Equinox

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Equinox

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1. One of the two points of intersection of the ecliptic and the celestial equator, occupied by the sun when its declination is 0�. Also called equinoctial point. </dd>
That point occupied on or about March 21, when the sun's declination changes from south to north, is called vernal equinox, March equinox, or first point of Aries; that point occupied on or about September 23, when the declination changes from north to south, is called autumnal equinox, September equinox, or first point of Libra.
Equinox is often used to mean vernal equinox, when referring to the origin of measurement of right ascension and celestial longitude. </dd>
2. That instant the sun occupies one of the equinoctial points. </dd>

References

This article is based on NASA's Dictionary of Technical Terms for Aerospace Use