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“Dark Time” Is the Right Time for the Perseids

08/09/2010

The last time the annual Perseid meteor shower happened during a run of moonless nights -- known by astronomers as “dark time” -- was in 2007; there is a new moon on August 10, so this is a prime time to view a spectacular sky show. Not only are the meteors fascinating to watch, they also leave short-lived streams of ionized gas in their wake. As hams have known for years, these meteor trails are excellent reflectors of radio waves.

The Perseid meteor shower is caused by debris from Comet Swift-Tuttle. Every 133 years the huge comet swings through the inner solar system, leaving behind a trail of dust and gravel. When Earth passes through the debris, specks of comet-stuff hit the atmosphere at 140,000 MPH and disintegrate in flashes of light. These meteors are called Perseids because they fly out of the constellation Perseus. Swift-Tuttle’s debris zone is so wide, Earth spends weeks inside it.

The show begins at sundown when Venus, Saturn, Mars and the crescent Moon pop out of the western twilight in tight conjunction. All four heavenly objects will fit within a circle about 10 degrees in diameter, beaming together through the dusky colors of sunset. No telescope is required to enjoy this naked-eye event. The planets will hang together in the western sky until 10 PM or so. When they leave, following the Sun below the horizon, the Perseid meteor shower begins. From that point on until dawn, meteors will flit across the starry sky.

The shower lasts for many days, but according to the International Meteor Organization, this year’s peak should occur during a half-day-long window centered on 0100 UTC on August 13; for us in North America, the best viewing will probably be late Thursday night and early Friday morning, August 12-13, or possibly the night before. Prime viewing for the Perseids is from about 10 PM onward (local time), until the first light of dawn. This is when the shower’s radiant -- its perspective point of origin -- is well up in your portion of the sky; the higher the radiant, the more meteors you will see.

Amateur Radio and the Perseids

If you own a 6 or 2 meter SSB/CW transceiver, you can get in on the action, bouncing your signals off Perseid meteor trails and making quick meteor scatter contacts over hundreds of miles, and possibly even as much as 1200 miles. Meteor scatter operation is particularly easy on 6 meters where 100 W and an omnidirectional antenna will do the job. On 2 meters, a directional antenna (such as a multielement Yagi) usually yields better results.

Some meteor scatter operators prefer to use SSB, making rapid exchanges of signal reports and grid squares. In recent years digital meteor scatter has been increasing in popularity. With the free sound-card-based WSJT software suite by Joe Taylor, K1JT, it is possible to make digital meteor scatter contacts almost any time of the day or night, not just during annual showers. Most WSJT scatter operators use a mode known as FSK441 and center their activities on calling frequencies at 50.260 and 144.140 MHz. They also announce their availability by using Web sites just as N0UK's Ping Jockey Central.

Watching the Perseids

To get the most enjoyment while watching for Perseids, find a dark spot with an open sky view, bundle up thoroughly in blankets or a sleeping bag and lie back in a reclining chair. Gaze into the stars and be patient. The best direction to watch is wherever your sky is darkest, usually straight up, perhaps with a little inclination toward the radiant. Sky & Telescope magazine reports that if you are at a very dark, rural site, “you can probably expect to see 100 or more meteors per hour when the radiant (in northern Perseus) is highest in your sky before the first light of dawn. Any light pollution will cut down on the numbers, as will the radiant’s lower altitude earlier in the night. But the brightest few meteors shine right through light pollution, and the few that happen when the radiant is low are especially long, skimming the upper atmosphere and flying far across the sky.”

Not all the meteors in the sky are Perseids. In addition to occasional random, sporadic meteors, the weaker Delta Aquarid shower is also active during Perseid season. The Delta Aquarids are slower, often yellower and track away from a radiant point in eastern Aquarius. Weaker still are the Kappa Cygnids, identifiable by their flight direction away from Cygnus in an altogether different part of the sky.



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