In 868 AD, a yet undiscovered comet crossed the Earth's orbit on the inside for the first time. The comet's orbit had been gradually changing in previous centuries. Shortly thereafter, in 902 AD, Chinese astronomers reported seeing the first Leonid storm. Numerous accounts would follow in the next centuries: "Stars fell like rain".
In the Americas observers are startled by many meteors. The reknowed German scientist Humboldt and companion Bonpland, who are in Cumana (Venezuela) at the time, record the event and make it widely known in the scientific community. Rumour has it that in 1766 a similar meteor storm was seen over Cumana.
"Tausende von Feuerkugeln und Sternschnuppen fielen hintereinander, vier Stunden lang. (…) Nach Bonplands Aussage war gleich zu Anfang der Erscheinung kein Stück am Himmel so groß als drei Mond Durchmesser das nicht jeden Augenblick von Feuerkugeln und Sternschnuppen gewimmelt hätte. Von 4h an hörte die Erscheinung allmählich auf; …" - Humboldt
In 1833, observers are somewhat familiar with Leonid storms. The storm that year is very intense and the event leads to the first formulation of a theory on the origin of meteors. "On the night of November 12-13, 1833, a tempest of falling stars broke over the Earth… The sky was scored in every direction with shining tracks and illuminated with majestic fireballs. At Boston, the frequency of meteors was estimated to be about half that of flakes of snow in an average snowstorm. Their numbers… were quite beyond counting; but as it waned, a reckoning was attempted, from which it was computed, on the basis of that much-diminished rate, that 240,000 must have been visible during the nine hours they continued to fall."
- Agnes Clerke's, Victorian Astronomy Writer
Ernst Tempel and Horace Tuttle independently discover a dim comet. After observing the comet for several weeks, an orbit was calculated. It was found that the orbit was of short period, 33.17 year. In November that year, a Leonid storm was anticipated. The sheer number of meteors startled observers in Europe, who scrambled to count the numerous meteors and determine the radiant position. An orbit was calculated for the meteoroids assuming a period of 33 years, and the similarity with the comet orbit was discovered.
It was now well established that the comet and the meteors returned to Earth every 33 years. The return of 1899 was eagerly awaited. That year, many meteors crossed the sky. Strong activity continued until 1902, with rates increasing to storm conditions in 1901. The sharp main peak of the Leonids was not observed. The comet also did not show itself again.
In 1932 rates went up again, but a big storm was not observed. It seems that the storm was simply missed by the more scientifically oriented observers because of bad weather. Such is not uncommon in the northern hemisphere in the middle of November.
Lost for nearly a century, the comet P/Tempel-Tuttle was rediscovered in 1965. Calculations would later reveal that this comet passed closer to the Earth's orbit (0.0032 a.u.) than on any occasion since 1833. On November 17th, 1966, a tremendous storm of tens of thousands of Leonids fell for a short interval timed by skywatchers in the central and western United States. This display probably rivaled the historic showers of 1799 and 1833. Within just two hours, observed rates increased from about 40 per hour to as much as 40 per second!
"We saw a rain of meteors turn into a hail of meteors too numerous to count," - Charles Capen in the San Gabriel Mountains of Southern California
"The meteors were so intense that we were guessing how many could be seen in a one-second sweep of the observers head… A rate of about 150,000 per hour was seen for about 20 minutes." - Dennis Milon, Kitt Peak in Southern Arizona
The first increase of Leonid rates announcing the return of the comet wasreported in 1994. On November 18 that year, the shower was as strong as the Perseids in August. The outburst lasted a little over a day and was rich in bright meteors. Observations of the Leonids in 1995 and 1996 confirmed the enhanced rates. The comet was finally recovered on March 10, 1997, following an accurate prediction of its orbit, and is expected to pass perihelion on February 28, 1998.
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