The equinoxes in March and September are the two times each year when day and night are of approximately equal duration. The vernal equinox is the beginning of spring (astronomically); the autumnal equinox is the beginning of autumn (astronomically).
In astronomy, an equinox is defined the moment when the sun reaches one of two intersections between the ecliptic and the celestial equator. The word "equinox" comes from the Latin for "equal night".
The March equinox is the vernal equinox in the northern hemisphere and is the autumnal equinox in the southern hemisphere; for the September equinox, those events are interchanged.
At the equinoxes, the sun rises due east and sets due west.
see also: solstice, precession