Jimmy Westlake: The vernal equinox

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Ahhh, springtime! The early signs are all here - mud, blackbirds, the return of the Big Dipper to our early evening sky, more mud, and the gradual lengthening of our daylight hours.

The season of spring officially arrives in the northern hemisphere at 6:07 p.m. MDT Tuesday. That's the instant the sun crosses the Earth's equator on its way north, what we call the vernal equinox. With each passing day, the sun rises a little bit earlier and sets a little bit later, increasing the number of daylight hours that we enjoy. Our daylight hours will continue to increase until the summer solstice on June 21, the longest day of the year and the first official day of summer.

Thanks to the 23 1/2-degree tilt of the Earth on its axis, the sun spends half of 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 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 night" and is used to describe these two special days when every location on Earth experiences exactly 12 hours of daylight and 12 hours of darkness, and the sun rises exactly in the east and sets exactly in the west. The autumnal equinox in 2007 is at 3:51 p.m. Sept. 23, when the sun again crosses the equator, but this time heading south. Equinoxes have always been celebrated by various cultures on Earth.

The ancient Druids went to great lengths to align gigantic stones at Stonehenge to point out the position of the rising sun on the dates of the equinoxes.

The Great Sphinx of Egypt also faces the direction of the rising sun on the date of the equinox. A coincidence? Probably not.

For Christians, the celebration of Easter is tied directly to the vernal equinox: Easter Sunday is the Sunday that falls immediately after the first full moon after the equinox.

Oh, and 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. Standing an egg on its end has nothing to do with the balance of cosmic forces on the date of the equinox. It has to do with the tiny bumps on the eggshell serve as little feet that the egg rests on. Be a myth-buster and try balancing that egg on a day that does not happen to be an equinox.

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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