Leonid
Observations:
1833 to present
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. 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 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 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 Denison 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 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.
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 America 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 Leonids possessed a period
of 33 or 34 years. He predicted a return in 1867.
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 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 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. After 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 display, 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.
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 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 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. Several 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. Larkin (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, William 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 per 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, Alphonso 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 deg, DEC=+22 deg. From the
above observations, it would seem the 1903 maximum came on
November 16.2 UT.
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 trains." 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 certainly 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 Observatory (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 to 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. Prentice 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 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 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 detect 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 information 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 Houston, 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 had 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 the 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 average 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 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.
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. (November 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 around 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 history, 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 publicised 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 polar 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 appeared 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 revealed 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 1971 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 generally 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 November 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 of 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-known 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 meteors, 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; however, 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 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 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 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." 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 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; however, 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 ZHR attained a maximum of 80 to 150 in
strong moonlight on the night of 1997 November 17. For the latter
display, the webmaster saw a spectacular fireball of magnitude -5
that suddenly flared to between magnitude -10 and -12.
Interestingly, despite the moonlight, this fireball briefly lit
up the landscape around me and it left a glowing train in the sky
that lasted over four minutes.
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