equinoxes, precession of the


equinoxes, precession of the

Motion of the points where the Sun crosses the celestial equator, caused by precession of Earth's axis. Hipparchus noticed that the stars' positions were shifted consistently from earlier measures, indicating that Earth, not the stars, was moving. This precession, a wobbling in the orientation of Earth's axis with a cycle of almost 26,000 years, is caused by the gravity of the Sun and the Moon acting on Earth's equatorial bulge. The planets also have a small influence on precession. Projecting Earth's axis onto the celestial sphere locates the northern and southern celestial poles. Precession makes these points trace out circles on the sky and also makes the celestial equator wobble, changing its points of intersection (equinoxes) with the ecliptic.

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