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
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(Marc Lester,AP Photo/The Anchorage Daily News)
(SPACE.com) The arrival of the autumnal equinox this Saturday (Sept. 22) signals a transition from northern summer to fall in an astronomical sense. But it also signals the start of aurora-watching season.
From now through the end of October, the chances of sighting the glow of the mysterious northern lightswill be reaching a peak.
In fact, auroras peak in frequency twice a year, with the other peak coming in the weeks before and after the Vernal Equinox, which marks the beginning of spring in the Northern Hemisphere. But why are aurora displays more common around the time of the equinoxes?
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