Fall for 2012 begins in the Northern Hemisphere on September 22, 10:49 A.M. EDT. Here’s more about the first day of fall—the autumnal equinox—plus, facts, folklore, and some fantastic fall photos! The word equinox comes from the Latin words for "equal
night." The fall and spring equinoxes are the only days of the year in
which the Sun crosses the celestial equator. From here on out, the temperatures begin to drop and the days start to get shorter than the nights.
Source Farmers Almanac
