The two annual periods when the sun crosses over the earth's equator either north to south or south to north, thereby making a day and a night of equal length: an unusual circumstance given the planet's tilt. The spring (or vernal) equinox is on March 21 or 22, and the autumn equinox on September 22 or 23. In other terms: the two periods when the ecliptic crosses the celestial equator.