Here come the Perseids!The 1999 Perseid meteor shower peaks on August 12th and 13th under the dark skies of a nearly new moon. |
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Right: A Perseid meteor from 1993. The colors are representative but digitally enhanced. As the meteor streaked across the night sky, different excited atoms emitted different colors of light. The origin of the green tinge visible at the right is currently unknown, however, and might result from oxygen in Earth's atmosphere. Credit & Copyright: S. Kohle & B. Koch (Astron. I., U. Bonn) [more information] In recent years the spectacular
Leonid meteors have attracted considerable attention, but historically
the Perseids are the best known of all meteor showers. It rarely fails to
provide a pleasing display and, because of its summertime appearance, it
tends to attract many astronomy novices.
Like most meteor showers, the Perseids are caused by comet debris. As comets enter the inner solar system, they are warmed by the sun and peppered by the solar wind, which produces the familar tails that stretch across the night sky when a bright comet is close to Earth. Comet tails are made of tiny pieces of ice, dust, and rock which are spewed into interplanetary space as they bubble off the comet's nucleus. When Earth encounters these particles on its journey around the Sun, they strike the atmosphere speeds exceeding 100,000 mph. (The average speed of Perseid meteoroids is 130,000 mph!) Most are observed as a bright streak across the sky that can last for several seconds, but occasionally a large fragment will explode in a multicolored fireball. Most of the streaks (popularly called 'shooting stars') are caused by meteoroids about the size of a grain of sand, but much less dense. Although they travel at high speeds, these tiny meteoroids pose no threat to people or objects on the ground. The Perseids were the first meteors ever associated with a particular comet. From 1861 to 1863, observers noted a great increase in the number of August Perseids. As many as 215 per hour were seen in 1863. The Italian astronomer Giovanni Virginio Schiaparelli (better known for giving the name "canali," or "channels," to the dark linear markings on Mars) calculated the orbits of some Perseid meteoroids and discovered that they closely matched that of periodic comet Swift-Tuttle, which had been discovered in 1862 during its close approach to Earth. Swift-Tuttle orbits the Sun once every 135 years. The last time it passed near Earth was in December 1992. The proximity of the comet once again caused an increase in Perseid activity and, in August 1993, observers in Central Europe were treated to 200 to 500 meteors per hour. Swift-Tuttle won't make another swing through the inner solar system until 2126, but when it does the comet itself is expected to be an impressive sight as seen from Earth, rivalling Comet Hyakutake in 1996 or Comet Hale-Bopp in 1997.
Perseid observing tipsPerseid meteors can be seen anytime after the
sun has set and the constellation Perseus is above the horizon (which is
nearly all the time from observing sites in the northern hemisphere). In
practice, watching for meteors between sunset and midnight is rarely
profitable. The best time to look is between about 2 a.m. and dawn. That's
when the local sky is pointing directly into the meteoroid debris stream (see
the diagram below). The early morning hours of August 12 and August 13 should
be good times to watch if you live in the northern hemisphere.
Above:The rate of meteor activity is usually greatest near dawn because the earth's orbital motion is in the direction of the dawn terminator. Earth scoops up meteoroids on the dawn side of the planet and outruns them on the dusk side.
For northern hemisphere observers at
latitudes higher than about 35 degrees Perseus is circumpolar -- it is always
above the horizon. Unfortunately, due to the high declination of the radiant,
the Perseids are not a good shower for skywatchers south of the equator.
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Experienced
observers suggest the following viewing strategy: Bring a reclining chair, or
spread a thick blanket over a flat spot of ground. Lie down and look up
somewhat toward the north. You don't need to stare directly at the radiant --
the meteors can appear anywhere in the sky. Their trails will tend to point
back toward the radiant, pictured as a red dot in the sky map below.
Binoculars and telescopes are not essential. The naked eye is usually best
for seeing meteors which often streak more than 45 degrees across the sky.
The field of view of most binoculars and telescopes is simply too narrow for
good meteor observations. 
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