Observing the Leonids
Location of the Leonids
Location of the Leonids
The night of November 12-13, 1833, not only marks the discovery of the Leonid meteor shower, but it marks the actual birth of meteor astronomy. During the hours following sunset on November 12, some astronomers noted an unusual number of meteors in the sky, but it was the early morning hours of the 13th that left the greatest impression on the people of eastern North America. During the 4 hours which preceded dawn, the skies were lit up by meteors.
Reactions to the 1833 display varied from the hysterics of the superstitious claiming Judgement Day was at hand, to just plain excitement by the scientific, who estimated that a thousand meteors a minute emanated from the constellation Leo. Newspapers of the time reveal that almost no one was left unaware of the spectacle, for if they were not awakened by the cries of excited neighbors, they were usually awakened by flashes of light cast into normally dark bedrooms by the fireballs.
At the time of the 1833 display, the true nature of meteors was not known for certain, but theories were abundant in the days and weeks which followed. The Charleston Courier published a story on how the sun caused gases to be released from plants recently killed by frost. These gases, the most abundant of which was believed to be hydrogen, "became ignited by electricity or phosphoric particles in the air." The United States Telegraph of Washington, DC, stated, "The strong southern wind of yesterday may have brought a body of electrified air, which, by the coldness of the morning, was caused to discharge its contents towards the earth." Despite these early, creative attempts to explain what had happened, it was D. Olmsted who ended up explaining the event most accurately.
After spending the last weeks of 1833 trying to collect as much information on the event as possible, Olmsted presented his early findings in January 1834. He noted the shower was of short duration, as it was not seen in Europe, nor west of Ohio [Author's note: We now know the shower was seen by numerous Native American tribes throughout the midwest and western United States, who frequently referred to the event as "the night the stars fell."] His personal observations had shown the meteors to radiate from a point in the constellation of Leo. Finally, noting that an abnormal display of meteors had also been observed in Europe and the Middle East during November 1832, Olmsted theorized that the meteors had originated from a cloud of particles in space. Although the exact nature of this cloud was not explained properly, it did lead the way to a more serious study of meteor showers.
The interest of the astronomical world began focusing on the predicted return of the Leonids as the decade of the 1860's began. Most important was H. A. Newton's examination of meteor showers reported during the past 2000 years. During 1863, he identified previous Leonid returns from the years 585, 902, 1582 and 1698. During 1864, Newton further identified ancient Leonid displays as occurring during 931, 934, 1002, 1202, 1366 and 1602. He capped this study with the determination that the Leonid period was 33.25 years and predicted the next return would actually occur on November 13-14, 1866.
The expected meteor storm occurred in 1866 as predicted, with observers reporting hourly rates ranging from 2000 to 5000 per hour. The Leonids were again seen in 1867, when moonlight reduced the rates to 1000 per hour. Another strong appearance of the Leonids in 1868 reached an intensity of 1000 per hour in dark skies.
The year 1867, marked an important development in the understanding of the evolution of the Leonids. On December 19, 1865, E. W. L. Tempel (Marseilles, France) had discovered a 6th-magnitude, circular comet near the "Big Dipper." After an independent discovery was made by H. Tuttle (Harvard College Observatory, Massachusetts, USA) on January 6, 1866, the comet took the name of "Tempel-Tuttle". After it passed closest to the sun on January 12, 1866, the comet began fading so rapidly, that it was not seen after February 9. Orbital calculations revealed the comet to be of short period, and, as 1867 began, T. von Oppolzer had more precisely calculated the period to be 33.17 years. Using observations from the 1866 Leonid display, U. J. J. Le Verrier computed an accurate orbit for the Leonids, and Dr. C. F. W. Peters, G. V. Schiaparelli, and von Oppolzer independently noted a striking resemblance between the orbits of this comet and Leonids.
Numerous confident predictions were put forth that the Leonids would next be at their best in 1899, and an early sign of returning enhanced activity was detected in 1898, when hourly rates reached 50-100 in the United States on November 14.
What C. P. Olivier called "the worst blow ever suffered by astronomy in the eyes of the public," was the failure of a spectacular meteor shower to appear in 1899. Predictions had been made and newspapers in Europe and America made the public well aware that astronomers were predicting a major meteor storm. Although the "storm" failed to appear, the Leonids did exhibit maximum hourly rates of 40 on November 14---at least indicating some unusual activity. Later investigations revealed the stream to have experienced close encounters with both Jupiter (1898) and Saturn (1870), so that the stream's distance from Earth in 1899 was nearly double that of the 1866 return. As it turned out, the Leonids were strongest in 1901, when a couple of observers in the western half of the United States estimated rates of 300-400 per hour. Enhanced activity finally subsided after 1903.
Predictions were provided for the 1932-1933 return of the Leonids, but these were not as well publicized as in 1899. Rates did rise to maximum values of over 100 per hour in 1930 (in moonlight), about 150 per hour in 1931, and over 200 per hour in 1932. Interestingly, the Leonids seemed to decline slower than normal after 1932, as maximum rates remained between 30 and 40 meteors per hour from 1933 through 1939. This meant that greater than normal activity persisted from 1928 to 1939, or 12 years. The previous periods of enhanced activity occurred during 1898-1903, 1865-1869 and 1831-1836, which amounted to only 5 or 6 years.
Throughout the 1940's and 1950's rates retained their "normal" character of about 10 per hour. However, the period was highlighted by a new advance in astronomy---radar studies. Jodrell Bank Radio Observatory was the first station to detect the Leonids, with maximum observed rates being 24 in 1946, but only 3 to 11 during the period of 1947 to 1953.
Visual observers generally ignored the Leonids during the late 1950's, and this state of neglect caused many to completely miss the unexpected arrival of enhanced activity in 1961, when a group of observers in Texas reported maximum Leonid rates of 51 per hour on the morning of November 16 and 54 per hour on the morning of November 17. Similar rates were reported elsewhere. The 1962 and 1963 displays were near normal, with hourly rates of 15 or 20, while the 1964 display perked up with enhanced rates of 30 per hour. During 1965, observers in Hawaii and Australia were treated to one of the best displays since 1932. From the Smithsonian tracking station at Maui (Hawaii) hourly rates peaked near 120 on November 16.
Although astronomers were still a year away from the predicted Leonid maximum, optimism did not run high concerning the appearance of a meteor storm. Judging by the 1899 and 1932 returns, the stream orbit had obviously been perturbed so that a close encounter with Earth's orbit seemed no longer possible. About as far as astronomers were willing to gamble was to say that rates would probably be greater than 100 per hour. For much of the world, this is the best that was seen, but for the western portion of the United States, it was a night to be remembered, as experienced observers in Arizona and California estimated that, for about 10-15 minutes, meteors were falling at a rate of 40-50 per second on the morning of November 17!
In the years following the 1966 display, hourly rates for the Leonids remained high. From 1967 through 1969, observers continued to detect rates of 100-150 per hour. After a return to normality in 1970 (15 per hour), rates jumped to 170 per hour in 1971 and 40 in 1972. Rates returned to normal thereafter.
The study of the Leonid meteor shower took a notable turn in 1981. D. K. Yeomans (Jet Propulsion Laboratory, California, USA) studied the relationship between the Leonids and the comet Tempel-Tuttle. He mapped out the dust distribution surrounding the comet by "analyzing the associated Leonid meteor shower data over the 902-1969 interval." He noted that most of the ejected dust lagged behind the comet and was outside its orbit, which was directly opposite to what was expected based on the way comets are known to operate. Yeomans suggested this indicated that the sun's own solar wind, as well as perturbations by the planets controlled the evolution of the Leonid stream. Concerning the occurrence of Leonid showers, Yeomans said "significant Leonid meteor showers are possible roughly 2500 days before or after the parent comet reaches perihelion but only if the comet passes closer than 0.025 AU inside or 0.010 AU outside the Earth's orbit." He added that optimum conditions will be present in 1998-1999, but that the lack of uniformity in the dust particle distribution still makes a prediction of the intensity of the event uncertain.
The Leonids began again drawing the attention of observers shortly after the 1990's began, but notable activity did not appear until 1994. In that year both visual and radio-echo observers detected rates that were above normal on the night of November 17-18, with visual hourly rates reaching about 40. Observers worldwide covered the 1995 return quite well. The period of maximum was rather broad and lasted about 24 hours, with the maximum rate reaching about 20 per hour; however, there was a short-lived outburst which produced about 30 per hour a few hours before the normal maximum. Observations obtained during 1996 indicated a maximum a rate of about 40 per hour, with numerous fireballs present. The 1997 Leonids were affected by strong moonlight; however, observers reported maximum rates of over 100 per hour. The 1998 Leonids occurred in moonless skies and rates over Europe were between 200 and 300 per hour. From the Canary Islands, it appears that counts over two-minute intervals indicated bursts which translated to 1000-2000 per hour!
Early in 1999, David Asher and Robert McNaught published a paper giving details on how to predict meteor storms for the Leonids. In general, they noted that each time the comet Tempel-Tuttle approached the sun it released a large amount of dust. The gravitational pull of Jupiter would pull the comet into a slightly different orbit as it exited the sun's vicinity or as it made its next approach. The result was that the comet was laying out a series of ringlets or filaments with clouds of dust. Asher and McNaught noted that after a while the old filaments would diffuse and merge together, but recent filaments would contain fairly dense clouds of dust. They subsequently predicted that a meteor storm would occur on November 18 as Earth passed through a filament produced by the 1899 return of Tempel-Tuttle. They predicted the peak would occur at 2:10 Universal Time, with a peak rate of 1000 per hour. As it turned out the actual peak occurred at 2:06 UT, which was remarkably close to the prediction! Interestingly, the actual rate was over 3000 per hour, so the prediction missed this by quite a bit. Enhanced rates continued from 2000-2002 and the predictions continued to show a high rate of success.