Every August, just when many people go vacationing in the country where skies are dark, the best-known meteor shower makes its appearance.
The annual Perseid meteor shower is expected to be at its best this year, producing one or two meteors per minute during peak hours.
"It's going to be a great show," said Bill Cooke of NASA's Meteoroid Environment Office at the Marshall Space Flight Center in Alabama.
History of fiery tears
August is also known as the month of "The Tears of St. Lawrence."
Laurentius, a Christian deacon, is said to have been martyred by the Romans in 258 AD on an iron outdoor stove. It was in the midst of this torture that Laurentius cried out:
"I am already roasted on one side and, if thou wouldst have me well cooked, it is time to turn me on the other."
The Saint's death was commemorated on his feast day, Aug. 10. King Phillip II of Spain built his monastery place, the "Escorial," on the plan of the holy gridiron. And the abundance of shooting stars seen annually between approximately Aug. 8 and 14 have come to be known as St. Lawrence's "fiery tears."
We know today that these meteors are actually the dusty remains left behind by the comet Swift-Tuttle.
Discovered back in 1862, and most recently observed in 1992, 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 rams into our atmosphere at approximately 37 miles per second (60 kps) and creates bright streaks of light in our midsummer night skies.
According to the best estimates, in 2007 the Earth is predicted to cut through the densest part of the Perseid stream sometime around 2 a.m. ET on Monday, Aug. 13. That corresponds to 11 p.m. PT on Aug. 12 for those living in the Western United States or Canada.
The interval when the meteors will be falling at their highest rates will likely last several hours or more on either side of these times.
As a result, it is the late-night hours Sunday, on through the first light of dawn Monday that holds the greatest promise of seeing a very fine Perseid display.
The moon, whose bright light almost totally wrecked last year's shower, will have zero impact this year. The moon will be new on Sunday, meaning that there will be no interference from it at all.
What to expect
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 yields 90 or 100 meteors per hour. However, observers with exceptional skies often record even larger numbers. Typically during an overnight watch, the Perseids are capable of producing a number of bright, flaring and fragmenting meteors, which leave fine trains in their wake.
On the night of shower maximum, the Perseid radiant is not far from the famous "Double Star Cluster" of Perseus (hence the name, "Perseid"). Low in the northeast during the early evening, it rises higher in the sky until morning twilight ends observing. Shower members appearing close to the radiant have foreshortened tracks; those appearing farther away are often brighter, have longer tracks, and move faster across the sky.
About five to 10 of the meteors seen in any given hour will not fit this geometric pattern, and may be classified as sporadic or as members of some other (minor) shower.
Plan your time
Perseid activity increases sharply in the hours after midnight, so plan your observing times accordingly. We are then looking more nearly face-on into the direction of the Earth's motion as it orbits the Sun, and the radiant is also higher up.
Making a meteor count is as simple as lying in a lawn chair or on the ground and marking on a clipboard whenever a "shooting star" is seen. Watching for the Perseids consists of lying back, gazing up into the stars, and waiting. It is customary to watch the point halfway between the radiant (which will be rising in the northeast sky) and the zenith, though it's perfectly all right for your gaze to wander.
Counts should be made on several nights before and after the predicted maximum, so the behavior of the shower away from its peak can be determined.
Usually, good numbers of meteors should be seen on the preceding and following nights as well. The shower is generally at one-quarter strength one or two nights before and after maximum. A few Perseids can be seen as much as two weeks before and a week after the peak. The extreme limits, in fact, are said to extend from July 17 to August 24, though an occasional one may be seen almost anytime during the month of August.
In addition to the Perseids, some skywatchers will have an opportunity to view another potentially strong meteor display at the beginning of September: the Aurigid meteors. We'll have more details on this in next week's Night Sky, so stay tuned!
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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.