about: the astronomical event when the sun is directly above the equator

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.

Names

• Vernal equinox and autumnal equinox: these classical names are direct derivatives of Latin (ver = spring and autumnus = autumn).
• March equinox and September equinox: a usage becoming the preferred standard by technical writers choosing to avoid Northern Hemisphere bias (implied by assuming that March is in the springtime and September is autumnal—true for those in the Northern Hemisphere but exactly opposite in the Southern Hemisphere).
• Northward equinox and southward equinox: names referring to the apparent motion of the Sun at the times of the equinox.
• Vernal point and autumnal point are the points on the celestial sphere where the Sun is located on the vernal equinox and autumnal equinox respectively (again, the seasonal attribution is that of the Northern Hemisphere).
• First point (or cusp) of Aries or Libra are archaic names used by navigators and astrologers. Navigational ephemeris tables record the geographic position of the First Point of Aries as the reference for position of navigational stars. Due to the precession of the equinoxes, the astrological signs where these equinoxes are located no longer correspond with the actual constellations once ascribed to them.