Cosmic dust, some of which has been wafting through space since the American Civil War, is renewing a summer ritual by slamming into Earth's atmosphere, lighting up the night sky with shooting stars whose numbers are building to a peak on Aug. 12.

The return of the Perseid meteor shower marks one of the most rewarding skywatching events of the year. No matter how many other night sky shows fail to meet expectations, the Perseids rarely falter. Only two weeks straight of cloudy skies can completely spoil the shower.

The Perseids began in mid-July, with one or two meteors streaking through the sky each evening. Activity is slowly picking up in advance of the peak, when dozens of shooting stars will be visible each hour from dark skies.

A meteor shower is Earth's way of swallowing up leftovers from the solar system's formation. Each year, the planet passes through a stream of debris left behind by the infrequent passages of the comet Swift-Tuttle, which crosses the inner solar system once every 128 years as part of its elongated orbit around the Sun.

The debris is caused when a stream of charged particles zooming out from the Sun burns material off the comet. The leftover particles, most no larger than grains of sand, collide with Earth's atmosphere at up to 130,000 mph, where they burn up and produce meteors, commonly known as shooting stars.

Because most meteors in a shower are tiny, there is little threat to spacecraft, and virtually none to people on the ground.

But some comets, like Swift-Tuttle, carve an orbit that is much nearer, and their trips around the Sun have been noted more than once.

Swift-Tuttle last passed by in 1992, freshening the debris stream and bumping up the peak hourly meteor rate of the Perseids to as high as 500 the following two years.

Because the comet's path is slightly different on each pass, and because the dust that's left behind moves through space, some parts of the debris stream are more dense than others. Also, Earth's orbit varies each year and the planet moves through a different part of the overall debris stream.

So scientists are never sure exactly when the peak will occur or how strong it will be. Material left by Swift-Tuttle's pass in 1862 had also helped increase the Perseid activity in recent years, Conod said, but that influx may be waning now too.

"From every northern location in the world, a fair number of nice meteors will be seen," says Rainer Arlt, an astronomer at Astrophysikalisches Institut Potsdam in Germany. "The only hindrance is artificial light."

The Perseids regularly produce 50 to 150 meteors per hour -- more than 1 per minute -- under dark skies. There have been years when they produced only a handful, and other years when the count soared above 200 per hour. The first records of the shower date back to 36 A.D., with a Chinese account of "more than 100 meteors" being sighted one early morning.

This year's peak hourly rate is expected to be on the low end of the range, likely around 50.

The best times to watch will be the overnight hours on Aug. 11/12 and Aug. 12/13, astronomers say. The peak is forecast to occur Aug. 12, between 14h and 17h UT, or Universal Time, said Arlt. Unfortunately, that's 10 a.m. to 1 p.m. EDT. Observers in Hawaii should see the peak under dark skies in the very morning of August 12, and parts of eastern Asia will see the peak fall during night hours, Arlt said.

"Those who have dry transparent air may be able to see up to 50 Perseids an hour," during the peak, said Robert Lunsford of the American Meteor Society. Hazy humid conditions would reduce that count.

A dozen or more per hour could also be visible a night or two before the peak, and then a night or two after. The shower continues through about Aug. 22, by which time it will have wound back down to just 1 to 2 meteors per hour.

Also, up to 10 shooting stars not associated with the Perseids occur every hour of every night this time of year. These other meteors, which are typically not as bright as the Perseids, can approach from any direction in the sky.