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Leonids

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(Three-letter Code=LEO)


Copyright©1996 by Sirko Molau

This image was obtained by Sirko Molau (Germany) using a video camera system during the 1995 outburst of the Leonid meteor shower. The long streaks are Leonid meteors, while everything else in the image are stars (note Orion in the lower right quadrant of the image). Although the video equipment used was very sensitive, the image does closely match photographs taken during the Leonid display of 1966, and might represent what the visual observer might see at any one moment during the Leonid peak of 1998 an d 1999.

Observer's Synopsis

The duration of this meteor shower covers the period of November 14-20. Maximum currently occurs on November 17 (solar longitude=235 deg), from an average radiant of RA=153 deg, DEC=+22 deg. Although the maximum hourly rate typically reaches 10-15, th is shower is most notable for producing greatly enhanced activity every 33 years---events that are associated with the periodic return of comet Tempel-Tuttle. During these exceptional returns, the Leonids have produced rates of up to several thousand mete ors per hour. The Leonids are swift meteors, which are best known for producing many exceptionally bright meteors that leave a high percentage of persistent trains. The radiant's daily motion is +1.0 deg in RA and -0.4 deg in DEC.

How to Observe

The point from where the Leonid meteors appear to radiate is located within the constellation Leo and is referred to as the radiant. The radiant is located in the western portion of that constellation in what is commonly referred to as the "sickl e" or "backwards question mark." The radiant rises around 12:30 a.m. local time. Although a few Leonids can be observed prior to this, more will be seen after it rises. At about 3:00 a.m. the radiant is about 30 degrees above the horizon. T he radiant location with respect to the horizon is shown below.


(Image produced by the Author using Starry Night 2.0 and Adobe Photoshop 5.0. It represents the view from mid-northern latitudes at about 3:00 a.m. local time.)

To best observe the Leonids wear appropriate clothing for the weather. Lay outside in a reclining lawn chair with your feet pointing towards the east (the general direction of the radiant). Do not look directly at the radiant, because meteors directly in front of you will not move much and fainter ones might be missed. Instead, keep your center of gaze about 30 or 40 degrees above or west of the radiant. The Leonids can be observed right on into morning twilight, especially during years of enhanced ac tivity, i.e., 1998 and 1999. Other minor meteor showers will be going on at the time and stray meteors, more commonly called sporadics, will frequently be seen that do not belong to a meteor shower. When you see a meteor mentally trace it backwards and if you arrive at the "sickle" of Leo it is probably a Leonid.

History

The Leonids and the Birth of Meteor Astronomy

The night of November 12-13, 1833, not only marks the discovery of the Leonid meteor shower, but sparked 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 are varied from the hysterics of the superstitious claiming Judgement Day was at hand, to the just plain excitement of the scientific, who estimated that a thousand meteors a minute emanated from the region of Leo. Newspa pers 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 were 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 pl ants 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 stron g 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 Denis on 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. First of all, 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, the coordinates of which were given as RA=150 deg, DEC=+20 deg. Finally, noting that an abnormal display of meteors had also been observed in Europe and the Middle East during Nove mber 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.
One of the more significant findings of the 1833 Leonid storm was the determination of the meteor shower's radiant. As mentioned above, Olmsted had obtained a position, but on the same morning, Professor A. C. Twining (West Point, New York) and W. E. Aiken (Emmittsburg, Maryland) obtained more precise estimates of RA=148.4, DEC=+22.3 and RA=148.2 deg, DEC=+23.8 deg, respectively. This was the first time a shower radiant had ever been pinpointed more precisely than a simple direction in the sky or even a constellation.
New information continued to surface following the 1833 display which helped shed new light on the origin of the Leonids. First, a report was found concerning F. H. A. Humboldt's observation of thousands of bright meteors while in Cumana, South Americ a during November 12, 1799. Further digging around this date in other publications revealed the spectacle was visible from the Equator to Greenland. Next, in November 1834, the Leonids reappeared and, although they were not as plentiful as in the previous year, they did demonstrate that some annual activity might be present from this region. In the years that followed, Leonid displays continued to weaken. In 1837, Heinrich Wilhelm Matthias Olbers combined all of the available data and concluded that the L eonids possessed a period of 33 or 34 years. He predicted a return in 1867.

The Leonids of 1866

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 Hubert A. Newton's examination of meteor showers reported during the past 2000 years. During 1863, he id entified 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 pe riod 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 1867 display had the misfortune of occurring with the moon above the horizon, but observers still reported rates as high as 1000 per hour, meaning the shower may have actually been stronger than in the previous year. 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, Ernst Wilhelm Liebrecht Tempel (Marseilles, France) had discovered a 6th-magnitude, circular object near Beta Ursae Majoris. Aft er an independent discovery was made by Horace Tuttle (Harvard College Observatory, Massachusetts) on January 6, 1866, the comet took the name of Tempel-Tuttle. Perihelion came on January 12, 1866, afterwhich the comet began fading so rapidly, that it was not seen after February 9. Orbital calculations shortly thereafter revealed the comet to be of short period, and, as 1867 began, Theodor von Oppolzer had more precisely calculated the period to be 33.17 years. Using observations from the 1866 Leonid disp lay, Urbain Jean Joseph Le Verrier computed an accurate orbit for the Leonids, and Dr. C. F. W. Peters, Giovanni Virginio Schiaparelli and von Oppolzer independently noted a striking resemblance between the comet and meteor stream.
After a final notable display on November 14, 1869, when hourly rates reached 200 or more, the following years were notable only due to a fairly consistent rate ranging from 10 to 15 Leonids per hour.

The Leonids of 1899

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 Charles 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 possess maximum hourly rates of 40 on November 14---at least indicating some unusual activity. Later investigations r evealed 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 actual peak of activity for the Leonids came on November 14-15, 1901. In the British Isles, Henry Corder (Bridgwater), E. C. Willis (Norwich) and others reported hourly rates as high as 25 before morning twilight interfered. Seve ral hours later, the Leonid radiant was well placed for observers in the United States, and it was apparent that the activity had increased. On the east coast, Olivier (Virginia) and Robert M. Dole (Massachusetts) independently obtained hourly rates of 60 and 37, respectively. By the time the Leonids were visible over the western half of the United States, they had apparently reached their peak. At Carlton College (Minnesota) it was estimated that individuals could have counted about 400 per hour. E. L. L arkin (Echo Mountain, California) estimated that rates reached a maximum of 5 per minute (300 per hour). By the time the British Isles had the radiant back in view, hourly rates had apparently declined to about 20. After analyzing the available data, Will iam F. Denning concluded that the maximum of this shower came on November 15.48 Greenwich Mean Time (November 15.98 UT).
The Leonids were barely detected in 1902, due to moonlight, but there was a reappearance in 1903. On November 16, Denning estimated a maximum hourly rate of 140, and said that for 15 minutes following 5:30 a.m. (local time) meteors were falling at 3 p er minute. From plotted meteor paths, he found the radiant to have been 6 degrees in diameter, centered at RA=151 deg, DEC=+22 deg. John R. Henry (Dublin, Ireland) was also surprised by the intensity of the display, and he noted maximum rates near 200 per hour. Henry further noted that, at maximum, the Leonid meteors were pear-shaped and left rich trains. He noted, "Other members of the star shower dissolved in bright streaks, or made their appearance as vivid flashes of light...." Finally, Alph onso King (Sheffield, England) did not begin observations until 5:57 a.m. He noted that 18 Leonids were seen in the first five and a half minutes, while only 16 were seen in the next half hour. King plotted 10 meteors which indicated a radiant of RA=148 d eg, DEC=+22 deg. From the above observations, it would seem the 1903 maximum came on November 16.2 UT.

The Leonids of 1932-1933

The Leonids returned to normal in the years following 1903, with hourly rates ranging from 5 to 20 (average about 15). Despite having miscalculated the Leonid maximum in 1899, astronomers began to make predictions for the next return---the most likely date being 1932. Enhanced activity began early when, in 1928, maximum hourly rates reached 50 or more. During 1929, rates were lower, only 30 per hour, but moonlight was then a factor. During this latter year, members of the American Meteor Society (AMS) made fairly extensive observations, and Olivier's analysis revealed a radiant diameter of 5-6 degrees and a shower duration of 8-10 days.
The Leonids began to show great strength in 1930. Professor C. C. Wylie (Iowa City, Iowa) estimated maximum hourly rates of 120 shortly before dawn on November 17. Olivier said the shower contained "many brilliant meteors with long enduring train s." His analysis showed Leonids were first observed on November 13/14 and last seen on the 22nd. He confirmed that rates were "considerably over 100 per hour, despite moonlight...." The 1931 display showed a slight increase over 1930, but c ertainly not as great as expected considering the lack of moonlight. Olivier's analysis of AMS observations revealed rates between 130 and 190 per hour for observers in the United States during the pre-dawn hours of November 17.
The predicted meteor storm of 1932 was looked for with great anticipation by astronomers, but it had been realized that moonlight would interfer with observations. Nevertheless, the first detection of the rapid rise to maximum came at Helwan Observato ry (Egypt) during the pre-dawn hours of November 17. P. A. Curry was one of seven observers keeping a lookout for the expected storm, and the greatest hourly rates reached 51; however, it should be noted that the 5-minute counts showed a steady rise t o 9 at 4 a.m.---amounting to 108 per hour---followed by a rapid decrease in numbers thereafter. Members of the British Astronomical Association (BAA) were best placed for maximum, which came just a few hours after the Helwan observations. J. P. M. Prentic e obtained the highest rates of 240 per hour. Unfortunately, even after taking moonlight into account, it was obvious that a "meteor storm" comparable to those of 1833 and 1866 did not occur.
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 perio ds of enhanced activity occurred during 1898-1903, 1865-1869 and 1831-1836, which amounted to only 5 or 6 years.

The Leonids of 1966

Throughout the 1940's and 1950's hourly rates retained their "normal" character of 10-15 per hour. However, the period was highlighted by a new advance in astronomy---radar studies. Jodrell Bank Radio Observatory was the first station to det ect the Leonids, with maximum observed rates being 24 in 1946, but only 3 to 11 during the period of 1947 to 1953. Unfortunately, due to the weakness of the Leonids during the 1950's, the increasing sophistication of the equipment still could not obtain i nformation such as radiant positions or radiant diameters.
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. Dennis Milon was one of five amateur astronomers observing outside Hou ston, Texas, when 51 Leonids appeared between 3:10 and 4:10 a.m. on November 16 (about November 16.4 UT). The next morning the greatest one-hour interval produced a rate of 54 Leonids (about November 17.4 UT), bringing the Texas group to believe maximum h ad probably occurred late on the 16th. Similar rates were reported elsewhere. Norman D. Petersen (California) commented that the Leonids were blue-white, very rapid, and often left long-enduring trains 10 degrees in length.
The 1962 and 1963 displays were about 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 t he Smithsonian tracking station at Maui (Hawaii) hourly rates were near 20 on November 16.56 UT, but increased to about 120 by November 16.64 UT. Meanwhile, observers at the Smithsonian tracking station at Woomera (Australia) reported 38 Leonids of an ave rage magnitude of -3 between November 16.65 and 16.77 UT.
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 e ncounter 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.
On the night of November 17, 1966, expectations were high worldwide, but few observers got to see the Leonids as well as Dennis Milon and a dozen other amateur astronomers situated under the clear skies of Arizona. Observations began at 2:30 a.m. (Nov ember 17.35 UT) and 33 Leonids were detected during the next hour. After a short break, the next hour began at 3:50 a.m., with 192 Leonids being observed. The team had been keeping magnitude estimates during the early part of the shower, but this ended ar ound 5:00 and, by 5:10, the observers were detecting 30 meteors every minute, but the display was far from over. Rates at 5:30 were estimated as several hundred a minute and the team estimated a peak rate of 40 per second was attained at 5:54 (November 17 .50 UT)! The activity declined thereafter, and by 6:40 it was down to 30 per minute, despite the fact that astronomical twilight had begun 9 minutes earlier. To sum up, it would seem the 1966 return of the Leonids was one of the greatest displays in histo ry, with maximum rates being 2400 meteors per minute or 144,000 per hour.
The major peak of the 1966 display was also enjoyed by observers in New Mexico, Texas, and California. Observers in the two former states were somewhat hampered by twilight, but observers in California may have had the best view though not as well pub licised as the Arizona observations at the time. When Table Mountain Observatory's assistant astronomer James Young began his observations at 2:30 a.m. (local time) heavy clouds were present, but conditions had greatly improved by 3:30. From that time on Young and the four other observers present watched as meteor rates continued to climb. By 4:45 a.m. (local time) the group decided rates had reached about 50 per second, with this intensity being maintained for about 10 minutes before a noticeable decline had set in. By the time twilight had begun the group had photographed over 1000 meteor trails, including about a dozen fireballs!
Observers in the eastern portion of the United States did report rates of several hundred per hour, but other countries reported rates generally less than 200 per hour, since maximum had occurred during daylight. An exception was observers at a USSR p olar arctic station, who were able to monitor the shower at its peak. With the radiant only 8 deg above the horizon, the report from two observers said, "there was a continuous flight of meteors in a single direction, from north to south. Some appear ed in the zenith and curved over the southern horizon, some appeared from the northern horizon and disappeared in the zenith, and some flew across the entire horizon, leaving behind a bright trail." R. L. Khotinok's analysis of the complete report re vealed an observed maximum rate of 20,000 per hour, while a correction for the low altitude gave a rate of 130,000 per hour---agreeing quite well with the Arizona and California observations.
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 19 71 and 40 in 1972. The Leonids have remained between 10 to 15 per hour at maximum ever since.
One of the first Leonid studies involving an analysis of observational data, was published in 1932 by Alphonso King. The study was basically a look at his observations made during 1899-1904 and 1920-1931. King noted the diameter of the radiant to gene rally be less than 4 deg, and he determined a radiant ephemeris which indicated a daily motion of +1.0 deg in RA and -0.4 deg in DEC.
Some of the more interesting recent studies of the Leonids involved extensive observations by professional and amateur astronomers in the Soviet Union during 1971 and 1972. The first set of observations were made at Sudak and Simferopol during Novembe r 15-19, 1971. Although numerous observers participated, it was the more experienced observations of N. V. Smirnov and Yu. V. Lyzhin which were evaluated. Some of the various observed aspects of the meteors included 553 meteors with an average magnitude o f 3.40 and 171 color estimates indicating 74% were green, 20% were white, 1% were blue and 1% were orange. One of the most striking discoveries was the detection of multiple radiants. Although six radiants were determined, the most active was the long-kno wn radiant at RA=151.7 deg, DEC=+22.9 deg (based on 222 plotted meteors) and the authors noted that the total plots indicated activity primarily came from an area 2.5 deg x 8 deg centered on this radiant.
The 1972 visual survey was conducted during November 16-18, from the same locations given above. A magnitude breakdown was not given strictly for the Leonids, but for all meteors observed at Sudak. The average brightness ended up as 3.01 for 576 meteo rs, of which 335 were Leonids. On this occasion, six radiants were again determined from plots, with the main center being at RA=151.9 deg, DEC=+22.7 deg (based on 185 meteors). The radiants were generally grouped into an area about 10 deg across; how ever, it should be noted that two radiants within this area were distinctly detected in both years---one near Mu Leonis (RA=150 deg, DEC=+28 deg) and the other between Gamma and Eta Leonis (RA=151 deg, DEC=+17 deg).
During 1967, one of the first mathematical surveys of the perturbations suffered by the Leonid meteor stream was conducted. Using the orbit determined for the 1866 Leonid shower, E. I. Kazimirchak-Polonskaya, N. A. Belyaev, I. S. Astapovich and A. K. Terent'eva examined 12 hypothetical meteor groups situated around the orbit. One of the major findings was that Jupiter and Saturn were primarily responsible for altering the encounter conditions between Earth and the meteor stream. Earth itself was even found to have a strong effect on meteor bodies passing within several thousand kilometers of its surface by shortening the revolution period by several years, strongly altering the eccentricity and even changing the inclination.

The Leonids of 1998-1999

The most ambitious study of the relationship between Tempel-Tuttle and the Leonids was published in 1981. Donald K. Yeomans (Jet Propulsion Laboratory, California) mapped out the dust distribution surrounding Tempel-Tuttle by "analyzing the assoc iated 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 the theory of outgassing and dust ejection developed to explain the comet's deviation from "pure gravitational motion." Yeomans suggested this indicated "that radiation pressure and planetary perturbations, rather than ejection processes, control the dynamic evolution of the Leonid particles." Concerni ng 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 E arth'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 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 an analysis by Peter Jenniskens indicating a short burst with a ZHR of 70 to 80. Observers worldwide covered the 1995 return quite well. The period of maximum was rather broad and lasted about 24 hours, with the maximum ZHR reaching about 35; h owever, there was a short-lived outburst which produced about 50 per hour a few hours before the normal maximum. Observations obtained during 1996 indicated a maximum ZHR of about 60 per hour, with numerous fireballs present.
The latest observations of the Leonids occurred in strong moonlight on November 17, 1997, when numerous observations were reported worldwide. Observers indicated a peak zenithal hourly rate of between 80 and 150, around 10:50 UT on the 17th. The wide scatter was attributed to the moonlight-affected seeing. Numerous fireballs were seen. The Author observed a spectacular -5 fireball that lit up the sky when it suddenly flared to a magnitude of between -10 and -12. It left a glowing train that lasted ove r four minutes and became distorted by high-altitude winds. Other observers reported numerous fireballs in the range of -6 to -9. The Central Bureau for Astronomical Telegrams reported that a possible secondary peak in activity occurred during the period of 16:45 to 21:30 UT on the 17th. This latter "peak" was detected by monitoring the 50MHz HAM radio signals.

Conclusions

As indicated by Yeomans in 1981, inconsistencies within the dust cloud surrounding comet Tempel-Tuttle are expected. The particles encountered during the period of 1994-1996 are those that were probably released by the comet a few hundred years ago an d have had time to disperse. Obviously as the comet gets closer to the sun, the particles encountered during the Leonid display will be newer, with less time to disperse. Since Earth will pass quite close to the orbit of comet Tempel-Tuttle during 1998 an d 1999, anticipation is still high for a very strong display in those years.

Orbit

The orbit of the Leonid stream, based on 12 photographic meteors obtained from W1954, H1959 and MP1961, as well as 3 radar meteors obtained from S1970, is as follows:

AOPANiqea
173.1235.4 161.00.9830.9019.956

The 1965 orbit of comet Tempel-Tuttle, according to Yeomans' 1981 study published in Icarus is

AOPANiqea
172.6234.4 162.70.9820.90410.272

Other Interesting WWW Sites

  • Leonids '98 (P. Jenniskens)

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