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Annual celestial event returns

Perseid meteor shower to peak August 12

By Joe Rao
SPACE.com

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Perseid meteors can be traced to particles of dust from the tail of comet Swift-Tuttle.

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(SPACE.com) -- Every August, when many people go vacationing in the country where skies are dark, the best-known meteor shower makes its appearance -- the Perseids.

In 2005, the Perseids are expected to reach their maximum on August 12. Peak activity is unfortunately predicted for the daylight hours across North America.

Sky watchers are thus encouraged to watch during the predawn hours of Friday, August 12 and again during the early morning hours of Saturday.

Observers will be favored by an absence of bright moonlight during these intervals. At midnorthern latitudes, moonset occurs on the evening of August 11 at around 11-p.m. local daylight time and around 11:20 p.m. the following night.

Since dawn doesn't break until around 4:30 a.m., that means there will be about 5 to 5 1/2 hours of dark, moonless skies for the two best viewing nights for the Perseids.

Take full advantage of this year's favorable lunar circumstances. Next year, a bright waning gibbous moon will flood the after-midnight sky with its light and seriously hinder the Perseids.

Bits of a comet

We know today that these meteors are actually the dross of the Swift-Tuttle comet.

Discovered back in 1862, this comet takes approximately 130 years to circle the sun. And in much the same way that the Tempel-Tuttle comet leaves a trail of debris along its orbit to produce the Leonid Meteors of November, comet Swift-Tuttle produces a similar debris trail along its orbit to cause the Perseids.

Indeed, every year during mid-August, when the Earth passes close to the orbit of Swift-Tuttle, the material left behind by the comet from its previous visits, ram into our atmosphere at approximately 37 miles per second (60 kilometers per second) and create bright streaks of light in our midsummer night skies.

Comet Swift-Tuttle made its most recent appearance more than a dozen years ago, in December 1992. Its orbit is highly elongated and as such it takes roughly 130 years to make one trip around the sun. For several years before and after its 1992 return, the Perseids were a far more prolific shower, appearing to produce brief outbursts of as many as several hundred meteors per hour, many of which were dazzlingly bright and spectacular.

The most likely reason was that the Perseids parent comet was itself passing through the inner solar system and that the streams of Perseid meteoroids in the comet's vicinity were larger and more thickly clumped together. Hence the reason for the brighter meteors and much-higher-than-normal meteor rates.

In recent years, with the comet now far back out in space, Perseid activity has apparently returned to normal.

Meteor clumps

A very good shower will produce about one meteor per minute for a given observer under a dark country sky. Any light pollution or moonlight considerably reduces the count.

The August Perseids are among the strongest of the readily observed annual meteor showers, and at maximum activity nominally yield 50 or 60 meteors per hour. However, observers with exceptional skies often record even larger numbers.

But while 60 meteors per hour correspond to one meteor sighting every minute, keep in mind that this is only a statistical average.

In reality, what usually is seen is what some have called, "the clumping effect." Sometimes you'll see two or even three Perseids streak across the sky in quick succession, all within less than minute. This is usually followed by a lull of several minutes or more, before the sky suddenly bears fruit once again.



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