The annual Leonid meteor shower is predicted to reach its peak about noon on Tuesday, Nov. 17. The zenithal hourly rate (the "ZHR" -- an extrapolation of the average hourly rate of meteors observed to very dark, clear skies with the radiant of the shower at the zenith) is generally only 5 - 20, but this year is different enough to demand more attention. The comet responsible for the debris that provides the meteors for the shower, Comet Temple-Tuttle, passed closest to Sun in its 33.17-yr orbit on Feb. 28. Historically when our mid-November passage through the comet's orbit takes place soon after the comet's inner solar system passage, spectacular meteor "storms" appear.
Meteor storms are short lived, but during the hour or less in which they occur, the ZHR exceeds 3600 -- more than a meteor a second. In 1966, the last time Temple-Tuttle visited the inner solar system, observers in the American southwest saw meteors at a rate of 150,000 per hour with short intervals when the rate was estimated to reach 140 per second. One observer was reported to say that he felt as if he should duck because the meteors were coming in such thick swarms. Even outside of storms, though, the showers are richer than usual, so rising early to view this shower seems worthwhile this year.
In 1833 Leonid meteors were seen in North America at a rate of 14,000 per hour. These observations forced the recognition that meteors were from interplanetary space and not merely of "meteorological" origin. Until that time meteors were believed to be some phenomenon taking place in Earth's atmosphere, perhaps related to lightning. "Radiants," particular points in the sky from which the meteors seem to originate, were recognized at that time as well. Looking in the direction of the radiant is the same as looking down the "tube" of the comet orbit so that the parallel moving meteors seem to diverge from a small region of the sky. Some writers also believe that growth of religious movements across the country in the 1830's may have been stimulated by the Leonid meteor storm of 1833.
This year is ideal for early morning viewing with moonrise at 5:25 a.m. on Nov. 17 and after 6 a.m. the next morning. The radiant is in the Sickle of Leo. This rises about midnight, so the source of the meteors rises into view then. Of course the radiant is not the best place to look, for meteors in than region are moving nearly directly toward us and leave short trails. Looking away from the radiant, to the darkest part of the sky, is probably best. You can trace the trails back to see if they seem to intersect in the region of Leo.
There seems to be a small knot or sheet of meteoroids that follow closely behind Temple-Tuttle to produce the spectacular storms. Unfortunately for us, when Earth is predicted to pass through the region of space containing this swarm the radiant will be highest in the sky in Japan and China. Next year's prediction favors Europe and western Asia, so we will not be in a good position for a meteor storm then, either. In both years, though, there is expected to be an increase in the frequency of meteors in the shower, so it will be well worth our while to set our alarms and go out before dawn to look for meteors associated with this comet.
The meteors in the Leonid shower are moving almost opposite to the direction of Earth's motion and crash into our atmosphere with speeds up to 71 kilometers per second, or more than 150,000 miles per hour. This is about the maximum speed with which any meteor associated with a comet that is part of Sun's family can possibly crash into our planet. Consequently, the meteors are often bright, and many leave trails that have been seen to last for up to 8 minutes. Still, they are mostly no larger than grains of sand, and none has been known to avoid burning up completely and reaching ground as a meteorite.
