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Clear skies for Perseid meteors

The forecast looks terrific for Friday night's debut of the annual Perseid Meteor Shower. Only the nearly-full moon will diminish the show this year.

This annual event typically produces up to 100 "shooting stars" per hour. It's not the busiest of the annual meteor showers, but it is the most popular, thanks to the mild weather and the number of people on vacation who can afford to stay up late (or get up early) to watch it.

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The fly in the ointment this year is the moon. It will be just two days past full, rising about 9:36 p.m. and staying up for the rest of the night. The resulting glare will wash out most of the dimmer meteors, although the bigger, brighter ones will still be visible.

If nothing else, it will be a great night to be out under the stars. The air should be clear and relatively dry. From a dark location, the Milky Way will be in full glory. Jupiter is gleaming - the brightest star-like object in the southern sky. (Put some binoculars on it and see if you can spot its four largest moons, lined up in a row on either side of the planet.) If you're up early, you can watch for Venus and Mercury to rise above the eastern horizon after about 4:30 a.m.

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There's also this slim, but very cool possibility: NASA is reminding meteor-watchers that in the brief time between sunset and moons rise (between, say 8:30 p.m. and 10 p.m.), we can expect to see the very first of the Perseid meteors to come into the atmosphere at very low angles - skipping over the outer atmosphere like pebbles skipping across a pond instead of plunging straight in.  These "Earth-grazing Perseids" are rare, but they can put on quite a show.

So, pick out the darkest location you can find. Set out a lawn chair or a blanket in a spot where the moon will be behind you or behind a tree or building. Relax, let your eyes adjust to the dark, and watch for the Perseids.

They will appear to be coming from the Constellation Perseus (just below the "W"-shaped constellation Cassiopeia), which rises in the northeast after sunset. (That's because that's the direction the Earth is moving along its orbit, and therefore where the atmosphere first strikes the dust particles.) But you will be able to see them zipping by almost anywhere in the sky.

What you're seeing are bits of dust scattered along the trail of the Comet Swift-Tuttle, which returns to the inner solar system once every 130 years, the last time in 1992. The size of sand grains or peas, they're circling the sun along the comet's orbital pathway. We see them because, in its annual trip around the sun, the Earth runs through the dust trail like a car through a swarm of insects. As the Earth's atmosphere encounters them, the particles plunge into the air at 36 miles per second, heating the air molecules around them, vaporizing, and producing the brief glow we see as a "shooting star."

Good hunting! Oh, and if you miss the shower's peak on Friday/Saturday, try again the next night. The activity will taper off a bit each night for about two weeks.

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