Today is the first day of fall, and that's a concept we can all get our heads around. Out with the shorts, the sunglasses, the beach towels, the surfboards. In with the jackets, the dead leaves, the cold breezes, and the vampire costumes. But what about the autumnal equinox, that great indicator of the back-to-school season? Not such an easy nut to crack.
Here's your 60-second primer.
The autumnal equinox, for folks in the Northern hemisphere, arrives every September. On the bottom half of the globe, the orientation is flipped – the southern hemisphere’s vernal (spring) equinox occurs in September, while its autumnal (fall) equinox is in March.
The equinox occurs when the sun, moving southward, crosses the earth's equator. For us in the US, the event marks the official end of summer and the start of fall.
In 2009, the autumnal equinox will officially take place on Sept. 22, at 5:18 PM, Eastern Daylight Time.
It's often said that the autumnal equinox is a day of perfect equilibrium – a 24-hour cycle split neatly into 12 hours of light, and 12 hours of dark. That's not exactly true. In areas distant from the equator, the sun can take longer to rise and set; closer to the equator, the day lasts a little more than 12 hours. The real even split between day and night doesn't occur until later in the fall, according to the US Naval Observatory.
While we're at it, here's one more myth for you: During the autumnal equinox, it's easier to balance an egg. The thinking goes something like this: the gravity is equal, the hours of day and night are equal, and there's all sorts of balance in the air. (People believe the same thing about the vernal equinox, by the way.)
Now don't let us get in your way. Of course you can balance an egg on the equinox. "Just ask anybody who's ever tried," notes the BBC. "Of course, they'll probably neglect to tell you that you can also perform this feat any other day of the year. That's because it has nothing to do with celestial alignment."
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