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 "equinox" 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).

(... from Wikipedia on 2010-03-13 00:00:47 )