The first day of fall, they day when the Sun is 90 degrees above the horizon at the Equator at solar noon
by plahhhhhhhhhhhhhhhhhhh November 24, 2011
The moment at which the Sun passes over the equator, occurring around September 21st or 22nd, marking the beginning of Autumn or Fall.
The only times that the amount of daylight and night are equal is during either the Vernal or Autumnal Equinox.
by Wil Mundy January 27, 2008