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Predictions are that the annual Leonid meteor shower will be a "really big show" this year, peaking before dawn on the mornings of Nov. 17 and 18.
In most years, this is a minor shower producing about 10 meteors per hour. A meteor shower is considered to be a storm when it reaches a rate of one meteor per second or 3,600 meteors per hour. And every 33 years or so, the Leonids are capable of creating such a deluge.
The Leonids produced a magnificent storm on the nights of Nov. 12-13, 1833, when thousands of bright meteors were seen in the eastern U.S. for more than an hour. Until then, most astronomers had believed that meteors were an atmospheric oddity having no connection with the rest of the solar system. But the Leonid display generated intense interest and created a new branch of astronomy.
The last Leonid storm was in 1966, though it was not visible in most of the U. S. Astronomers are paying careful attention this year and next in an effort to predict the next meteor show. The odds favor 1998 and 1999, but the Leonids have defied prediction before, and there is no way to tell for certain which part of the planet will get to see the peak when it does happen.
In 1996, in a moonless sky, the peak rate of the Leonid shower rose to about 50 meteors per hour. Last year, it reached 80. So this year, there should be a fine meteor shower even if there is no meteor storm. Lesser numbers of Leonids will be visible for about a week around the dates of the peak.
If the weather provides a clear sky, viewing conditions will be ideal Nov. 17-18. The moon will be new, so moonlight will not interfere in the early morning hours when the constellation Leo the Lion rises in the East. This is when Earth will cross the orbit of Comet Tempel-Tuttle, the comet that provides the particles from which the Leonid meteors actually come. The meteor shower therefore seems to come from a point in Leo, which gives the Leonids their name.
Most meteor showers happen when Earth crosses the orbit of a comet. The meteors are caused by tiny particles of dust released from the comet's nucleus and left behind in space as the comet gets closer to the sun. As Earth plows through this stream of particles, each one collides with Earth's upper atmosphere at tremendous speed and burns up from friction with air molecules. The resulting heat momentarily creates a streak of glowing air molecules called a meteor or sometimes a shooting or falling star.
Comet Tempel-Tuttle passed Earth's orbit in March of this year, creating the possibility of thousands of meteors and fireballs. Any meteor storm will last no more than a few hours and perhaps less than one hour. Only observers in the region of the planet facing into the particle stream, and also in darkness, will be able to see the storm when it hits. This year, the best chance seems to be for eastern Asia, but if Earth passes through the particle stream several hours early or late, North America and Europe could rotate into the favored position.
Watch for meteors after midnight Nov. 17-18. By then, the shower's radiant point in Leo will be fairly high in the East. The higher a shower's radiant point is above the horizon, the more meteors appear all over the sky. The hour or two before morning twilight should be best.
Light pollution wipes out many meteors for observers, so choose a dark site with an open view of as much of the sky as possible. Jupiter will still be the brightest "star" of the evening during November, appearing high in the South at sunset and setting well after midnight.
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