The Equinox reference article from the English Wikipedia on 24-Jul-2004
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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 equinoxes in March and September are the two times each year when day and night are of equal duration. For terms of measuring the length of day on the equinox, sunrise starts when the sun is half way above the horizon and sunset is when the sun is halfway under the horizon. Using this definition, the length of the day is precisely 12 hours. At the equinoxes, the sun rises due east and sets due west.

The March equinox is the vernal equinox; the September equinox is the autumnal equinox.

see also: solstice, precession