Perseid Meteor Showers

(Sky & Telescope Aug. 7) - The Perseid meteor shower, an annual celestial event beloved by millions of skywatchers around the world, returns to the night sky this coming week. But moonlight will interfere somewhat with the view.

Sky & Telescope magazine predicts that the Perseid shower will reach its peak late on Friday and Saturday nights, August 11-12 and 12-13 (for viewers in North America). The rate of activity should pick up after midnight until the first light of dawn.

You'll need no equipment but your eyes. The moonlight in the sky will hide the fainter meteors, and so will artificial light pollution, but the brightest meteors should still show through.

Find a dark spot with a wide-open view of the sky. Bring a reclining lawn chair and a sleeping bag; the bag not only provides warmth against the late-night chill but also serves as mosquito armor in this era of West Nile virus. Cover your remaining exposed parts (including hair and clothing) with an effective mosquito repellent.

"Go out after about 11 or midnight or so, lie back, and gaze up at the stars," says Sky & Telescope senior editor Alan MacRobert. "Relax, be patient, keep the Moon out of sight, and let your eyes adapt to the dark. With a little luck you'll see a 'shooting star' every few minutes on average."

Perseids can appear anywhere and everywhere in the sky. So the best direction to watch is wherever your sky is darkest. Faint Perseids appear as tiny, quick streaks. Occasional brighter ones may sail across the heavens for several seconds and leave a brief train of glowing smoke.

If you trace each meteor's direction of flight backward far enough across the sky, you'll find that this imaginary line crosses a spot in the constellation Perseus, near Cassiopeia. This is the shower's radiant, the perspective point from which all the Perseids would appear to come if you could see them approaching from the far distance. The radiant is low in the north-northeast before midnight and rises higher in the northeast during the early-morning hours.

Don't give up if it's cloudy on the peak nights. The shower lasts for about two weeks, with fairly good rates in the predawn hours of August 10 through 15. (The radiant is always low or below the horizon for Southern Hemisphere countries like Australia, New Zealand, and South Africa; therefore few if any Perseids can ever be seen from these regions.)

The Perseid meteoroids are tiny, sand- to pea-size bits of rocky debris that were shed long ago by Comet Swift-Tuttle. This comet, like others, is slowly disintegrating as it orbits the Sun. Over the centuries, its crumbly remains have spread all along its 130-year orbit to form a sparse "river of rubble" hundreds of millions of miles long.

Earth's own path around the Sun carries us through this stream of particles every mid-August. The particles, or meteoroids, are traveling 37 miles per second with respect to Earth at the place where we encounter them. So when one of them strikes the upper atmosphere (about 80 miles up), it creates a quick, white-hot streak of superheated air.

For several years in the early 1990s the Perseids performed spectacularly, flaring with outbursts of up to hundreds of meteors visible per hour. The rubble streams responsible for these outbursts were probably shed during Comet Swift-Tuttle's swing by the Sun in 1862. In recent years, though, the shower has returned to normal.

For more information: SkyTonight.com.