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The Perseids meteor shower 2006, photograph by Jim Colyer
Tuesday 31 July 2007
Viewer's Guide to the Perseid Meteor Shower
by Robert Roy Britt
Senior Science Writer
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.
"Light pollution is a real killer when it comes to meteor showers," says Kevin Conod, an astronomer at the Newark Museum's Dreyfuss Planetarium in New Jersey. A trip to the country is the best way to assure good viewing.
The Perseids are an annual event that anyone can enjoy.
[image] Scores of meteors near the bowl of the Little Dipper, in a 10 to 12 minuteexposure by A. Scott Murrell during the 1966 Leonid storm. He used a 50-mm f/1.9lens and Tri-X film in a camera tracking the stars at New Mexico State UniversityObserv atory
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.
The instigators of meteor showers, comets, are primordial remnants from the time when the Sun and then the planets formed, some 4.5 billion years ago. Many comets inhabit a sphere of space called the Oort Cloud that stretches nearly one-fifth the way to the next nearest star. They are so far out there that most have not passed through the inner solar system in recorded history.
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.
When to watch
Though they vary, the Perseids are more predictable than most meteor showers. And while never grand on the scale of historic meteor storms caused by the November Leonid meteor shower, the Perseids are dependable.
"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.
The Perseid meteor shower gets its name from the constellation Perseus, because the meteors appear to emanate from a point, called the radiant, within Perseus.
Regardless of where you live in the Northern Hemisphere, the radiant begins the night low on the Northeast horizon, tossing very few shooting stars into view. As the night goes on, the radiant moves overhead, and opportunities improve. However, this year a bright last-quarter Moon will obscure fainter meteors in the pre-dawn hours, which otherwise is prime time for meteor watching.
"Despite the Moon it would be best to watch during the last few hours before morning twilight when the radiant lies high in the northern sky," Lunsford suggests. "It would be best to face away from the Moon, either to the north -- probably the best direction -- west or southwest."
The best time for most meteor showers is between 2 a.m. and dawn. This is when the part of the Earth you stand on is beginning to fly headlong into a debris stream, and so that's when most of the debris shows up in the portion of the night sky you can see.
The Perseids are an annual event that anyone can enjoy.
A little patience can help, too. Astronomers suggest taking time to allow your eyes to adjust to the darkness. Binoculars and telescopes are of no use, because meteors move too quickly. But a lounge chair can come in handy and relieve neck strain.
Perseid meteors are known for a curious feature that careful viewing should reveal: "About half of them leave behind a trail of smoke, which can be visible for several seconds or more after the meteor has burned up," Conod said. And an occasional Perseid meteor will explode into a fireball, a bright momentary flash, but this meteor shower is not known for producing an abundance of such events.
Because of the location of the radiant, the Perseids are mostly a Northern Hemisphere event, though a few Perseid meteors can be seen from below the equator.
You never know
Even the reliable Perseids can be unpredictable, at least within a range. In 1999, viewers had high hopes since there was no moonlight to spoil the show, but most people were disappointed by a low output of about 50 meteors per hour.
"I was fortunate to see 80 per hour during my last hour of observing on the West Coast," said Lunsford. "It was too light for a majority of North America to see this surge of activity."
But you never know.
"After the dismal show in 1999 there was not much hope in 2000 as the bright waxing gibbous Moon set only 2 hours before morning twilight on Aug. 12," Lunsford said. "Those who bothered to watch were treated to an excellent display as many observers counted 75 to 100 Perseids an hour."
Meteor shower forecasting is in its infancy, and the Perseids are one of only two showers (the other being the Leonids) for which reliable forecasts have been developed. Here's why the Perseids vary from year to year:
"Each return of comet Swift-Tuttle is not exactly in the same location as previous ones," said Lunsford. "Each August the Earth skims the outer portions of these orbits. These orbits have been moving slowly inward toward the Sun."
Esko Lyytinen, a Finnish researcher, says that in 2004 Earth will for the first time in two millennia pass directly through a concentration of dust that Swift-Tuttle cast off in 1992.
"I expect in 2004 a short outburst may be approaching a small storm, but this prediction is quite uncertain," Lyytinen said via e-mail. Unfortunately, the brief but intense peak of the 2004 shower will likely occur during daylight in North America.
Looking farther ahead, the year 2028 may dazzle. Lyytinen said the planet will move through a dense trail left behind in 1479. "I expect this to produce a real storm over the United States, although under unfavorable moonlight conditions," he said.
Later this year, a real meteor storm
This year's Perseids could be seen as a practice run, a chance to hone your meteor-watching skills for a truly spectacular event that could come later this year, in November.
Click here for a sneak preview of the Leonid meteor shower, predicted to provide "a grand display."
Orion SkyQuest XT8 IntelliScope Telescope $479
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The Perseids (PURR-see-idz) are a prolific meteor shower associated with the comet Swift-Tuttle. The Perseids are so called because the point they appear to come from, called the radiant, lies in the constellation Perseus. However, they can be seen all across the sky. Because of the path of Swift-Tuttle's orbit, Perseids are mostly visible on the northern hemisphere.
The shower is visible from mid-July each year, with the greatest activity between August 8 and 14, peaking about August 12. During the peak, the rate of meteors reaches a hundred or more per hour.
Meteor showers occur when Earth moves through a meteor stream. The stream in this case is called the Perseid cloud and it stretches along the orbit of the Comet Swift-Tuttle. The cloud consists of particles ejected by the comet as it passed by the Sun. Most of the dust in the cloud today is approximately a thousand years old.
However, there is also a relatively young filament of dust in the stream that boiled off the comet in 1862. The approximate rate of meteors originating from this filament is much higher than normal.
The famous Perseid meteor shower has been observed for about 2000 years, with the first known information on these meteors coming from the Far East. In early Europe, the Perseids came to be known as the "tears of St. Lawrence."
To experience the shower in its full, one should observe in the dark of a clear moonless night, from a point far outside any large cities, where stars are not dimmed by light pollution.
Labels: Perseids meteor shower