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.