THE PERSEID METEOR SHOWERS
Dr. Terry L. Goforth
Associate Professor of Physics, SWOSU
August 2002

Looking for a new activity that will let you enjoy the great outdoors? Perhaps it's time to try the Perseid meteor shower. The Perseids are an annual meteor shower that reaches its peak on or around August 12. Warm summer nights and a good chance of clear skies make the Perseids a favorite for casual meteor watching.

Meteor showers occur when the Earth passes near the orbit of a comet. (The Perseid shower is the result of the comet Swift-Tuttle which orbits the sun once every 130 years.) A comet is often described as a dirty snowball–a large sphere of icy material with tiny specks of dusty material embedded in it. Each time the comet swings around the sun, some of the ice is lost due to the sun's heat. The dusty particles are then free to continue orbiting the sun in nearly the same orbit as the parent comet. When the Earth encounters this swarm, the particles enter our atmosphere at very high speeds (about 132,000 mph for Perseid particles). Friction between the fast-moving particles and the air heats the particles to extremely high temperatures and makes them glow, leaving a streak in the sky until the particles completely disintegrate. (Most of the particles are the size of a grain of sand when they first encounter the atmosphere.) The glowing streaks are meteors, often referred to as falling stars.

Diagram showing Earth passing through dust strands

Since the particles all have a common source, they appear to radiate from a single point in the sky. The meteor shower is named for the constellation nearest this radiant. In the case of Comet Swift-Tuttle, the radiant is in the constellation Perseus, so the resulting shower is called the Perseid Meteor Shower. This doesn't mean all the meteors will be seen in Perseus–they will appear all over the sky and will seem to be moving directly away from Perseus. To make sense of this, imagine driving on the highway in a snowstorm. In the absence of any wind, all the snowflakes seem to be coming from directly in front of you. However, you see them in front, on the sides, and behind the car.

As with any meteor shower, the best time to observe will be between midnight (1 a.m. daylight savings time) and dawn. The car-in-a-snowstorm works to explain this too. Most of the snowflakes hit the windshield, a few hit the side windows, and very few hit the rear window. As the Earth moves along its orbit around the sun, the "windshield" lies where the time is 6 a.m. (7 a.m. DST). Midnight and noon (1 a.m. and 1 p.m. DST) are the side windows, and 6 p.m. (7 p.m. DST) is the rear window. Of course we can't see the meteors in the daylight hours, so the best time to view is before dawn when we are close to the Earth's "windshield" but still in darkness.

Diagram of Earth's rotational and orbital motion relative to comet dust streams

If you're not inclined to stay up late or get up early, you may still be able to catch a good show. From dusk until about 2 a.m. (3 a.m. DST), there will be fewer meteors, but those that occur tend to be "Earthgrazers" which travel long distances parallel to the horizon. These meteors tend to be brighter and last a little longer than those overhead and often leave long streaks of light behind them.

The Perseid dust stream is quite broad–we actually entered it in late July and will take most of the month of August to pass through it. You can observe a few Perseid meteors on any night during this time. The peak period with the highest occurrence of meteors this year is on August 12, so the best show should occur during the evening hours and morning hours of August 11-August 12 and again on August 12- August 13.

As with any meteor shower, the Perseids are best observed with the naked eye–no special equipment needed. To get the most out of the shower:

If you'd like more information, check out science.nasa.gov/headlines/y2002/19jul_perseids.htm and science.nasa.gov/headlines/y2002/07aug_horseflies.htm. Enjoy the show!

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Last update:  August 8, 2002