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.