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Tuesday, December 13, 2005

Meteors in Moonlight: Geminid Shower

From Sky and Telescope:

An old, reliable meteor shower is heading our way. The annual Geminid shower should reach its peak activity late on the night of Dec. 13-14: late Tuesday night and early Wednesday morning. However, the glare of the nearly full moon will hide all but the brightest meteors.

Along with the better-known Perseids of August, the Geminids are the strongest of the reliable annual meteor showers -- those that hardly change from year to year. Sky & Telescope magazine predicts that late on the peak night, you might see a "shooting star" through the moonlight every 5 to 10 minutes on average.

The time to watch will be anytime from about 10PM Tue., Dec. 13, until the first light of dawn Wed., Dec. 14. You'll need no equipment but your eyes. Find a spot with an open view of the sky and no bright lights nearby. Bring a reclining lawn chair, bundle up warmly, and bring a sleeping bag; clear nights get very cold.

"Arrange the chair so the moon is behind you out of sight, lie back, and watch the stars," says Sky & Telescope senior editor Alan MacRobert. "Be patient."

In years when there's no moon on the peak night, meteor watchers under dark skies can see Geminids as often as once a minute. This year only the less frequent bright ones will be visible. But the best of these may sail across the heavens for several seconds, leaving brief trails of glowing smoke.

To see the Geminids without the moon's glare interfering, you'll have to watch Tuesday morning, between 4:30AM and dawn, when the moon is at or below the horizon. Under these conditions, you could see dozens of meteors.

If you trace each meteor's direction of flight backward far enough across the sky, you'll find that the imaginary line you're drawing crosses a spot in the constellation Gemini near the stars Castor and Pollux. This spot is the shower's radiant, the perspective point from which all the Geminids would appear to come if you could see them approaching from the far distance. The radiant is well up in the east by 10PM nearly overhead around 2AM, and high in the west by the first light of dawn. But you don't have to look there. Just watch whatever part of your sky offers the darkest view.

The Geminid shower is active for several days, not just on its peak night. You may see a few meteors per hour for two or three nights beforehand and one night after.