Set your alarm clock for the wee hours between midnight and dawn next week to watch the summer's best dazzle of shooting stars: the Persied meteor shower.

This show of streaking lights is named for the constellation Perseus, the area the meteors appear to radiate from, located in the Milky Way high overhead between the tight cluster of the Pleiades and the sideways W-shape of Cassiopeia.

The Perseids, one of the most reliable meteor showers of the year, occur when Earth brushes through the debris trails left behind in space as Comet Swift-Tuttle orbits the sun every century or so.

(This comet was last seen in 1992, and before that in 1862, the year it was first described, during the Civil War. Astronomers think Swift-Tuttle, named for Americans Lewis Swift and Horace Tuttle, who "discovered" the comet, has been orbiting the sun for at least 2,000 years and may have been observed by humans as early as 69 BC.)

As Comet Swift-Tuttle speeds on its elliptical orbit around the sun, it sheds bits of debris from its nucleus. Each time the ball of ice and what astronomer Chet Raymo calls "celestial construction junk" passes, it sheds more; these trails of space detritus are what create the Perseid meteor showers.

The ephemeral streaks of light that we call shooting stars occur when this comet-dust, ranging from sand-grain-size particles to bits the size of peas or marbles, slams into Earth's atmosphere.

Swift-Tuttle's detritus is particularly speedy as meteors go: it hits our outer atmosphere at around 133,000 miles per hour, or a bit under 40 miles per second. (These shooting stars don't risk speeding tickets though: they zip by far too fast for a standard radar gun to measure.)

At that speed, the meteors compress the air in front of them so quickly that it heats to temperatures as hot as 3,000 degrees F, and the particles vaporize almost instantly, producing those fiery trails across the night sky.

Most meteors never hit the Earth, instead burning out not long after they smack into our atmosphere around sixty miles overhead; those that do reach the planet's surface are called meteorites.

(It was a giant meteorite about the size of Comet Swift-Tuttle's nucleus, which scientists estimate measures six miles across, that slammed into Earth back in Paleozoic time, creating the massive dust clouds that cooled the planet almost instantly, killing the dinosaurs and many other species.)

Prime nights for viewing the Perseids this year will be August 11 and 12, next Monday and Tuesday, but they're visible this week too.

Best viewing times are in the wee hours of the morning after the sun sets.

Meteor-viewing in North America is always better after midnight, since that's when Earth rotates to place our continent on its "leading" side as it moves through space, meaning that our hemisphere is aiming directly into any space detritus.

So find yourself a dark place away from city lights, and get out your lawn chairs or lie on the ground, and look up at the sky. Locate Perseus by finding the silvery ribbon of the Milky Way, running more or less southwest to Northeast at this time of year. Follow the Milky Way toward the northeastern horizon, passing the sideways W-shape of Cassiopeia, and watch for brilliant streaks of light.

Be patient: this year's show is expected to bring 50 to 100 meteors an hour, or somewhere around one a minute. But they'll be spectacular, since Perseids are noted for their speed and thus, their tendency to vaporize in dazzling fireballs. 

So set your alarm clocks for the wee hours and haul yourself out before dawn to see the Perseids, summer's best meteor show.

Copyright 2008 Susan J. Tweit
First printed in the Salida, Colorado, Mountain Mail, and aired on KHEN-FM community radio.