ETYM Old Eng. equinoxium, equenoxium, Latin aequinoctium; aequus equal + nox, noctis, night: cf. French équinoxe. Related to Equal, and Night.
Either of two times of the year when the sun crosses the plane of the earth's equator and day and night are of equal length.
Date on which sun crosses equator. autumnal equinox, such date in autumn (about Sept. 23). vernal equinox, such date in spring (about March 21).
The points in spring and autumn at which the Sun’s path, the ecliptic, crosses the celestial equator, so that the day and night are of approximately equal length. The vernal equinox occurs about 21 March and the autumnal equinox, 23 Sept.