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Tuesday, August 03, 2004

2004: An Excellent Year for the Perseids :

A bright Perseid flashed across the horns of Taurus before dawn on August 13, 2002, while Vincent Varnas was photographing from Stevenson, Washington. He used a 35-mm f/2 lens and Fujicolor Press 1600 film for this lucky 20-second exposure, one of many he took during 2½ hours of sky shooting. To the meteor’s lower left is Mars; to its right are Aldebaran and the Hyades, with the Pleiades above.

Circumstances will be nearly ideal for watching the annual Perseid meteor shower at its predicted maximum late on the night of August 11–12. Many families on August vacations at dark, country sites discover these meteors on their own, and late-summer campers often pull their sleeping bags out of their tents to enjoy this Old Faithful shower.
The Perseids are one of the two strongest and most dependable annual meteor showers (the Geminids of December are the other). Earth’s orbit carries us through the densest part of the Perseid meteoroid stream every year around August 11th or 12th, so these “shooting stars” appear almost like clockwork. Their rates, however, can vary a lot from year to year. An observer under a dark sky might typically see more than 60 Perseids per hour between midnight and dawn. Since the waning crescent Moon will be only three days from new at the time of shower maximum, this is an opportune year for watching them.
But that is only one reason why anticipation is running high among meteor observers. How the 2004 shower will actually perform is anybody’s guess — but it will probably be better than normal, and there’s a chance it could be spectacular.
Meteor Basics
The meteoroids of the Perseid stream range in size from pebbles to sand grains and have a consistency like bits of ash. They ram into our upper atmosphere at a speed of 60 kilometers per second, creating incandescent trails of shocked, ionized air as they vaporize.
On the peak night, the Perseids will appear to diverge from a patch of sky between Perseus and Cassiopeia just east of the famous Double Cluster. The meteors’ apparent divergence from this radiant point is an effect of perspective; the meteoroids are actually traveling in parallel through space. Meteors appearing near the radiant will display short trails because we see them nearly end on, while those far from the radiant, seen broadside, look much longer.
In the early-evening hours the radiant is low in the north-northeast, so the meteors strike the upper atmosphere at a low angle — and therefore we see comparatively few of them per square kilometer at the atmosphere’s top. As the night advances, the radiant rises higher in the northeast, the meteors arrive more nearly straight down, and so we see more of them. By the time morning twilight begins, the radiant has climbed to around 60° altitude for observers at midnorthern latitudes.
If it’s cloudy on the peak night, don’t despair; for one or two nights before and after, rates are still roughly a quarter to half of the maximum. In fact, the first few forerunners of the shower may show up as early as July 20th; the last stragglers have been recorded as late as August 24th.
Interference by moonlight this year will be minor, especially after the peak date, permitting night-after-night monitoring of the shower’s buildup and decline. On the morning of August 10th the thick waning crescent Moon won’t rise until about 1 a.m. local daylight time for observers in midnorthern latitudes. On the 12th, a thinner crescent will come up about 2:30 a.m. The Moon is new on the 15th, and for a week afterward the waxing crescent will set before midnight.
Meteor watching is simple. Pick an observing site that’s free of glary lights nearby, has an open view of the sky, and preferably is as far as possible out from under city light pollution. Don’t forget the mosquito repellent. Bundle up in blankets or a sleeping bag, lie back, and gaze into the stars. The direction to watch is wherever your sky is darkest, usually straight up. For more observing hints, see


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