A MID-SUMMER NIGHT'S SHOW
Dr. Terry L. Goforth
Associate Professor of Physics, SWOSU
August 2004

As the dog days of summer wear on, we approach the peak of an annual sky show. The Perseid meteor shower peaks each year around August 11-12, giving us a nice excuse for finding a dark spot, tossing a sleeping bag on the ground, and enjoying the outdoor comfort of a summer night. (Don't forget the bug spray!)

Diagram of earth moving among dust lanes in comet's orbit

Meteor showers occur when the Earth passes through the orbit of a comet. Comets are bodies of mostly icy materials with some rocky matter imbedded, resulting in the oft-use description as a "dirty snowball." The orbits of comets are very long and narrow, carrying them very close to the sun at one end of the orbit and very far away at the other. When far from the sun, the comet is just exactly as described, a big ball, perhaps 10 miles in diameter, of dirty ice. However, when the comet moves in closer to the sun, some of the icy material on its surface will sublime. (Sublimation occurs when icy materials convert from a solid, frozen state directly to a gaseous state without becoming a liquid. Dry ice, which is just carbon dioxide in the solid form, sublimes, which is why we call it dry ice.) The released gases and the bits of rocky dust imbedded in it are then released to space, forming the familiar tail of the comet. This released gas and dust doesn't just go away. It remains in orbit around the sun in the same orbit as the comet. When the Earth passes through these streams of dust, many of the dust particles–most about thousandth of an inch in size–will plow through the Earth's atmosphere. These particles are traveling very fast relative to Earth, typically 25,000 mph or faster. At this speed, friction between the particles and the atmosphere cause them to heat up very rapidly; as a result, they glow, causing the bright streaks in the sky that we call meteors or "falling stars."

There are lots of random dust particles in space. On an average night, you might observe six to eight meteors per hour. During meteor showers, this number increases. How much it increases depends on the density of the dust particles left behind by the comet. In late July and early August, Earth passes through the orbit of Comet Swift-Tuttle. Typically, observers can expect to see 40 to 60 meteors per hour at the peak of the Perseid shower around August 11 or 12. The orbit of Swift-Tuttle is broad, so meteors associated with it actually begin in mid-July and continue to the end of August. However, fewer meteors will be seen away from the peak dates.

This year should be a good year for observing the Perseid meteor shower for two reasons. The first is certain–the moon will be a small, waning crescent. Thus moonlight will not diminish the brightness of the meteors. The second reason is a bit more iffy. Meteor "forecasters" suggest that Earth will pass through a particularly dense stream of dust on Wednesday, August 11, at 4 p.m. CDT. Unfortunately for most of the U.S., if this time prediction is correct, we'll miss out on a flurry of meteors, perhaps as many as 200 per hour. (However, if you plan to be in Europe or Asia, make plans to watch the show.) For North America, the best viewing time will be on the evening of August 11 or the pre-dawn hours of August 12.

Earth's rotation and orbital motion relative to the comet orbit and the sun's position

The meteors are traveling along parallel paths. However, just like the snow flakes in a snow storm, they will seem to radiate out from a point in front of the earth. (Think how snowflakes seem to be coming from a point in front of your car when you drive.) Meteor showers are named for the constellation closest to this "radiant point." The Perseids are named for Perseus, which rises in the northeast around 10 p.m., so before midnight you may see "earthgrazers"–meteors that travel parallel to the horizon and often leave long streaks. Later, as Perseus rises higher, you'll have a chance to see more meteors scattered over a larger portion of the sky. The intensity of meteors usually increases after midnight (or 1 a.m. daylight savings time) because you are on the "front edge" of the Earth in its orbit around the sun. (Again, the car-in-a-snowstorm analogy works. After midnight, you are riding on the windshield, which gets hit by more snowflakes than the back window.)

If you plan to get out for the shower, here are a few tips:

Sources of information:

"An Excellent Year for the Perseids?," Joe Rao, Sky and Telescope, August 2004, p 78. Also online at http://skyandtelescope.com/observing/objects/meteors/article_1289_1.asp

science.nasa.gov/headlines/y2004/25jun_perseids2004.htm



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Last update: July 28, 2004