The precise time when the sun enters one of the equinoctial points, or the first point of Aries, about March 21st, and the first point of Libra, about September 23rd, making the day and the night of equal length. These are called the vernal and autumnal equinoxes.

These points are found to be moving backward or westward, at the rate of 50" of a degree in a year.

This is called the precession of the equinoxes.