Northern Hemisphere
For Northern Hemisphere observers, August is usually regarded as "meteor month" with one of the best displays of the year reaching its peak near midmonth. That display is, of course, the annual Perseid Meteor Shower beloved by everyone from meteor enthusiasts to summer campers. The name derives in part from the word Perseides (Περσείδες), a term found in Greek mythology referring to the sons of Perseus. The stream of debris is called the Perseid cloud and stretches along the orbit of the comet Swift-Tuttle. The cloud consists of particles ejected by the comet as it travels on its 130-year orbit. Most of the dust in the cloud today is around a thousand years old. However, there is also a relatively young filament of dust in the stream that was pulled off the comet in 1862.
But sky watchers beware: You will be facing a major obstacle in your attempt to observe this year’s Perseid performance, namely, the moon. This year, the moon will be full on Aug. 13just in time to seriously hamper, if not all but prevent, observation of the peak of the annual Perseid meteor shower, which is predicted peak overnight on Aug. 12 and 13.
Bright moonlight will flood the sky all through that entire night and will certainly play havoc with any serious attempts to observe these meteors. But there are still some chances, including three dark nights this week, for skywatchers to search for Perseids, weather permitting.
The Perseids are already around, having been active only in a very weak and scattered form since around July 17. But a noticeable upswing in Perseid activity is expected to begin during the second week of August, leading up to their peak night. The Perseids are typically fast, bright and occasionally leave persistent trains. And every once in a while a Perseid fireball will blaze up bright enough to be quite spectacular. The meteors are more than capable to attract attention even in bright moonlight.
Southern Hemisphere
With this as a background, perhaps the best times to look this year will be during the predawn hours several mornings before the night of full moon. That's when the constellation Perseus (from where the meteors get their name) will stand high in the northeast sky. In fact, three "windows" of dark skies will be available between moonset and the first light of dawn on the mornings of Aug. 9, 10 and 11.
Generally speaking, there will be about 2 1/2 hours of completely dark skies available on the morning of the Tuesday (Aug. 9). This shrinks to about 1 1/2 hours on Wednesday, and to less than a half hour by Thursday morning. The 2011 Perseid meteor shower viewing table here shows prime skywatching times for some selected US cities. All times are a.m. and are local daylight times. "Dawn" is the time when morning (astronomical) twilight begins. "Window" is the number of minutes between the time of moonset and the start of twilight.
Here's an example: When will the sky be dark and moonless for Perseid viewing on the morning of Wednesday, Aug. 10 from Houston? Answer: there will be an 89-minute period of dark skies beginning at moonset (3:49 a.m.) and continuing until dawn breaks (5:18 a.m.). Perhaps up to a dozen or so forerunners of the main Perseid display might appear to steak by within an hour's watch on these mornings.
In the absence of moonlight a single observer might see up to 100 meteors per hour on the peak night, a number that sadly cannot be hoped to be approached in 2011.
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