The date (near September 22 in the northern hemisphere) when night and day are nearly of the same length and Sun crosses the celestial equator (i.e., declination 0) moving southward (in the northern hemisphere). In the southern hemisphere, the autumnal equinox corresponds to the center of the Sun crossing the celestial equator moving northward and occurs on the date of the northern vernal equinox. The autumnal equinox marks the first day of the season of autumn.

The above plots show how the date of the autumnal equinox shifts through the Gregorian calendar according to the insertion of leap years. The table below gives the universal time of the autumnal equinox. To convert to U. S. Eastern daylight saving time, subtract 4 hours, so the autumnal equinox occurs at September 23, 1998 at 1:34 a.m. EDT; September 23, 1999 at 07:23 a.m. EDT; and September 22, 2000 at 13:11 (1:11 p.m.).

Note that the times below were calculated using AutumnalEquinox[] in the Mathematica application package Scientific Astronomer, which is accurate to within only an hour or so, and in practice gives times that differ by up to 15 minutes from those computed by the U.S. Naval Observatory (which computes September 23, 1999 at 11:31 UT instead of 11:23 UT and September 22, 2000 at 17:27 UT instead of 17:11).

Here is a QuickTime movie illustrating the tilt of the Earth's equatorial plane relative to the Sun which is responsible for the seasons. The dates of maximum tilt of the Earth's equator correspond to the summer solstice and winter solstice, and the dates of zero tilt to the vernal equinox and autumnal equinox.