Equinox

The two times of the year when the sun crosses the equator and night and day are of equal length; usually occurs on March 21st (spring equinox) and September 23 (fall equinox).

The time when the sun crosses the earth's equator, making night and day of approximately equal length all over the earth and occurring about March 21 (the spring or vernal equinox) and September 22 (autumnal equinox).

Equinox Either of the two points of intersection of the Sun's apparent annual path and the plane of the Earth's equator. In the Northern Hemisphere the spring (vernal) equinox falls on or about 21 March, and the autumnal equinox on or about 22 September.