Perseid shower to light up the sky

August 11, 2007|By Frank D. Roylance | Frank D. Roylance,sun reporter

A lucky alignment of celestial geometry and the weather might make this weekend one of the best opportunities in years for Marylanders to watch the annual Perseid meteor shower.

The Perseid shower -- named for the constellation Perseus from which these "shooting stars" seem to emerge -- is always one of the most active and reliable of the year's annual meteor showers.

But too often, the display is weakened by bright moonlight, bad weather or by a peak that arrives during our daylight hours.

Not this time. For once there will be no moonlight to interfere. The shower will peak at 1 a.m. Monday. And the weather forecast, for now, seems promising.

"This is about as ideal as you can get," said Alan M. MacRobert, a senior editor at Sky & Telescope magazine, who planned to watch from the dark skies of New Hampshire.

Meteor showers occur when the Earth, in its annual orbit around the sun, passes through dust trails left by comets as they pass by on their own orbits around the sun.

The comet dust -- described by another Sky & Telescope editor as the size and texture of Grape Nuts cereal -- strikes the Earth's atmosphere like bugs on a windshield. The friction heats the air molecules, which glow, creating a brief trail of light across the sky until the particles vaporize.

The annual Perseid shower occurs each August when the Earth passes through dust left by comet Swift-Tuttle, which last flew around the sun in December 1992.

That return visit recharged the comet's debris trail, and the annual Perseid shower, for a time.

"It was great in the early and mid-1990s, weaker in the 1960s and '70s," MacRobert said. "It's weaker now than the '90s, and will be for a couple of decades."

Swift-Tuttle's next turn around the sun isn't until July 2126. But there is plenty to see until then.

Viewed under dark skies, the Perseid showers typically produce between 50 and 100 meteors per hour at their peak -- roughly one a minute. But they can accelerate, briefly, to rates as high as 200 per hour.

Though it's expected to peak at 1 a.m. Monday, the shower has already begun. Swift-Tuttle meteors begin to strike the planet as early as July 17, rise in frequency until around the 11th or 12th of August, then decline until Aug. 24.

Can't stay up that late on a work night? Try looking tonight.

"A day beforehand, the Perseids are typically at maybe half, or somewhat less, than their peak strength," MacRobert said. "This is sort of variable, but you will see some ... The whole week before and after [the peak] there are at least occasional ones."

While the moon won't be a hindrance, meteor watchers will still have to contend with light pollution. Porch lights, street lights, cars, billboards and such send copious amounts of light into the sky every night, obscuring the dimmer stars and meteors.

"If you do have light all around you, get a tree or a house between you and the lights, as a shield," said Jim O'Leary, director of the Maryland Science Center's Davis Planetarium.

Wherever you go, you'll need 20 minutes or so for your eyes to adjust to the dark. You also need a spot with as broad and unobstructed a view of the sky as possible.

The naked eye is the best way to see a meteor shower. A reclining chair or a blanket will spare your neck and make for a more comfortable experience.

The show begins for Marylanders as soon as the constellation Perseid rises above the northeast horizon, at 10 or 11 p.m.

As the night advances, MacRobert said, Perseid will appear to rise higher in the sky, and we'll see more and more of the meteors as they smash into the atmosphere at 37 miles per second.

There's no need to find Perseus in the sky, though. It's better to scan the whole sky, watching for meteors that flash past us at wider angles from Perseus. The best are called "Earth-grazers" -- the ones that skip across the atmosphere like stones across a pond, often leaving long trails behind them. The show will continue until the sky begins to brighten before dawn.

O'Leary said he'll be watching from a dark spot of his own.

"Absolutely. I always go look for the Perseids," he said. "It's always been the most dependable of meteor showers, the annual Old Faithful."

Looking for dark skies

Meteor showers are best seen from dark-sky locations far from urban lighting.

Amateur astronomers like Tuckahoe State Park, just north of Route 404 on the Caroline/Talbot county line; Alpha Ridge State Park off Route 99 in Howard County; and Point Lookout State Park in St. Mary's County.

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