Jimmy Westlake: Equinox

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Autumn officially arrives for the northern hemisphere at 10:03 p.m. Mountain Standard Time on Friday. How do astronomers determine the precise moment that marks the beginning of autumn?

Autumn begins the instant the sun crosses the equator on its way south. Thanks to the 23 ½-degreee tilt of the Earth's axis of rotation, the sun spends half the year shining straight down on the northern hemisphere and the other half of the year shining straight down on the southern hemisphere. Separating these two extremes are two days called the equinoxes - six months apart - when the sun shines straight down on the Earth's equator.

Equinox is a word that means "equal nights," and is used to describe these two special days when every location on Earth experiences exactly 12 hours of sunlight and 12 hours of darkness. These also are the only two days of the year when the sun rises exactly in the east and sets exactly in the west.

You've probably noticed recently that the sun is rising later and setting earlier than it did in mid-summer, and if you are very observant, you also might have noticed that it is rising and setting much farther south than it was in mid-summer. Here in the northern hemisphere, we have just enjoyed six months of long, sun-filled days, but now it's the southern hemisphere's turn. The sun will sink lower and lower in our daytime sky from now until the winter solstice on Dec. 21. Then, it will begin moving northward again and cross the equator on March 21, this time bringing spring with it.

Oh, and one more thing - that business about balancing an egg on its end only on the day of the equinox? It's a myth. You can just as easily balance that egg on any day of the year. But don't take my word for it. Give it a try and see for yourself.

I hope your equinox is a happy one.

Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published on the Web sites of CNN.com, NASA's "Astronomy Picture of the Day," Spaceweather.com, Space.com, Discover.com, and in Sky & Telescope, Astronomy, Night Sky, Discover and WeatherWise magazines.

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