Equinoxes occur twice a year, when the tilt of the Earth's axis is oriented neither from or to the Sun, causing the Sun to be located over 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.
More technically, equinox is a term from spherical astronomy referring to the intersection on the celestial sphere of the celestial equator by the plane of the ecliptic. The term is thereby used as a name for a specific junction on a set of spherical coordinates and also, being that these coordinates derive directly from the rotation of the earth and its inclination on its axis as it orbits the sun, a date for the arrival of the earth-sun system on the annual calendar which causes the aforesaid intersection.
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).
