Equinox, During the course of a year, the sun appears to move northward for about six months and southward for about six months. The times when the sun appears to pass over the earth's equator (on or about March 21 and September 23) are called the equinoxes. The word comes from two Latin words meaning equal night; on these dates, the length of the night (and day) is nearly the same everywhere over the earth. At other times during the year, the length of the night (and day) varies with latitude.
In the Northern Hemisphere, the March equinox is called the spring, or vernal, equinox and marks the beginning of spring. The September equinox is the fall, or autumnal, equinox. (In the Southern Hemisphere the March equinox marks the beginning of fall and the September equinox the beginning of spring.)