Bruce and Sally Witt

An equinox occurs twice a year, when the tilt of the Earth’s axis is inclined neither away from nor towards the Sun, the Sun being vertically above a point on the Equator. The term equinox can also be used in a broader sense, meaning the date when such a passage happens.

The name is derived from the Latin aequus (equal) and nox (night), because around the equinox, the night and day are approximately equally long. It may be better understood to mean that latitudes +L and -L north and south of the equator experience nights of equal length.

The word is also used for the same event happening on other planets and in setting up a celestial coordinate system; see equinox (celestial coordinates).

At an equinox, the Sun is at one of two opposite points on the celestial sphere where the celestial equator (i.e. declination 0) and ecliptic intersect. These points of intersection are called equinoctial points: the vernal point and the autumnal point. By extension, the term equinox may denote an equinoctial point.

In the Northern Hemisphere spring officially begins at 7:44 a.m. ET on Friday, March 20, 2009—the vernal equinox, or spring equinox (see vernal equinox pictures).

But don’t be fooled by the old rumor that on the vernal equinox the length of day is exactly equal to the length of night.

The true days of day-night equality always fall before the vernal equinox and after the autumnal, or fall, equinox, according to Geoff Chester, a public affairs specialist with the U.S. Naval Observatory in Washington, D.C.

“Exactly when it happens depends on where you are located on the surface of the Earth,” he said.