Urban Nature

Perseid Meteor Shower

August 8, 2008

Late summer in the Bay Area often brings the first warm nights of the year, and with them a yen to be outside long after dark. Fortunately, the season also gives a perfect excuse to do just that: the Perseid meteor shower. Named for the constellation Perseus, from which the meteors appear to radiate, this display is among the most spectacular of the annual meteor showers. This year, the viewing is good August 8 to 14, and best on August 12.

This annual light show is fueled by debris from comet Swift-Tuttle, which orbits the sun every 130 years. The sun’s rays evaporate ice in the comet’s nucleus, which expels a steady stream of flotsam. Much of this material settles into the comet’s oblong orbit, creating a vast oval-shaped path of dust and ice. In mid-August, the earth plows through a segment of this oval.

Cast-off comet particles, most no bigger than grains of sand, hit our atmosphere traveling between 30 and 40 miles per second, producing a shockwave with enormous pressure and heat. It is this heat (up to 3,000 degrees Fahrenheit) that creates the bright, fiery streaks in the sky. When the particles are larger, about the size of a piece of gravel, they can “splatter” on impact with the atmosphere. The resulting fireball leaves a lingering trail, and its popping, spluttering passage across the sky can sometimes be heard from the ground.

The Perseids are best watched against a dark sky while you’re sitting in a comfortable chair. Meteor showers are most impressive after midnight, and ideally a few hours before dawn: Before midnight, we’re on the “trailing” side of the globe, and this year the moonset on August 12 isn’t till 1:30 a.m. After that, skies will be darker and we’ll be facing into the oncoming stream of particles and will generally see many more meteors. At the peak night of the shower, as the earth enters the densest part of the stream, you should be able to see 60 or more meteors per hour.

So, get away from city lights, look for Perseus rising in the northeast just before midnight, and settle in for the show!

About the Author

San Francisco native David Carroll just embarked on a six-month sojourn in the Galapagos Islands.

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