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Updated March 14, 2011
(Follow this link for information on the 2003 Leonids.)
Leonids: November 14-20; Peak November 19, 2002
The Leonids are among the most interesting and important of all meteor showers. In recent years the Leonids have put on some stunning displays, adding to a rich history of periodic storms. Although it will be hampered by bright moonlight, the last of the current series of Leonid squalls is predicted for the early morning hours of November 19, 2002.
The Leonids were the first meteor shower to be identified as such. Numerous anecdotal accounts date back to at least the sixth century C.E., including an observation from South America of thousands of bright meteors by F.H.A. Humboldt in 1799.
On November 13, 1833, a great Leonid meteor storm was observed in eastern North America. Although he was wrong on some of the specifics, Denison Olmsted correctly concluded that a cloud of particles was responsible.
The 1860s saw the birth of modern meteorics. Daniel Kirkwood suggested that meteor showers were caused by the debris of old comets. He was spectacularly proven right within five years. First, the derived orbit of Comet Swift-Tuttle, discovered in 1862, was found to be in close agreement with the characteristics of the well-known Perseid shower. Then Comet Tempel-Tuttle was discovered in late 1865, and its orbit compared favorably with that of the Leonid meteoroid stream, particularly in its 33-year period that was already associated with Leonid storm activity. Another great storm was indeed observed in November 1866, and the causal relationship was verified.
Although anticipated storms did not materialize in 1899 or 1933 (due to orbital perturbations caused by Jupiter), the cycle was resumed in 1966 with a spectacular deluge estimated by astonished observers in the American southwest at 40 meteors per second. While never approaching this prodigious rate, the Leonids put on good to excellent shows in the years following the latest return of Tempel-Tuttle in early 1998, including a memorable storm of >1,000 meteors per hour as seen from Alberta in 2001. This series of displays has also resulted in major improvements in prediction mechanisms, as meteor astronomers learn to interpret Earth’s interactions with the individual filaments of material associated with successive returns of the parent comet.
While there is a Leonid shower every mid-November, the normal yield is only 10 or so meteors per hour. However, when Comet Tempel-Tuttle is in the neighbourhood, that figure can increase by a factor of hundreds or even thousands. While there is a debris cloud forming a torus (doughnut-shape) around the entire orbit of the comet, the densest portions of the cloud by far were cast off during recent perihelion passes of the comet, and therefore remain clumped in its immediate proximity.
In recent years extensive theoretical modeling has been done of the meteoroid swarms which cause Leonid storms. Teams of experts, including Fort McMurray native Peter Brown, have identified elongated ribbons of comet debris related to specific passes over the last several centuries. They have grown increasingly successful in predicting to within a few minutes, Earth’s passage through the most favourably placed of these.
In 2002 these experts agree that there should be two separate peaks about six hours apart, each of which should yield a theoretical rate of roughly 3,000 meteors per hour. Residents in western North America are favourably situated for the second of these, when Earth plows through the filament of material associated with Comet Tempel-Tuttle’s 1866 perihelion. Centred around 3:40 a.m. MST, high rates of meteors can be expected for at least an hour both before and after the peak.
Current models anticipate that 2002 will present not only the last storm of the current era, but that further interactions with Jupiter will preclude future series of storms associated with either the 2031 or 2065 returns of Tempel-Tuttle. A century from now, conditions will again be favourable.
Not all of the news is good. The Moon is full within hours of the maximum, and will therefore be bright and fairly high in the sky throughout the night. The resultant brightening of the sky will reduce observed rates by a factor of three or more. According to Gary Kronk in his definitive book Meteor Showers, following the great 1866 storm which yielded 2,000-5,000 meteors per hour, “the 1867 display had the misfortune of occurring with the moon above the horizon, but observers still reported rates of 1,000 per hour, meaning the shower may have actually been stronger than in the previous year.” We can hope for a similar showing this year.
Leonids do have a tendency of being bright, in large part due to their tremendous speed of 71 km/s. These head-on collisions with Earth occur near the theoretical maximum velocity for particles constrained to the solar system. Roughly half leave lingering luminous trains, and the brighter ones may have a blue or green hue.
Observers are encouraged to block the Moon from their line of sight using a local obstruction such as a car, a tree, or better yet, a mountain. A viewing locale on the Eastern Slopes would be ideal. Meteors will appear to radiate from a point in the Sickle of Leo, near the bright telescopic double star Algieba (gamma Leonis). Fortunately, the Moon will be about 100° removed from the shower’s radiant point, allowing the observer to comfortably see the latter while blocking the former.
Groups of observers are encouraged to observe different sections of the sky. Scientifically useful counts can be maintained at ten-minute intervals – five if activity warrants – using a watch with a timer function and a microcassette recorder to facilitate uninterrupted viewing. While a single person can perform both functions for a group, the count of each individual observer should be recorded, not a group total. Qualitative remarks about the brightness, colour, or train of individual meteors should be recorded as a matter of course.
Neither phase of Moon, nor cold, potentially cloudy November weather, has any effect on radio observations. The easiest method is to detect signals emanating from FM transmitters beyond the horizon which are briefly reflected off ionization trains of meteors. Last year David Cleary achieved excellent results while testing equipment and methodology for Sky Scan, using frequency 92.1 MHz, which has strong transmitters in both Calgary and Winnipeg. For further information, see: http://www.skyscan.ca/LeonidResults.htm
If you are fortunate enough to get clear skies, a memorable observing session can be further augmented by tuning your car radio to any FM frequency with no local station. And in the event of inclement weather, the shower can still be enjoyed in this fashion, a silver lining for the thickest of clouds.
For further information
More on the Leonids is available at the following websites: