planet’s rotation axis and its orbital plane. Precession changes the axis direction, while tilt variation changes the angle itself.
Earth's axial tilt : The angle between Earth’s rotation axis and the perpendicular to its orbital plane, currently about 23.4 degrees. The Sun and Moon exert torques on Earth’s equatorial bulge because the axis is tilted.
Sidereal year : The time Earth takes to complete one orbit relative to the distant stars, about 365.2564 days. The difference from the tropical year measures the annual effect of precession.
Ptolemy : A second-century CE astronomer who organized Greek mathematical astronomy in the Almagest. His account preserved the ancient estimate and model of equinoctial precession.
Celestial sphere : An imaginary sphere centered on an observer, used to describe the apparent positions of celestial objects. Precession describes how Earth-based reference directions shift against this sky framework.
Milankovitch cycles : Long-term variations in Earth's orbit and rotation that alter the seasonal distribution of sunlight. Precession is one component of these climatic cycles, alongside tilt and orbital-shape changes.
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