EQUINOX, in Astronomy, the time when the sun enters one of the equinoctial points.
The equinoxes happen when the sun is in the equinoctial circle, and the days are equal to the nights throughout the world, which is the case twice a-year; namely, about the 21st of March and the 23d of September, the former being called the vernal and the latter the autumnal equinox. See ASTRONOMY, vol. iv., p. 14; and PRECESSION OF THE EQUINOXES.