On Saturday, Sept. 22, the sun crosses the celestial equator heading south. This is the autumnal equinox in the northern hemisphere and the vernal equinox in the southern hemisphere. CREDIT: Geoff Gaherty/Starry Night Software/SPACE.com View full size image

When is the first day of the northern autumn in 2012? A carefully worded answer is that on Saturday, Sept. 22), fall begins astronomically in the Northern Hemisphere, while spring begins in the Southern Hemisphere. The exact time of the event will be 10:49 a.m. EDT (1449 GMT).

This season-changing equinox, like a similar event on March 20 that heralded northern spring, is gets its name from the Latin for “equal night,” alluding to the fact that day and night are then of equal length worldwide. But this is not necessarily so.

The definition of the equinox as being a time of equal day and night is a convenient oversimplification.  For one thing, it treats night as simply the time the sun is beneath the horizon, and completely ignores twilight. 

Not so equal

If the sun were nothing more than a point of light in the sky, and if the Earth lacked an atmosphere, then at the time of an equinox the sun would indeed spend one half of its path above the horizon and one half below. But in reality, interference by Earth's atmosphere (which refracts the sun's light) raises the sun’s disk by more than its own apparent diameter as it rises or sets. [Earth's Equinoxes & Solstices (Infographic)]

http://www.space.com/17710-fall-equinox-seasons-change-explained.html