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Related to equinoxes: precession of the equinoxes
1. (equinoctial points) The two points on the celestial sphere at which the ecliptic intersects the celestial equator. They are thus the two points at which the Sun in its apparent annual motion crosses the celestial equator. The Sun crosses from south to north of the equator at the dynamical equinox, still informally called the vernal equinox, symbol:, which lies at present in the constellation Pisces. The Sun crosses from north to south of the equator at the autumnal equinox, symbol: ≊, which at present lies in Virgo. The dynamical or vernal equinox is the zero point for both the equatorial and ecliptic coordinate systems, although in star catalogs a catalog equinox is now used. The equinoxes are not fixed in position but are moving westward around the ecliptic as a result of precession of the Earth's axis; the advance is about 50″ of arc per year. See also mean equator and equinox; true equator and equinox.
2. The two instants at which the Sun crosses the equinoctial points, on about March 21 (dynamical equinox) and Sept. 23. On the days of the equinoxes the hours of daylight and of darkness are equal.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006