equinox

equinox , either of two points on the celestial sphere where the ecliptic and the celestial equator intersect. The vernal equinox, also known as "the first point of Aries," is the point at which the sun appears to cross the celestial equator from south to north. This occurs about Mar. 21, marking the beginning of spring in the Northern Hemisphere. At the autumnal equinox, about Sept. 23, the sun again appears to cross the celestial equator, this time from north to south; this marks the beginning of autumn in the Northern Hemisphere. On the date of either equinox, night and day are of equal length (12 hr each) in all parts of the world; the word equinox is often used to refer to either of these dates. The equinoxes are not fixed points on the celestial sphere but move westward along the ecliptic, passing through all the constellations of the zodiac in 26,000 years. This motion is called the precession of the equinoxes . The vernal equinox is a reference point in the equatorial coordinate system .

equinox the time or date (twice each year) at which the sun crosses the celestial equator, when day and night are of equal length (about 22 September and 20 March).
See also precession of the equinoxes.

equinox Either of the two days each year when day and night are of equal duration. They occur on the two occasions, (one spring, one autumn) when the Sun crosses the celestial equator. In the Northern Hemisphere, the Vernal (spring) Equinox occurs around March 21 and the Autumnal Equinox around September 23.