i. That point of intersection of the ecliptic and the celestial equator occupied by the sun as it changes from a south to a north declination, on or about March 21. Also called the
March equinox or
the first point of Aries.
ii. That instant the sun reaches the point of zero declination when crossing the celestial equator from south to north.
Positions of sun and the earth at spring and autumnal equinoxes.
. One of the two points of intersection of the ecliptic and the celestial equators, occupied by the sun when its declination is 0°. Also called an
equinoctial point. That point occupied on or about March 21, when the sun's declination changes from south to north, is called the
vernal equinox, spring 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 the
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. At the time of equinox, the duration of day and night is the same or equal.
ii. That instant the sun occupies one of the equinoctial points.
