The autumnal equinox occurs around 22 September each year, although this is only autumn for observers in the Northern Hemisphere. On this day, the Sun crosses from the north to the south of the celestial equator at the descending node of its orbit, and the length of daylight and night are the same for observers everywhere on the Earth (with minor latitude corrections due to refraction).

The autumnal equinox is also known as the First Point of Libra, since about 2,000 years ago, the point on the celestial sphere where the ecliptic and celestial equator crossed was in the constellation Libra. However, due to the precession of the equinoxes, the autumnal equinox now occurs in Virgo.