equinox

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Position of the earth relative to the sun 
on the two equinoxes.

The equinox is the date when the sun's vertical rays strike at 0^o latitude (the equator). On this day there is equal day and night (12 hours daylight and 12 hours of night) at all place. The spring or vernal equinox occurs on about March 21. The autumnal equinox occurs on about September 23.  See also solstice.