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An equinox in astronomy is the event when the Sun can be observed to be directly above the Earth's equator, occurring around March 20 and September 23 each year. More technically, the equinox happens when the Sun is at one of two opposite points on the celestial sphere where the celestial equator and ecliptic intersect. In a wider sense, the equinoxes are the two days each year when the center of the Sun spends an equal amount of time above and below the horizon at every location on Earth. The word equinox derives from the Latin words aequus (equal) and nox (night).
http://en.wikipedia.org/wiki/Equinox