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THE LEONIDS: King of the Meteor Showers!
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LATE EVERY AUTUMN around November 18th, in the still, silent hours before dawn, dedicated meteor watchers have long kept a vigil. In the cold, early morning darkness the sky glimmers with a preview of early spring constellations. Leading them up in the east is Leo. Its familiar Sickle asterism, a backwards question mark, seems especially full of meaning these nights, for on the cutting edge of the Sickle's curved hook is the emanation point of the greatest of all showers. Most of the time nothing happens. Only occasionally does the watcher, lying in blankets against the cold, glimpse a meteor somewhere in the sky whose flight direction betrays its invisible origin in the Sickle. Perhaps 8 or 10 Leonids per hour make it onto the watcher's clipboard. As the first hint of dawn appears he checks the time, makes one final note, and gets to his feet, pleased with the night's gleanings of data but saying as always, "Not this year."
But in 2006, there is a "chance" of catching a brief burst of Leonid activity, with hourly rates perhaps 10 to 15 times greater than normal. This enhanced activity might be seen from western sections of Africa and Europe, as well as eastern Canada, Bermuda and the northeastern U.S.
The Leonid meteors at their peak are capable of producing the grandest display of celestial fireworks known to man. In most years, only a scattered few Leonids can be seen. Normally that is all there is to the Leonid meteor shower. But every third of a century, for four or five Novembers in a row, there is a chance that the Leonids may fall by the hundreds, or even the thousands per hour. Just what will happen in those years, no one can say. It may be nothing, or it may be everything.
HISTORY: The Leonids historically are active between November 14th and 21st. The source of the Leonids was first identified in 1866 by the Italian astronomer, Giovanni Schiaparelli (of Martian canali fame). Other experts in celestial mechanics, notably Urbain Le Verrier and Theodor von Oppolzer, independently spotted a striking resemblance of the orbit of the Leonids to that of the then-newly discovered Periodic Comet Tempel-Tuttle. This comet appears to orbit the Sun at intervals of roughly 33 years. Today, even though not all prominent meteor showers have been matched to known comets, the relationship is clear: meteor streams are the debris of crumbling comets.
The Leonids are the swiftest of all shower meteors, at 44 miles per second -- almost the highest theoretical speed for meteors belonging to the solar system -- because their orbit runs nearly head-on to the Earth's. No Leonid has been known to escape entire vaporization and reach the ground as a meteorite. Many are bright, some are fireballs; greenish or bluish; about two out of three leave trains, which can sometimes persist for five minutes or more! They first appear at altitudes of between 96 and 61 miles up; they belong to Zdenek Ceplecha's meteor structure classification above C2 (meaning that the Leonids apparently originate from the snow-and-dust surface of the Tempel-Tuttle Comet and likely have the consistancy of cigar ash).
Apparently, it is in those years leading up to, and just after the passage of this comet through the inner solar system, that spectacular Leonid showers are possible. Records indicate great Leonid displays as far back as AD 902 (Arabic "Year of the Stars," night of the death of Ibrahim II, Aghiabid emir of Tunisia and Sicily); 931 (first Chinese record of the shower); 934; 967 (first Japanese record); 1037; 1202; 1366; 1532 (first Korean record); 1533; 1799 (seen by Alexander von Humboldt when he landed in Venezuela to begin his South American explorations); 1832; 1833 (estimates of 240,000 over a nine-hour interval over North America, forced the recognition of the interplanetary, not "meteorological" nature of meteors); 1866 (8,000/hr in Europe, the greatest shower ever observed there, and the first linked with the comet); 1867; 1868; 1869. Then, the comet and the associated Leonid swarm were perturbed by Jupiter away from the Earth. The displays of 1899, 1932 and 1933 were for the most part, disappointing and it was felt that the Leonids had matured into an even and thin meteor stream. But in 1961 the Leonids began to revive; a strong shower was observed in 1965 over Hawaii and Australia; then on November 17, 1966, over central and western sections of North America meteor rates of up to 40 per second were noted (144,000 per hour!).
Comet Tempel-Tuttle returned to the inner solar system in 1998. In that year, the Leonids roared to life, producing 340/hr -- many were brilliant fireballs. In 1999, for about an hour, Leonids fell at rates of nearly 4,000/hr as seen from Europe and the Middle East. In 2000, despite the bugaboo of bright moonlight, rich displays, but no meteor storms were observed. What we got were peak zenithal hourly rates (ZHRs) of 290 over Europe and 480 over North America. In 2001, Leonid storms of 1,600 to 2,500/hr. were observed over both North America and the Far East, and in 2002, storms of 2,400 to 2,700/hr. were seen from Europe and North America.
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H.L. Trouvelot accumulated a night's worth of meteors in one painting after monitoring the Leonids on November 13-14, 1868. Two years after the parent comet (Tempel-Tuttle) had passed through the inner solar system, the Leonids were still putting on a very respectable show.
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WHAT TO EXPECT: Work by astronomers Robert McNaught (Australia) and David Asher (Ireland) have been quite successful in forecasting Leonid activity since 1999. Together with an independent prediction made by Jeremie Vaubaillon (France), all now suggest a brief Leonid outburst in 2006. Predicting exactly just how any meteor display will "perform" can be rather difficult. As ever, we must wait and see -- but we can always hope too!
THE FORECAST FOR 2006: There is a chance for a short-lived outburst timed for skywatchers in Western Europe and perhaps eastern North America. Back in 1999, McNaught and Asher published a technical paper that included a forecast that Earth would encounter a trail of dust ejected by Comet Tempel-Tuttle from 1932. The Earth had previously passed through this same trail of dust back in 1969, when Leonids fell at rates of 2 to 4 per minute for about half an hour. But in 2006, activity is predicted to be only half as strong, so rates of 1 or 2 per minute may be rule. Maximum activity shoud last only half an hour, centered on 11:45 p.m. Eastern Standard Time on Saturday, November 18th. Unfortunately for the central and western U.S., this will be before the constellation Leo has risen, so the outburst will not be visible there. Another caveat will be that the majority of the 2006 Leonids are forecast to be rather faint and difficult to see.
However, observers should be on guard for any renegade fireballs or "Earth-grazing meteors" which will streak from low out of the east-northeast sky on very long, majestic paths across the greater part of the sky. Seeing even just one of these spectacular meteors will make a cold evening of meteor observing worthwhile!
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"LEONID METEOR STORM" by Shigemi Numazawa
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H O W T O W A T C H
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The golden rule of meteor observing is to be comfortable. Take the advice of someone whose teeth have chattered on many a late-autumn night -- wrap up much more warmly than you think is necessary! A sleeping bag placed atop a lawn chair is ideal for comfort and a thermos mug of hot coffee, tea or soup will keep you awake and warm.
Find a clear, dark place with a view of as much of the sky as possible. Don't stare at any one place but keep your eyes moving all over the sky. Begin your watch by 11 p.m. and carry on until the first light of dawn. The meteors will be radiating from the Sickle of Leo (the above chart is good for 12 to 2 a.m.) which will be rising out of the east-northeast sky. Keep alert for any brilliant fireballs or bolides (exploding meteors). Optical aid is not necessary for observing the Leonid shower, but a pair of binoculars sometimes comes in handy. Some Leonid trains remain visible for several minutes or more. If you follow a train in binoculars, time how long it remains visible and note any changes you see in its shape.
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METEOR/LEONID/ASTRONOMY LINKS
METEOROBS (Ongoing discussions by meteor observers)
SKYWAY, INC (Service highlights upcoming celestial events)
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LISTEN TO THE NASA METEOR RADAR! CLICK ON RADARDOME
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