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Star Talk: Perseid meteor shower shaping up to be a good show

Sunday, July 28, 2013
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It’s time again for the annual Perseid meteor shower. This display of “shooting stars” always takes place during mid-August and this year will be a good one for the show.

In about two weeks, the peak number of “falling stars” for the Perseid meteor shower will occur. Although the greatest number of meteors per hour will occur during the afternoon of Aug. 12 this year, that’s not really bad news. The shower’s rise to maximum and its following decline will mean that we will have two opportunities to watch the meteor shower — the best displays will be during the nights of Aug. 11-12 and Aug. 12-13.

The Perseid meteors are bits of material left behind as a debris trail by Comet Swift-Tuttle. Once a comet enters the inner solar system and the sun warms its exterior, minute particles are vaporized from the surface of the comet. Once released, these flecks drop back and reflect sunlight to create the comet’s long, graceful tail.

This interplanetary flotsam continues to disperse and eventually expands to reach Earth’s orbit. Every year during mid-August, Earth travels within this region and rams through these comet fragments at an effective speed of more than 130,000 miles per hour.

At such hypervelocity speeds, the comet grains vanish as they rocket through Earth’s thin, upper atmosphere, heating the rarefied air to momentary incandescence. From the ground, we observe this brief flash of light as the comet crumb streaks to its end.

Comet confusion

Comet Swift-Tuttle was discovered in July 1862 and found to swing around the sun every 130 years. It last visited the inner solar system in 1992. At that time, Swift-Tuttle became bright enough to be viewed with binoculars. However, during its next passage in 2122, the comet will swing closer to Earth and be bright enough to be a naked-eye comet, possibly similar in appearance of Comet Hale-Bopp.

Since meteor showers come from grains left behind by comets, this probably contributes to the confusion about the terms “meteor” and “comet.” Meteors are minute pieces of material that usually disintegrate about 50 miles above Earth’s surface; comets are millions of miles away and are made of ices with space “dust” mixed in. Because of the dusty, lightweight makeup of comet material, astronomers commonly refer to comets as “dirty snowballs.”

About a half-dozen random meteors can be seen from a dark location on any clear night. These “chips off the old block” originate from boulders and larger objects in the asteroid belt, a region between Mars and Jupiter that is more than 100 million miles from Earth. Most asteroids have a rock and metal composition.

Lunar crescent

The evening of Aug. 11 will start with a conspicuous lunar crescent in the west-southwest. Then, as the sky darkens and other space objects come into view, the planet Saturn will appear to the upper left of the moon and bright Venus will be shining in the west to the far right of the lunar crescent.

These three celestial objects will be delightful telescope targets while waiting for the moon to set and the sky to darken. In addition, to the immediate upper left of the moon that evening will be Spica, a bluish-white star and the brightest star of Virgo. However, Spica might be difficult to see given that the moon will be adjacent to it and almost 4,000 times brighter than the star.

The moon will set just after 10 p.m. on Aug. 11, leaving the sky dark for the annual shooting-star spectacular.

As evening arrives on Aug. 12, the lit side of the lunar crescent will have noticeably grown from the previous evening; also, the moon will be positioned farther to the southwest than it was during the preceding nightfall. Now the lunar crescent will appear just to the lower right of Saturn and be quite a ways to the left of Venus. Spica will then be to the lower right of the moon and about 10,000 times fainter than the 6-day-old, waxing lunar crescent. Moonset will be about 45 minutes later than on Aug. 11.

Although, there are always some Perseid meteors to see during the evening when this event takes place, with this meteor shower, the rate of meteors sighted always increases after midnight and continues to intensify until dawn. This happens because after 12 a.m. an observer’s location begins to turn directly into the source of the Perseid meteors — the radiant for the shower.

The Perseid meteors all seem to fly away from the constellation Perseus, a star pattern that takes until midnight to finally edge completely above the northeastern horizon. As Perseus climbs higher into the sky, the radiant of the Perseid shower gains altitude and we face more squarely into the oncoming meteors.

Perseverance required

It is important to understand that the widely publicized, predicted peak of 100 meteors per hour for the Perseids is not what you will see. This number, called the zenith hourly rate, is derived for an observer under ideal conditions with no light pollution and the radiant overhead — not conditions that occur in the Northeast.

The best place to watch any meteor shower is from a field with an open view of the sky. Sit back in a reclining lawn chair, bring along some blankets — as well as mosquito repellent — and watch the sky. Patience, perseverance and good use of peripheral vision will make for a rewarding meteor sighting experience.

Richard Monda is an astronomer living in the Capital Region.

 
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