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( Vernal equinox)
Equinoxes occur twice a year, when the tilt of the Earth's axis is oriented neither from nor to the Sun, causing the Sun to be located vertically above a point on the equator. The name is derived from the Latin aequus (equal) and nox (night), because at the equinox the night and day are equally long. The term equinox can also be used in a wider sense, as the date (day) that such a passage happens. The word is also used for the same event happening to other planets, and in setting up a celestial coordinate system—see Equinox (celestial coordinates). At an equinox, the Sun is at one of two opposite points on the celestial sphere where the celestial equator (i.e., declination 0) and ecliptic intersect. These points of intersection are called equinoctial points—the vernal point and the autumnal point. By extension, the term equinox may be used to denote an equinoctial point. An equinox happens each year at two specific moments in time (not a whole day) when the centre of the Sun can be observed to be vertically above the Earth's equator, occurring around March 20 or 21 and September 22 or 23 each year.
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