The Autumnal Equinox

According to Infoplease:

At the autumnal equinox (Sept. 23, 2006, 12:03 A.M. EDT), the sun appears to cross the celestial equator, from north to south; this marks the beginning of autumn in the Northern Hemisphere

On the date of either equinox, the sun is above the equator and night and day are of approximately equal length; the word equinox is often used to refer to either of these dates.

The equinoxes are not fixed points on the celestial sphere but move westward along the ecliptic, passing through all the constellations of the zodiacin 26,000 years. This motion is called the precession of the equinoxes. The vernal equinox is a reference point in the equatorial coordinate system.