Annual Perseid Meteor Shower

By Scott CravenThe Republic | azcentral.comFri Aug 9, 2013 11:20 PM

If you’re prone to staring off into space, or sometimes find yourself out after midnight in a very dark place, well, this is your weekend.

The annual Perseid meteor shower has begun and will peak Sunday and Monday nights, sending about 90 fireballs per hour streaking across the sky.

With clear skies expected and the moon setting well before midnight, the stage is set for the best meteor shower of the year for Valley astronomy fans.

The Perseid shower is created by the Earth moving through the tail of Comet Swift-Tuttle, its cosmic dust pelting the atmosphere to provide an unparalleled aerial show.

Although it’s easy to imagine that each fiery trail is created by a rock the size of a basketball, the pyrotechnics actually are the result of something much smaller.

“About the size of a grain of sand,” said Bob McMillan, associate research scientist at the University of Arizona’s Lunar and Planetary Laboratory. “A few might be pea-sized, some perhaps gravel-sized.”

Particle size matters when it comes to brightness, McMillan said, but just as important is velocity. The faster the particles travel, the more dazzling they are as they collide with Earth’s atmosphere and incinerate.

Velocity varies from year to year based on the Earth’s orbit, entering the comet’s tail at a 90-degree angle or catching it from behind, McMillan said.

This year, particles will be hitting the atmosphere at about 40 miles per second. That’s 144,000 mph, on the high side of the meteoric speedometer, McMillan said, which should provide a good show.

The best time to view the shower is after midnight, when the dark side of Earth is turned directly toward the cometary stream, McMillan said. The moon sets well before midnight and won’t interfere with viewing.

But city lights will. You’ll want to be miles from urban light pollution for optimum conditions, though all is not lost for those in their suburban backyards. McMillan estimated that city-dwellers may see about 10 meteors an hour.

“If you’re sticking around city lights, be patient,” he said. “It could get pretty boring at times.”

Mother Nature is cooperating this year. A dry spell across the Southwest means odds are excellent for clear skies, said Charlotte Dewey, a National Weather Service meteorologist in Phoenix.

Scattered high clouds are a possibility, she said, but nothing likely to interfere with watching the Perseids.

Dewey even has enough faith in the forecast that she is planning to view the show herself — and far from city lights.

“It definitely looks worth it this weekend to get out of town and stay up late,” she said.

The Perseid shower will be at its most active 11 a.m. Monday, so Arizona and much of the Western Hemisphere will be in the wrong place at the right time. But there still will be plenty of activity before and after the apex, McMillan said.

“It takes a few days to pass through the tail,” he said, “so there’s ample time to catch it.”