Sunday, December 10, 2006
Meteor watching is one of my favorite starry-night activities. No special equipment is needed, and the excitement of seeing a brilliant "falling star" is enough to keep me awake anticipating the next one.
What causes a meteor? Before entering the Earth's atmosphere, meteors are properly called meteoroids. Most of them are so small that you could hold 1,000 of them in the cupped palm of your hand. Meteoroids, like tiny planets, follow orbits around the sun and move at very high speeds, typically 30 to 40 miles per second. In space, things can move that fast because there is no air to retard their motion, but when a meteoroid enters the Earth's upper atmosphere, it encounters so much resistance that it heats a column of air several feet in diameter to the point of glowing white hot. In the blink of an eye, the meteoroid vaporizes with a bright flash of light that we call a meteor.
On any given moonless night, a single observer can expect to see about a half dozen meteors each hour, but there are certain nights of the year when that number increases dramatically.
These are the nights of our annual meteor showers, when the Earth crosses paths with an old comet whose orbit has filled with dusty debris. The two most reliable annual meteor showers are the August Perseid shower and the December Geminid shower.
A meteor shower is named after the constellation from which its meteors seem to originate.
For example, the December Geminids seem to fan out from the constellation Gemini. Single-observer rates for the Geminids are typically 60 to 90 an hour, but can be as many as 120 an hour.
Geminid meteors have been observed every year since at least 1862 and are unique because they are the only meteors associated with an asteroid instead of a comet. In 1983, astronomers noticed that the orbits of the Geminid meteors closely matched that of the asteroid 3200 Phaethon. Perhaps a recent impact on Phaethon kicked up lots of dust particles that we now see as Geminid meteors, or it's possible that Phaethon is a burned-out comet masquerading as an asteroid. The relatively slow speed of the Geminid particles as they enter Earth's atmosphere create long, slow, graceful meteors.
This year's Geminid meteor shower is due to peak in the early morning hours of Dec. 14, but the entire night of Dec. 13 will offer great meteor watching.
A fat crescent moon will rise about 1 a.m. and slightly diminish the number of faint meteors visible, but the Geminid shower offers lots of bright fireballs that will shine through the moonlight. The meteors will streak across all parts of the sky, but when traced backwards, all of their trails will converge on the constellation Gemini, rising in the northeastern sky around 9 p.m. The best time to look will probably be the dark two-hour interval between 11 p.m. and 1 a.m.
Wrap yourself up in a warm sleeping bag and keep plenty of your favorite hot beverage nearby. Then, sit back and watch the best meteor shower of the year.
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.