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.