A big night to catch some shooting stars

Meteors: The annual Perseid shower promises to light up the sky beginning at midnight tomorrow.

August 10, 2004|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

The stars are aligning this week for what astronomers are saying could be one of the best displays in years of the annual Perseid meteor shower.

Where skies are clear and dark, sky watchers in Maryland could see 60 to 80 meteors per hour when the shower peaks between midnight tomorrow and dawn Thursday.

And if the meteor forecasters' computer models are correct, there's even a chance for a 40-minute "outburst" of up to 200 per hour tomorrow.

If there isn't, "they'll have to go back to the drawing boards and come up with new models to predict how meteor showers evolve," said Gary Kronk, an Missouri-based amateur astronomer and author of four books on meteor showers and comets.

The Perseid meteor shower occurs annually in mid-August as the Earth passes through the broad dust trail left in space by the passage of Comet 109P/Swift-Tuttle.

Swift-Tuttle orbits the sun once every 130 years and has been making the trip for thousands for years. "This comet was seen as long ago as 69 B.C.," Kronk said, referring to a recorded sighting.

It last cruised through this part of the solar system in December 1992, its first visit since 1862, during the Civil War.

In all those visits, the dust trail Swift-Tuttle has left behind has gradually spread into a broad orbital avenue of dust particles the size of pebbles or sand grains.

And every summer, as the Earth crosses through that region on its annual orbit around the solar system, those particles slam into the atmosphere at 130,000 mph. The energy released causes the air around the particle to glow, creating a bright, fleeting trail as they vaporize.

"The Perseids produce a lot of really bright meteors ... because of their high speed," Kronk said.

Perseids are identifiable because they appear to radiate from the constellation Perseus, which rises in the northeast after 11 p.m.

Look for it just below the more familiar W-shaped constellation Cassiopeia.

They arrive as early as July 20, increasing nightly toward their peak around Aug. 11 and 12. Then their numbers fall off to nothing by the third week in August.

The Perseids constitute one of the year's most active and reliable meteor showers. And they're popular because of the mild weather in August, unlike, say, the Geminids, which are spectacular, but relatively unwatched because they return in December.

All that's required to watch the shower are clear, dark skies, a lawn chair and a broad view of the night sky.

Just point your feet toward the south, and stretch out on your lawn chair or sleeping bag. Let your eyes adjust to the dark for 15 minutes or so, and scan the sky between the zenith (directly overhead) and halfway down toward the horizon.

The first bit of good news for this year's Perseids is that the moon will not interfere with the spectacle.

The moon won't rise until 2:30 a.m. Thursday. When it does, it will be a slender crescent.

"That's kind of neat because for the last two years the Perseids have been ruined" by a bright moon, Kronk said.

Another twist this year are predictions made independently by two experts that the shower will have an initial peak tomorrow that is briefer but more intense than Thursday morning's peak.

This "outburst ... could possibly reach upward of 200 an hour," Kronk said. One forecast pushes the possibilities to 1,000 per hour.

Astronomers Esko Lyytinen of Finland and Tom Van Flandern of Washington, D.C., say the outburst - if it occurs -would result from the Earth's passage through a dense skein of debris laid down during Swift-Tuttle's passage in 1862.

"The planets are always tugging on stuff," Kronk said, "and whenever a comet appears, it's in a slightly different orbit."

With each passage, he said, the dust left behind "remains separate for a while, until hundreds of years go by. The Aug. 11 stuff is a more recent cloud. It's spreading, but it hasn't merged with the main Perseid debris field yet."

The bad news is that Lyytinen and Van Flandern are predicting tomorrow's outburst will occur about 5 p.m. EDT, in broad daylight here.

If they're right about the timing, it would be a triumph for their computer modeling of how comet trails form and disperse in space and a thrill for skywatchers from eastern Europe to China.

But they would have to be wrong by six hours or more for Marylanders to get a look.

The other fly in the celestial soup is the weather forecast. Meteorologists are calling for partly cloudy skies and a chance of thunderstorms on both days. If clouds ruin the view, the only consolation is that Perseid meteors will continue to fall each morning through Aug. 24, although at much lower rates.

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