posted August 12, 2010 02:18 PM         

The annual Perseid meteor shower is already putting on an excellent show, and the celestial fireworks have yet to peak. The main event is tonight. Meanwhile, a delightfully tight configuration of planets graces the evening sky.

Rarely has there been a better time to go out, look up and enjoy easy-to-watch cosmic spectacles.

Across the Northern Hemisphere, the best time to watch the Perseid meteor shower will be tonight through the pre-dawn hours local time Friday, regardless of where you live. Weather permitting, patient skywatchers could see a shooting star every minute or so.

The Perseids are always reliable and sometimes rather spectacular. Only bad weather or bright moonlight can put a damper on the event, and this year the moon — a thin crescent that will set right after the sun — is not a factor.

Astronomers are expecting the best, and skywatchers around the globe are seeing encouraging, sometimes explosive signs. Several of the Perseids' characteristic fireballs and exploding meteors, called bolides, have been spotted.

"On Saturday night, one bolide lit up the field," said Steve Lieber of the Astronomical Society of Long Island. "Looked like a flash going off. Saw the vapor trail for 15 to 20 seconds after that."

Vapor trails are striking and sometimes colorful streams, looking like smoke, that can linger after particularly bright meteors. Most last just a few seconds.

"What has struck me so far about this year are not so much the overall number of meteors people have seen, but the number of reported fireball meteors," said SPACE.com's skywatching columnist, Joe Rao. "It seems there have been more such sightings than usual this year. Hopefully that will keep up right on through the maximum of the shower."

Meanwhile, Venus, Mars and Saturn are clustered in the evening sky and will be joined tonight and Friday by the graceful crescent moon. Anyone with clear skies can easily spot the foursome looming above the western horizon as soon as darkness falls. It's a great opportunity, using SPACE.com's planet alignment map, to find and identify planets you otherwise might mistake for stars.

But wait, there's more. In the predawn all week, Jupiter is a brilliant jewel high in the southern sky and impossible to miss — nothing nearby is even close to being as bright.

How the meteor shower works

The Perseids are bits of debris from Comet Swift-Tuttle, which has laid down several streams of debris, each in a slightly different location, over the centuries as it orbits the sun.

Every August, Earth passes through these debris streams, which spread out over time. Most meteors are the size of sand grains, with a few as large as a pea. They vaporize as they enter Earth's atmosphere, creating brilliant streaks across the sky.

Earth began entering the Perseid stream in late July. Over the past several nights, increasing numbers of shooting stars, including some dazzling fireballs, have been reported. Rates of around 20 per hour (3 per minute) were already being noted early Tuesday, according to the International Meteor Organization. That bodes well for a spectacular show at the peak.

The Perseids appear to emanate from the constellation Perseus, which rises high in the sky around midnight and is nearly overhead by dawn. As with most meteor showers, the hours between midnight and daybreak are typically the best time to watch, because that's when the side of Earth you are on is rotating into the direction of Earth's travels through space, so meteors are being "scooped up" by the atmosphere at higher rates, much like a car's windshield ends the lives of more bugs than does the rear bumper.

Astronomers expect 80 meteors per hour and perhaps even more in short bursts of up to 15 minutes or so. Rates will be much lower for those in urban areas. [Top 10 Perseid Facts]

When and how to watch

But the shower's peak, when Earth is moving through the densest part of the debris trail, favors tonight through dawn. "If skies are clear, skywatchers can expect to see dozens of meteors per hour between midnight and dawn," according to the editors of StarDate magazine.

The best location is far from city and suburban lights. Scan as much of the sky as possible. The meteors can appear anywhere, heading in any direction. If you trace their paths backward, they'll all point to the constellation Perseus. (Meteors that don't point to Perseus are not part of the shower. They're called background meteors and are the sort you can see on any August night.)

People in locations that can become chilly should dress warmer than they think necessary, to allow for prolonged viewing.

Seasoned skywatchers advise using a blanket or lounge chair for comfort, so you can lie back and look up for long periods. Allow at least 15 minutes for your eyes to fully adjust to the darkness. Then expect meteors to be sporadic: You might see two in a row, or several minutes could go by between shooting stars.

"Relax, be patient and let your eyes adapt to the dark," says Sky & Telescope editor in chief Robert Naeye. "With a little luck you'll see a shooting star every minute or so on average."

Avid meteor watchers may try scanning the northeastern horizon from 9 p.m. to 11 p.m. local time (your local time, wherever you are) today for Perseids that graze the horizon. These earthgrazers, as they are called, are rare and remarkable.

"Earthgrazers are meteors that approach from the horizon and skim the atmosphere overhead like a stone skipping across the surface of a pond," says Bill Cooke of the Meteoroid Environment Office at NASA's Marshall Space Flight Center, in Huntsville, Ala. "They are long, slow and colorful – among the most beautiful of meteors."

If you wish to spend just one stretch of time meteor-watching, set your alarm for around 3 a.m. Friday and spend an hour or two gazing skyward.

Planet spotting

While the best meteor-watching will be late night through daybreak, it's well worth going outside just after sunset tonight and Friday. Venus is so bright in the western sky you can't miss it, and a thin crescent moon will be just below it this evening. As darkness falls, Mars and Saturn will come into view. If you have hazy skies or live in an urban area, you may need binoculars to see Mars and Saturn.

These worlds — just points of light to the naked eye — shine by reflected sunlight, unlike stars, which make their own light. If there is turbulence in the atmosphere, you may notice the stars twinkling, but likely not the planets.

While the planets and our moon are all very far apart in space, they appear lined up this week thanks to a special circumstance of orbital mechanics. The outer planets, Mars and Saturn, take much longer to go around the sun than the inner planet Venus. Venus "laps" the outer planets frequently, and it never strays far from the sun from our vantage point.