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;
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 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.