BLOOMINGTON, Ind. -- The annual Perseid meteor shower, which will peak the night of Aug. 11-12, is one of the most popular every year because it happens on warm summer nights, when gazing at the starry sky is always enjoyable. Unfortunately, this year the moon will interfere with the shower's peak for mid-northern observers.
In a dark sky the shower may produce as many as 150 meteors per hour, but this time moonlight will wash out the fainter ones. Still, 15 to 30 of the brightest meteors should be visible each hour, some with smoke trails that will last several seconds after the meteor has vanished.
The Perseids will be visible for most of August, though there will be fewer meteors to see the farther from the peak date you watch. If the peak is hidden by clouds, 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 halfway up the sky from the horizon. You won't need binoculars or a telescope because the meteors move much too fast for those devices. 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 these "shooting stars" 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 star Capella and the Pleiades star cluster below it. Meteors near the radiant will have short trails because we see them nearly straight on, while those far from the radiant will look longer because they are seen from the side.
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 30 miles 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. All of this happens about 60 miles above the ground, regardless of how close some meteors may appear.
As darkness falls during August, Jupiter will appear low in the west-southwest, setting in late evening. Nearby to its left (south) will be the bright white star Spica.
Saturn will be highest in the south in late twilight this month. The planet's famous rings will tilt more than 26 degrees to our line of sight. To its right (west) will be the bright red-orange star Antares.
Venus will rise about three hours before the sun in August, long before the first light of dawn. It will reach its greatest altitude on Aug. 2 for observers at mid-northern latitudes, when it will be about 30 degrees above the horizon.
Mars and Mercury will be too close to the sun for viewing in August.
A total eclipse of the sun will occur Aug. 21, visible in North America in a narrow path from Oregon to South Carolina. All those in the Americas north of the equator will see at least a partial eclipse. Detailed information and maps of the eclipse's exact route can be found at the NASA eclipse website.
If you look at the constellation Cassiopeia in the northeast on a clear summer night and can't see the Milky Way sprawling high across the sky from the northern to the southern horizon, it means your sky has significant light pollution, which is the case for about two-thirds of the world's population. This dimming of the night sky is caused by excessive artificial lighting, much of which is wasted. See the International Dark-Sky Association website for more information.
The moon will be full on Aug. 7, at third quarter on Aug. 14, new on Aug. 21 and at first quarter on Aug. 29.