precession of the equinoxes

The precession of the equinoxes is a cyclical change in the equinoxes (the directions into the celestial sphere that are the intersections of Earth's equatorial plane and the ecliptic), which is due to a precession in Earth's axis of rotation, i.e., its axial precession. It constitutes a continuous movement regarding the direction toward which the Earth's poles point (i.e., the celestial poles), which align most closely with various stars as the precession progresses. The poles each basically follow a circle in the sky with an angular radius matching Earth's obliquity, about 23 degrees. Earth's precession rate (the precession constant) is very roughly 1/26000 precession cycles per year, shifting the direction of the Earth's north pole on the order of an arcsecond per month (~19.5 arcseconds per year, with each equinox precessing around the entire 360 degrees, shifting direction about 50 arcseconds per year). The precession is caused by the gravity of the Sun on the Earth's tilted rotation-axis as well as similar effects from the Moon and planets, along with the Earth's orbital eccentricity. The precession constant is roughly 1/25772 at present, but is not truly constant, the precession's period currently growing slowly.

The precession of the equinoxes has been known for thousands of years, but modern analysis has determined it to be a combination of two precessions: that due to the Sun and Moon which affects Earth's axis and equator, termed the precession of the equator or lunisolar precession, and a second precession of, Earth's orbital plane, termed the precession of the ecliptic or planetary precession, due to the planets, primarily Jupiter and Venus. The combination of the two precessions is termed the general precession. The ecliptic's precession makes each ecliptic pole trace a loop that is not so circular, currently shifting the ecliptic about a half an arcsecond per year.

The precession of the equinoxes is a major factor regarding equatorial coordinate systems, constituting a major reason for associating them with dates, e.g., J2000.0 equinox.