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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 northernautumnal equinox. The vernal equinox marks the first day of the season of spring.
http://scienceworld.wolfram.com/astronomy/VernalEquinox.html