Here Come the Perseids!
August 9, 1999: The attention of the world will be riveted on the heavens this week as the last total solar eclipse of the 20th century takes place on August 11. However, for most skywatchers outside the path of totality, the best sky show won't occur until a day and a half later, on Thursday, August 12, and Friday, August 13. That's when the annual Perseid meteor shower reaches its peak with 50 to 150 shooting stars per hour.
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.
This year should be a good one for viewing Perseids. The shower's maximum takes place on August 12, 1999, under dark skies just one day after the new moon. Although the Perseids officially begin in late July, the shower builds rather slowly in intensity until there is a series of sharp peaks in activity near mid-August.
The figure below shows the expected activity profile for 1999 based on observations in 1997. The shower's maximum is centered around 21 UT (2 p.m. PDT) on Thursday, August 12, 1999. The level of activity remains relatively high for nearly 24 hours afterward, so the early morning hours before dawn on Thursday, August 12 and on Friday, August 13 should be good times to observe in most places [click for more observing tips]. The other two times indicated in the figure (0700 UT on August 12 and August 13) are launch windows for the Science@NASA Perseids Live! high altitude balloon flight which will transmit a live webcast of the meteor shower from the stratosphere.
Left: 1997 Perseids activity based on visual records from The International Meteor Organization. The vertical axis is the "zenithal hourly rate" of visual meteors, or the hourly rate of meteors an observer would witness under ideal conditions with the meteors appearing directly overhead. The horizontal axis is the solar longitude of Earth, and may also be regarded as time increasing from left to right. If the activity profile in 1999 is similar to 1997, then maximum activity will occur around 2100 UT on August 12, 1999
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.
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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.
Left: Comet Swift-Tuttle, shown here in false color, is the largest object known to make repeated passes near the Earth. It is also one of the oldest known periodic comets with sightings spanning two millennia. Last seen in 1862, its reappearance in 1992 was not spectacular, but the comet did become bright enough to see from many locations with binoculars. To create this composite telescopic image, four separate exposures have been combined, compensating for the motion of the comet. As a result, the stars appear slightly trailed. The inset shows details of the central coma. The unseen nucleus itself is essentially a chunk of dirty ice about ten kilometers in diameter. Credit: D. McDavid (Limber Observatory), D.C. Boice (SwRI). [more information].
Perseid observing tips
Perseid 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.
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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.
The sky map below represents a view of the sky looking northeast from a mid-latitude viewing site at 3:00 a.m. (local time wherever you live). The radiant, in the constellation Perseus, is located almost midway between Jupiter in the east and Polaris in the north. Jupiter (magnitude -2.6) and Saturn (magnitude +0.3) will be very bright and easy to spot.
The image indicates the general region of the sky from which the Perseid meteors appear to emanate (red dot). This point, called the radiant, is really an optical illusion - the meteors are moving along parallel paths, but appear to come from a single point, just as a stretch of parallel railroad tracks will appear to meet at a point on the horizon.