Either of two moments in the year when the Sun is exactly above the Equator and day and night are of equal length all over Earth; also, either of two points in the sky where the ecliptic and the celestial equator (see celestial sphere) intersect. The vernal equinox, when spring begins in the Northern Hemisphere, occurs about March 21, when the Sun moves north across the celestial equator. The autumnal equinox falls about September 23, as the Sun crosses the celestial equator going south. See also solstice.

This entry comes from Encyclopædia Britannica Concise.
For the full entry on equinox, visit Britannica.com.

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