The date (near March 21 in the northern hemisphere) when night and day are nearly the same length and Sun crosses the celestial equator (i.e., declination 0) moving northward. In the southern hemisphere, the vernal equinox corresponds to the center of the Sun crossing the celestial equator moving southward and occurs on the date of the northern autumnal equinox. The vernal equinox marks the first day of the season of spring. The right ascension at the vernal equinox originally was in the constellation Aries and the point of crossing was known as the first point in Aries (now actually in Pisces because of precession ).

The above plots show how the date of the vernal equinox shifts through the Gregorian calendar according to the insertion of leap years. The table below gives the universal time of the vernal equinox. To convert to U. S. Eastern standard time, subtract 5 hours, so the vernal equinox occurs on March 20, 2001 at 8:14 a.m. EST.

Note that the times below were calculated using VernalEquinox[] 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 March 21, 1999 at 01:46 UT instead of 01:36 UT and March 20, 2000 at 07:35 UT instead of 07:25).

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.