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Weather looks good for tonight's meteor shower

High pressure and dry air out of Canada is the recipe we want for viewing tonight's Perseid meteor shower. Too often in the Chesapeake region we get nominally clear skies for this reliable annual event, but high humidity still washes out much of the display.

Not this time. Forecasters out at Sterling say the upper-level low responsible for yesterday's clouds and storms is pulling away off the Jersey coast. It's being replaced by all this terrific cool, dry air. It's still just 73 degrees at The Sun as I write, up from an overnight low of 63 degrees. The airport dipped to 58 degrees overnight. This seems to be the mid-August break in the weather we've been waiting for. And it's here just in time for the Perseid shower.

That's not to say things are perfect this year. For the early part of the night we still must contend with the glare of the moon, now just four days short of full. It won't set until 1.47 a.m. in Baltimore.

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Still, the skies should be clear, and the brightest meteors should begin to be visible after Perseus - the constellation from which the meteors seem to emerge - rises well above the northeastern horizon around 11 or 12 midnight tonight.

The best time to look will be between moonset and dawn tomorrow morning. Here's a nifty photo gallery of last year's Perseids.

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The Perseid shower occurs each year as the Earth, in its annual trip around the sun, crosses the dusty trail of the Comet Swift-Tuttle. The comet itself, on its 130-year orbit around the sun, is cruising somewhere out near the orbit of Uranus this year. But the dust it leaves behind is still orbiting all along the comet's path. And this is the night when the Earth crosses the densest portion of that trail.

As our planet smacks into those dust  grains and pebbles, they streak into the thin air at the top of the atmosphere at 37 miles per second, heating the air and making it glow until the dust is vaporized. We see it as a bright, fleeting trail across a portion of the sky.

The Perseids are remarkably reliable, producing as many as 60 meteors an hour at their peak. And because mid-summer is a pleasant time to be out under the stars, this is probably the most-watched annual meteor shower of the year - although it is not usually the best.

The first meteors to show up will be the so-called Earth-grazers. These are the meteors that streak across the sky late in the evening before the shower's early-morning peak. They're skimming the top of the atmosphere from east to west just as Perseus is rising in the northeast.

Later, as Perseus rises higher in the sky, the Perseid meteors seem to radiate in all directions from the constellation. They're not really coming from the constellation itself, of course. That's just the direction in which the Earth is traveling at this time of year. Perspective makes the meteors appear to be flying toward us, like snowflakes in the headlights of a moving car.

The bottom line here is that you can really look in any direction for these meteors. Although you may be able to trace the paths of many of them - the true Perseids - back to the constellation. They will appear almost anywhere in the sky. Until the moon sets, however, it may be best to watch with your back to the moon's glare. (If you see some that appear NOT to fly out of Perseus, they are probably "erratics" bits of dust and debris unrelated to Swift-Tuttle.

Once the moon has set, the added darkness should reveal more of the fainter meteors.

So, find a place with a dark sky, away from urban lighting. There are some suggestions for dark-sky locations at the end of the article at this link.

Take a blanket or a sleeping bag. We're looking for lows in the 50s tonight. Stretch out on the ground, or in a reclining lounge chair of some kind, and just watch the sky. Allow 10 or 15 minutes for your eyes to adjust to the dark.

But beware. Meteor-watching can be addictive, and will definitely cost you sleep!  Be sure to stop back here in the morning and share your experience with all the sleepyheads who passed it up.

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