EQUINOX, a term used to express either the moment at which, or the point at which, the sun apparently crosses the celestial equator. (From the Lat. aequus, equal, and nox, night.) Since the sun moves in the ecliptic, it is in the last-named sense the point of intersection of the ecliptic and the celestial equator. This is the usual meaning of the term in astronomy. There are two such points, opposite each other, at one of which the sun crosses the equator toward the north and at the other toward the south. They are called vernal and autumnal respectively, from the relation of the corresponding times to the seasons of the northern hemisphere. The line of the equinoxes is the diameter of the celestial sphere which joins them. The vernal equinox, alternatively called the First Point of Aries, is the initial point from which the right ascensions and the longitudes of the heavenly bodies are measured (see ASTRONOMY; Spherical; also PRECESSION OF THE EQUINOXES) .