The measurement of 180 degrees of right ascension on the celestial equator or of celestial longitude on the ecliptic measured from the Vernal equinox (determined as 0 Aries, therefore the autumnal equinox is determined as 0 Libra. However, due to precession the synetic autumnal point is now in Virgo. In the southern hemisphere this marks the Vernal Equinox, thus contradicting any ideas based on a seasonal zodiac. In the northern hemisphere, this occurs around September 23, and in the southern hemisphere around March 21.
Comments: Autumnal Equinox