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Hal Kibbey

Last modified: Tuesday, August 2, 2011


Aug. 2, 2011

BLOOMINGTON, Ind.-- The annual Perseid meteor shower of August is one of the most popular every year because it happens on warm summer nights, when gazing at the starry sky is always enjoyable. In a clear, dark sky there may be as many as 100 bright meteors per hour, some with smoke trails that last several seconds after the meteor has vanished.


Photo courtesy of NASA

This year, however, the full moon will interfere with viewing the Perseids during their peak on the night of Aug. 12-13. Bright moonlight will wash out the fainter meteors, and instead of 100 per hour, observers may see only about 20. Blocking the moon with a building or tree will reduce the glare.

The Perseids will be visible for most of August, though there will be fewer meteors to see the further from the peak date you watch. If the peak on Aug. 12-13 is hidden by clouds, for example, try looking for meteors again as soon as the night sky is clear.

To minimize the effect of local light pollution, which can obscure as many as half of the meteors, try to avoid artificial lights. Face east if you have a clear view in that direction, and look about half way up the sky from the horizon. You won't need binoculars or a telescope because the meteors move much too fast for those. The chances of seeing a fireball will be greatest near dawn, when Earth will be moving head on into the meteor stream.

The Perseids may appear anywhere in the sky, but they will seem to originate from a point called the radiant in the constellation Perseus, which gives the meteors their name. The higher the radiant is above the northeastern horizon, the more meteors will be visible. Perseus is just north of the W shaped constellation Cassiopeia in the Milky Way, with the bright stars Capella and Aldebaran and the Pleiades star cluster below it. Meteors near the radiant will have short trails because we see them nearly end on, while those far from the radiant will look longer because they are seen from the side.

A computer simulation of meteors streaking from the Perseid shower's radiant can be seen at

Most meteor showers happen when Earth crosses the orbit of a comet; the Perseids come from Comet Swift Tuttle. The meteors are caused by particles released from the comet's nucleus and left behind in space. As Earth plows through this stream of debris, ranging in size from sand grains to pebbles, each particle slams into our atmosphere at a speed of more than 50 kilometers per second and burns up almost instantly from friction with air molecules. The resulting heat momentarily creates a streak of glowing air that we see as a meteor (sometimes called a "shooting star" or "falling star"). All of this happens about 50 miles above the ground, regardless of how close some meteors may appear.

More information about the Perseids and other meteor showers is available at


At the beginning of August, Saturn will be fairly low in the west-southwest an hour after sunset. It will be only about a quarter as high by month's end. Any telescope will show Saturn's biggest moon, Titan, which will be south of Saturn on Aug. 7 and 23, and north of the ringed planet on Aug. 15 and 31.

Jupiter will rise around midnight local time in early August and two hours earlier at the end of the month. Wait until the planet is reasonably high above the eastern horizon, when it will be a splendid sight in a telescope. Jupiter will outshine everything except the moon as it moves through the constellation Aries the Ram.

Mars will rise around 3 a.m. local time and cross the constellation Gemini the Twins during August.

Mercury will pass between Earth and the sun on Aug. 17, when it will be invisible in the solar glare. It will reappear in the morning sky on Aug. 27, though it will be difficult to find until it gets higher. On Aug. 31, the planet will appear 10 degrees above the eastern horizon a half hour before sunrise.

Venus will be out of sight during August, passing behind the sun on Aug. 16.

Light pollution

If you look at the constellation Cassiopeia on a clear summer night, and you can't see the Milky Way sprawling high across the sky from the northern to the southern horizon, then your sky has significant light pollution, which is the case for about two thirds of the world's population. See for information on this dimming of the night sky caused by excessive artificial lighting, much of which is wasted.

Moon phases

The moon will be at first quarter on Aug. 6, full on Aug. 13, at third quarter on Aug. 21 and new on Aug. 28.