September 22, 2008

Autumnal Equinox

Fall begins at 11:44 A.M. (Eastern Time) today. The autumnal equinox is defined as the point at which the Sun appears to cross the celestial equator from north to south. The celestial equator is the circle in the celestial sphere halfway between the celestial poles. It can be thought of as the plane of Earth’s equator projected out onto the sphere. The word equinox means “equal night,” when night and day are of the same duration.