An equinox in astronomy is that moment in time (not a whole day) when the centre of the Sun can be observed to be directly above the Earth's equator, occurring around March 20 and September 23 each year. These are the Autumnal Equinox and the Vernal Equinox. Which is which depends on which hemisphere you live in.

Equinox derives from latin meaning "equal night" because at the time of the Equinox day and night are of equal in length.

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