Equinoxes occur twice a year, when the tilt of the Earth's axis is oriented neither from nor to the Sun, causing the Sun to be located vertically above a point on the equator. The name is derived from the Latin aequus (equal) and nox (night), because at the equinox the night and day are equally long. The term equinox can also be used in a wider sense, as the date (day) that such a passage happens.

The word is also used for the same event happening to other planets, and in setting up a celestial coordinate system—see Equinox (celestial coordinates).

(... from Wikipedia on 2009-01-01 03:04:38 )