or AEQUINOX, in Astronomy, the time when the sun enters one of the equinoctial points.

The equinoxes happen when the sun is in the equi- noctial circle; when of consequence the days are equal to the night throughout the world, which is the case twice a-year, viz. about the 20th of March and the 23rd of September, the first of which is the vernal and the second the autumnal equinox.

It is found by observation, that the equinoctial points, and all the other points of the ecliptic, are continually moving backward, or in antecedentia, that is, westward. This retrograde motion of the equinoctial points, is that famous and difficult phenomenon called the precession of the equinoxes. See Astronomy Index.