An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the Sun being vertically above a point on the Equator. The term equinox can also be used in a broader sense, meaning the date when such a passage happens. The name "equinox" is derived from the Latin aequus (equal) and nox (night), because around the equinox, the night and day are approximately equally long. It may be better understood to mean that latitudes +L and -L north and south of the equator experience nights of equal length.
The word is also used for the same event happening on other planets and in setting up a celestial coordinate system
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 denote an equinoctial point.
An equinox happens each year at two specific moments in time (rather than two whole days), when there is a location on the Earth's Equator where the centre of the Sun can be observed to be vertically overhead, occurring around March 20/21 and September 22/23 each year.
