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

One of two points at which the ecliptic cuts the celestial equator.

The vernal equinox is the point in which the Sun, traveling on the ecliptic, crosses the celestial equator from south to north.