The Leonid meteor shower is back this month and poised to hit its peak next week. But there's a long history associated with the annual skywatching event.
It all began on the night of Nov. 12, 1833, when the Western Hemisphere unexpectedly came under attack by a firestorm of shooting stars that were reportedly silent, but overwhelming filled the sky.
During this historic display, which was seen under clear skies across the eastern United States, an estimated 240,000 meteors were observed. So heavy was the concentration of meteors that to those gazing skyward it was visually obvious that they were fanning out from a spot within the star pattern known as the Sickle in the constellation of Leo, the Lion. [Top 10 Leonid Meteor Shower Facts]
Following this realization, the meteors were given the Latin family name for their apparent place of origin: the Leonid meteor shower.
Leonid meteor shower is born
This spectacular 1833 Leonid meteor storm made a deep and terrifying impression on the American people.
According to newspaper reports almost everyone saw it, awakened either by the commotion in the streets or by the moving glare of fireballs shining into bedroom windows.
This point of emanation of the meteors (called the "radiant") was in the same place for all observers and remained so as the night wore on and the sky turned. Here was proof that the meteors were traveling parallel to each other from somewhere outside of our atmosphere.
Up until only some years earlier, astronomers had refused to believe that meteors — those streaks of light so commonly seen in the upper atmosphere — could have any astronomical connection at all.
This remarkable finding, that meteors are visitors from astronomical realms, was striking in its own way as the shower itself. It sparked intense study into this new field of astronomy.
After 1833, many astronomers researched the history of the Leonids in ancient European, Arab and Chinese documents. In 1837, the German physician and astronomer Heinrich Olbers suggested that better-than-average displays occurred in cycles of 33 or 34 years.
Other accounts subsequently came to light. In 1799, Alexander von Humboldt — the great German naturalist and explorer — watched a stupendous display of brilliant fireballs during his explorations in Venezuela.
"There was not a space in the heavens equal to twice the moon's diameter which was not filled every instant by shooting stars," Humboldt wrote.
The Leonids had apparently been observed for almost 1,000 years. [Gallery: Spectacular Leonid Meteor Shower Photos]
Particularly impressive displays were found to have taken place in the years of 1533, 1366, 1202, 1037, 967 and 934. Arab historical accounts have called the year of 902 A.D. the "Year of the Stars," as Leonid meteors lit up the night sky during which Ibrahim, king of Tunisia and Sicily, lay dying.
The Leonids comet connection
After astronomers began studying the Leonid meteor shower, they ultimately traced its origins to a so-called dense "knot" of matter that revolved around the sun in a period that, in 1866, was determined at 33.25 years. That same year, the likely source of meteor streams was established by Italian astronomer Giovanni Schiaparelli: The Leonids came from a comet.
Schiaparelli's Leonids origins discovery came after he established that the orbit of another famous shower — the August Perseids — closely matched that of Periodic Comet Swift-Tuttle. In that same work, Schiaparelli published his calculations for the orbit of the Leonid stream.
Other experts in celestial mechanics noticed a striking resemblance of the Leonid orbit to that of the newly discovered Periodic Comet Tempel-Tuttle. This relatively small comet is in an orbit that at its closest point to the sun almost coincides with the Earth's orbit and also moves through space in a direction opposite to Earth.
So when we meet its dusty trail in mid-November these particles collide with us at the maximum possible speed — 45 miles (72 kilometers) per second. From our perspective on Earth, the meteors come at us from the direction of the constellation of Leo, the lion, which in mid-November appears dead ahead of us in our path around the sun.
This means that we have to be on the forward side of the Earth to see them coming — that is, we must be up during the hours between midnight and dawn. Leonids tend to be quite bright and are tinged with green or blue because of their great speeds.
About half of the Leonids create bright, luminous trails — in the most extreme cases, hanging in the air for eight or even 10 minutes. The meteors begin to flow when they are still nearly 100 miles (160 km) high because they, like the Orionid meteors of October, are thin flakes from the nucleus of their parent comet. So the relationship became clear: meteor streams are the debris of crumbling comets.
Jupiter, the spoiler
After Schiaparelli's 1866 discovery, the Leonid meteor storms returned on schedule that same year and in 1867, though not nearly as abundant as what had been seen in 1833. The meteor rates reported in those years were about to be 5,000 per hour from Europe in 1866, and about 1,000 per hour from North America in 1867.
Another great shower was confidently expected for 1899 and fairly wide publicity was given to the possibility of a re-enactment of the events of 1867 and especially 1833.
But instead, the anticipated Leonids spectacular failed to materialize. It was later determined that their orbit had been significantly perturbed by Jupiter — as happens to most comets and meteors sooner or later — or perhaps, as some suggested, the meteors in the Leonid stream were becoming more evenly and thinly spread along their orbit.
The once great Leonids had seemingly become just a minor stream. Even worse, Comet Tempel-Tuttle, the source of the meteor debris, failed to be seen both in 1899 and 1932 and was presumed to be lost.
But this was not so.
They're back ...
By 1961, the Leonid meteor shower began to revive, unexpectedly reaching rates of up to 50 per hour. Then in 1965, Comet Tempel-Tuttle, lost for nearly a century, was at long last rediscovered.
Observers that same year in Hawaii and Australia witnessed Leonid rates of 120 per hour, many of these appearing as spectacular fireballs. Then one year later, on Nov. 17, 1966, a tremendous storm of tens of thousands of Leonids fell for a short interval timed for skywatchers in the central and western United States.
The rate was estimated by some as 40 per second!
That works out to 144,000 per hour. It was a veritable storm of Leonid meteors, the greatest meteor display ever seen at any rate in historical time.
Comet Tempel-Tuttle swept through the inner solar system once again in early 1998. New studies using computer modeling indicated that it was not a singular knot, but rather several dense trails of material imbedded within the Leonid stream that gave rise to spectacular meteor storms.
A spectacular display of hundreds of fireball meteors appeared in 1998. This was followed by meteor storms numbering in the thousands per hour in 1999, 2001 and 2002.
More than a decade has now passed since the comet's most recent visit, and the Leonids are now producing far more modest displays. At their peak, perhaps a dozen or more meteors might be seen streaking from out of the Sickle of Leo.
It seems that we'll have to wait until Tempel-Tuttle returns to the vicinity of the inner solar system in 2031 for the next great Leonid cycle.
However, long-range orbital computations indicate that a close encounter with Jupiter in 2028 is expected to once again seriously perturb both the comet's path and its accompanying dense trails of material, making storms of historic magnitude (meteor rates in the thousands per hour) unlikely for many decades to come.
About the best we can hope for are Leonid displays numbering not in the thousands, but maybe only the hundreds per hour for several years beginning perhaps in the year 2033. The Leonids have the potential to be quite impressive in those years — a display of, say, 500 meteors per hour would be more meteors than most people would see in an entire lifetime.
But any hope of a repeat of the amazing spectacles of 1799, 1833 or 1966 does not appear likely until sometime after the start of the 22nd century; something for our great-grandchildren to look forward to.
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