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They return year after year. For many of us, they spell holidays and warm summer nights. But who — or what — are they? The Perseid meteors, of course!

Observers will be sorry to learn that, in 2009, the Perseid meteor shower will be mostly washed out by the presence of the last quarter moon. The moon rises shortly after astronomical twilight ends, and it will be high up in the sky during the last part of the night. The "dark sky" observing window, necessary to appreciate the Persids to their fullest, will be extremely short. All in all, the observing conditions for this year's Perseid display will be rather poor.

This year, peak activity for the Perseids is expected around 2:00 p.m. Eastern Time on the afternoon of August 12. This means that, regardless of other limiting factors, observers in eastern North America are guaranteed to miss out on the few hours where the Perseid meteor shower reaches its greatest intensity. The best observing windows will occur during a one hour period after the end of astronomical twiligh, around 9:30 p.m., but just before moonrise (around 10 p.m. on August 11, 10:30 p.m. on the 12.) During that time period, one shouldn't expect to see more that 15 meteors per hour from moderately dark sites. During the last part of the night, the moon will be high up in the sky and drown all but the very brightest meteors — too bad, really, because this is also the time when the Perseids are usually more active, because the radiant (the area of the sky, located near the constellation Perseus, from which the Perseids seem to originate) is highest in the sky.

To sum up, the best evenings for observing the Perseids will be on August 11 and 12, between 9:30 and 10:30 p.m. (after the end of twilight, but before moonrise.) You may also be able to catch a few Perseids — although in lesser numbers — outside the peak period that ranges from August 10 to 13: indeed, Perseids are active from the end of July through the third week of August.

Have your wish-list ready, and enjoy the Perseids!