Viewer's Guide: Perseid Meteor Shower Peaks Aug. 12

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

It is also 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."

What to expect

In 2005, the Perseids are expected to reach their maximum on Aug. 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, Aug. 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 Aug. 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 5½ 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.

When and where to look

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.  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.

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 its 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 Aug. 24, though an occasional one might be seen almost anytime during the month of August.    

Photographing meteors

The Perseids are an excellent meteor display to attempt to photograph.  Meteor photography is popular.  However, the chance of your recording a meteor is enhanced by using a fast lens (f 2.8 or better) and ultrafast film (ISO 400 to 1600).  It makes no difference whether the camera is clock-driven or fixed on a tripod.

If your camera has an electronic shutter see if it also has a long time exposure mode that doesn't draw current. Otherwise, put in fresh batteries and plan on replacing them the next day. Mechanical shutters are preferable for long time exposures for this reason.

If all you have is a digital camera, then give it a try. Otherwise, use a film-based camera instead. The reason for this is that digital cameras suffer from thermal noise during exposures of more than a second or so.

In a dark sky, exposures of 10 to 20 minutes long can be made, but should be kept much shorter if background light threatens to fog the film.  Slight moonlight, twilight or city glow can be tolerated, as they have little to do with the efficiency of a particular lens-film combination in recording bright meteors. 

A successful photograph has many added values if an observer has witnessed and described the same meteor.  Also, the chance of obtaining a good meteor picture can be increased by pointing the camera well away from the radiant.

No danger

Many years ago, a phone call came into New York's Hayden Planetarium. The caller sounded concerned after hearing a radio announcement of an upcoming Perseid display and wanted to know if it would be dangerous to stay outdoors on the night of the peak of the shower (perhaps assuming there was a danger of getting hit).  These meteoroids, however, are no bigger than sand grains or pebbles, have the consistency of cigar ash and are consumed many miles above our heads.  

The caller was passed along to the Planetarium's Chief Astronomer who commented that there are only two dangers from Perseid watching: getting drenched with dew and falling asleep!

Whether you plan to take photographs, make detailed meteor counts or just lie back and watch nature put on a show, there should be plenty to see late on the nights of Aug. 11 and 12. As one long-time meteor enthusiast once noted:  "Meteor observing is relaxing and enjoyable, potentially dramatic and just plain fun!"

• Meteors and Meteor Showers: How They Work
• Does Anyone Ever Get Hit by Meteors?
• Meteor Observing Tips and Shooting Star Glossary
• Meteor Shower Photography

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.