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Mercury, July/August 1996 Table of Contents

by James C. White II, Middle Tennessee State University

(c) 1996 Astronomical Society of the Pacific

Every August, Earth swings through the dusty path of Comet Swift-Tuttle. The tiny grains of dust leap to their fiery deaths in our atmosphere -- and we are witness to the year's finest meteor shower.

This Month's Project: observing the Perseid meteor shower

Comet Hyakutake wowed us this spring [see Guest Observer, May/June, p. 11]. Comet Hale-Bopp teases us now for a potentially spectacular show next spring. And have you forgotten the humiliation that Comet Shoemaker-Levy 9 inflicted on mighty Jupiter two years ago? You had better put on a helmet to protect yourself, because bits of the sky are going to be falling soon. You guessed it: Another of those darned comets is to blame!

I exaggerate. Earth is indeed going to be pelted by cometary debris, but don't worry. Instead, be happy. We north of the equator will have a front-row seat for an annual astronomical spectacle.

From about July 17 through Aug. 24, Earth will pass through the debris spit out by Comet Swift-Tuttle. Like all comets, Swift- Tuttle is a shameful litterbug, leaving a trail of dust and pebbles as it orbits the Sun every 130 years. Each August, our planet crosses this dust-strewn path.

Swift-Tuttle last passed through the inner solar system in 1992, chucking a fresh load of cometary cigarette butts, so a good show may be in store this year. The bits of material, most smaller than a grain of sand, will plunge into our atmosphere at 59 kilometers per second (130,000 miles per hour) [see "The Stars Are Falling," March/April, p. 6]. Because of our orbit around the Sun, the shooting stars will seem to streak out from the direction of the constellation Perseus -- hence the name Perseids.

Some people were disappointed at last year's performance. Blame the bright Moon: It drowned out all those faint meteors. This year, the Moon won't interfere. Meteor-watchers estimate we'll see 200 meteors per hour from dark locations.

In recent years, meteor-watchers have noticed two peaks in the number of Perseids per hour. The first is a burst of meteors which occurs as Earth sails through a tight, dense stream of centuries-old debris. Half a day later comes the second, "traditional" maximum. Asia and Europe, brace yourselves for prime viewing of the first maximum on the evening of Aug. 12. North America has the best vantage point for the traditional maximum.

Observing Guidelines

A meteor shower is more than a show. It's an experience. All you need is a clear sky and a willingness to sacrifice sleep. Find an open space with a good view of the sky. Don't fret if you live in the city; your cousins in the country, away from city lights, will have a better view, but you will still be able to see plenty of meteors. Once you have staked out your space, unfold that lounge chair or blanket and crack open that thermos of hot chocolate.

The best time to observe a meteor shower is after midnight, as your nighttime location moves head-on into the path of the cometary debris. You may see some non-Perseid meteors (so- called sporadic meteors) before midnight. Perseid meteors will have trails that you can trace back to a point in the sky near the lovely Double Cluster in Perseus [see SkyTalk, p. C-4].

As observers, we are interested in the number of Perseids per time interval. Recruit a viewing companion, and choose an interval whose length you feel comfortable with. Why not try successive 15-minute intervals, with short breaks in between? You must concentrate on the sky during the intervals, and short (2- to 3-minute) breaks between intervals give you a chance to stretch.

At the beginning of each interval, have your companion record the time. Then, simply lie there for 15 minutes, looking straight up, and count. Each time you cry, "Meteor!" your observing buddy should make a mark. Don't look away during these observations or else you'll miss some. At the end of the interval, record the local time and switch jobs with your companion. If you can't convince someone to observe with you, use a tape recorder to record your time readings and observations; you can add up your meteor counts later by playing back the recording.

Complete as many of the observing intervals as you care to; the more you make, the more useful your data will be. Tabulate the number of meteors observed per interval, and make sure to label the table with the local time and date of each interval.

I encourage you to begin your observations about a week before the night of Aug. 11-12 and continue until about a week after. On each evening of your observations, you should assess how bright the sky is. To do this, estimate the magnitude of the dimmest star you can see. Sky charts provide you with magnitudes of easily identified stars. In Nashville, say, the faintest star may have a magnitude of 4.0. In rural Tennessee, it may be 6.0. The brightness of the sky will determine how many meteors you can see -- and will allow us to compare counts from different locations.

If you'd like to try something different, listen for meteors using an FM radio [see "Falling Stars of August," May/June 1993, p. 93].

Incorporate your observations into a document with the following information: name of the project (such as, "Observing the Perseids"), your name or the name of your group, latitude and longitude of the observing location, mailing address, telephone number, and email address, if available. In your report, please provide accurate time and date information, as well as written commentary.

Please submit your completed report by Sept. 30, 1996 by email to 2032694@mcimail.com or by regular mail to John Isles, Attn: Guest Observers, 1016 Westfield Drive, Jackson, Mich. 49203-3630. The selection committee will evaluate the reports and choose the Guest Observer for the January/February 1997 issue.

JAMES C. WHITE II is a professor in the Department of Physics and Astronomy at Middle Tennessee State University in Murfreesboro. His email address is jwhite@physics.mtsu.edu.