Cosmic Log

Alan Boyle writes: This year's Perseid meteor shower is shaping up as a beaut. The big night is next Thursday, but anytime now is a great time for skywatching - not only to see shooting stars, but to see the planets as well.

The Perseids are among the year's best-known meteor showers, especially for mid-northern latitudes. Here's why: The show begins ramping up in late July and hits its peak around Aug. 12-13, when it's usually pleasant to hang around outdoors in the northern hemisphere. Perseid meteors appear to radiate from the constellation Perseus, which is high up in the sky at about 3:30 a.m. in northern latitudes - prime time for meteor watching.

But the big attraction comes down to how many shooting stars you can see: During this time of year, Earth plows through the trails of space grit that have been laid down by Comet Swift-Tuttle as it makes its 130-year orbit around the sun. When those particles of grit zip through the upper atmosphere, they heat up and create those bright streaks we all know and love.

Fortunately for meteor-watchers, there's a lot of grit out there.

"The whole shower, we think, is about 160,000 years old," Peter Jenniskens, a meteor astronomer at the California-based SETI Institute, told me today. "The bulk of the shower you see is 5,000 years old."

Skywatchers have tracked the Perseids for centuries. In some circles, the meteors are known as "the Tears of St. Lawrence," because the show reaches its peak around Aug. 10 - the feast day of St. Lawrence, a third-century Christian martyr. It wasn't until 1867 that scientists figured out that a comet was behind the meteoric display.

The sky conditions are nearly ideal for this year's show, because the moon will be just a few days past its new phase. When the moon is full, its glare overwhelms the meteor flashes in the night sky, making viewing problematic. But this year's crescent moon will be far below the horizon by midnight, when the meteor show enters prime time.

You'll be good to go as long as you can get away from cloudy skies and the glare of city lights. Find an open area that gives you as wide a view of the sky as possible. Lie back on a blanket or chaise lounge, and give your eyes time to get accustomed to the darkness. You might want to bring along something warm to drink, to help you stay awake. It's a lot more fun if you go out with a group. If you're on your own, you can plug in to some tunes or an audiobook as you gaze into the night. But you may want to take in the silence instead: Some people swear they can hear the sounds of meteors zooming past.

NASA

Perseid meteors appear to emanate from a point in the constellation Perseus, as shown in this graphic depicting the northeastern sky at around midnight. Although the meteors can appear in any part of the sky, their tails can be traced back to that point..

If viewing conditions are absolutely perfect, you could see a meteor every minute at the height of the shower, which generally comes around 3:30 to 5:30 a.m., depending on your latitude. Is that past your bedtime? Don't sweat it; there's actually a lot to be said for watching the skies in the early evening. During that time frame, the Perseid meteors streak at a narrow angle through the atmosphere. "You don't get as many meteors, but you get these long streaks - very nice!" Jenniskens said.

Starry Night Software via Space.com

A planetary triangle in western skies after sunset..

This year, early evening is also prime time for seeing a pretty grouping of planets: Venus, Mars and Saturn can be identified as the sparkling points of a triangle in western skies between sunset and about 10 p.m. local time. Around midnight, bright Jupiter rises in the east and starts making its way toward the zenith. And in the wee hours of the morning, North Americans can spot the International Space Station's stately procession across the sky. (Check out Heavens-Above for planetary positions and NASA's satellite sighting website for the space station's schedule.)

Jenniskens said Earth is due to pass directly through a grit trail laid down by Swift-Tuttle in the year 1479 at 16:49 GMT Aug. 12. That doesn't do North American observers any good, because it's daylight at that time. But it does mean observers in, say, Hawaii or Japan could see twice as many meteors as they would under normal conditions.

If Thursday night doesn't work for you, that's OK. The Perseids are known for having a gradual ramp-up and fade-out, so there's the potential for seeing a good show anytime in the next couple of weeks. In fact, NASA's Marshall Space Flight Center reports that a dramatic Perseid fireball, six times as bright as Venus, was sighted over Arkansas on Tuesday.

"It's a very good start to this year's Perseid meteor shower," NASA's Janet Anderson writes. Amen to that!

Here are additional online resources that help you make the most of this prime skywatching season:

• The International Meteor Organization, which keeps track of shooting-star sightings, is already reporting that some observers have spotted as many as 30 meteors an hour.
• Meteorobs, an e-mail discussion forum for skywatchers, is beginning to buzz with Perseid news. Don't miss Robert Lunsford's outlook for the week ahead.
• The American Meteor Society has a meaty guide to the Perseids. Other viewing guides are available from Space.com, Sky & Telescope, Astronomy magazine, Universe Today, StarDate Online and EarthSky.
• NASA's Fluxtimator gets Jennisken's vote as the most useful Java-based tool for figuring out how many meteors you can see on which day. Be sure to fill out the form completely with the meteor shower ("7 Perseids"), your location, viewing conditions, date (make sure to select "2010") and time zone (DST or not?).
• The Clear Sky Chart shows you where the darkest, clearest skies can be found nearby.
• SpaceWeather.com and NASA Science News are providing updated information about the buildup to the Perseids, plus a Web page that lets you "listen" to the radio echoes created by passing meteors.
• MeteorWatch is a year-old website that lets you share Perseid sighting reports via Twitter.
• Top 10 tips for meteor watchers, written by yours truly during the 2007 Perseids.
• The science behind the meteor show: Interactive graphic explains the genesis of meteor showers.