First Day Of Fall - The Autumnal Equinox

 

Fall begins on September 23, early in the morning at 4:21 A.M. The autumnal equinox is when the Sun appears to cross the celestial equator from north to south. (The celestial equator is the circle in the celestial sphere halfway between the celestial poles. It can be thought of as the plane of Earth’s equator projected out onto the sphere.)
More info at
http://beta.almanac.com/content/first-day-fall-autumnal-equinox