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Armagh Observatory, 13 November 2001:
The Leonid meteors, or shooting stars, are produced by debris from comet Tempel-Tuttle. The comet swings past the Sun every 33.2 years, and during each close approach emits a dense stream of dust and small particles. In the course of time, these dust trails extend along the whole of the comet's orbit, but the trails remain very narrow and concentrated in space for hundreds of years before spreading out. Every year, between 15th and 20th November, the Earth encounters a broad stream of ancient debris, which leads to an annual meteor shower visible in the early hours towards the north-east after midnight between these dates. Because the cometary dust particles move in very similar orbits around the Sun their orbits are very nearly parallel to one another, and by perspective they seem to come from a single point in the sky known as the radiant. This lies in the constellation Leo, hence the term Leonids.

Leonid Dust Trails 2001
Leonid Dust Trails

This year, however, according to calculations by Armagh Observatory astronomer Dr David Asher, the Leonid meteor shower will be exceptional in having several storm-level peaks on Sunday, 18th November superposed on the normal background level of activity. Peak activity is expected to occur for an hour or so either side of 9:55 a.m., 5:24 p.m. and 6:13 p.m. These times will favour observers in north and central America for the first peak and Australasia for the second and third peaks, and it is expected that up to 800, 2000, and 8000 meteors per hour will be seen. These bursts of meteoric activity are caused by the Earth passing through or close to the centres of dust trails released by the comet in 1767, 1699 and 1866 respectively.

Comet Tempel-Tuttle revolves around the Sun in the opposite direction to the Earth, so when the Earth encounters the dense, discrete trails of dust particles, they enter the atmosphere at very high speeds, about 160,000 miles per hour. Most of the dust grains are very small, vaporizing in a few seconds at heights in excess of 60 miles as they run into the Earth's atmosphere, producing the familiar streaks of light called meteors.

Although this year the times of peak activity do not favour Northern Ireland meteor observers, meteor prediction is an uncertain science, and the Leonids may still provide a surprise. For example, at around 5:00 p.m. after sunset on 17th November the Moon is still passing through a dense trail of dust released by the comet in 1965, leading to the intriguing possibility (given clear weather) of observing occasional flashes of light on the dark portion of the Moon, produced by rare, large Leonid particles as they hit our nearest neighbour. Another chance to observe the same thing happens the following evening at about the same time, with the Moon this time running into the dust released by Comet Tempel-Tuttle in 1833.

In any case, whatever happens to the Moon, it will be worth observing the Leonids in the early hours after midnight especially on the nights of 17/18th and 18/19th November, not just to see the background meteor shower but in case there are any surprise bursts of activity. Those who wish to observe the Leonids are advised to wrap up warm with plenty of layers of clothing, find a clear dark site, and recline in a comfortable chair facing the general direction of the radiant, roughly towards the north-east after midnight.

Prof. Mark Bailey or John McFarland at the Armagh Observatory.
Tel.: 028-3752-2928.

See also:
Armagh Observatory Leonid Pages

Leonid Dust Trail Theories (PDF Format) by David Asher

Viewing Meteor Showers

Last Revised: 2001 November 14th
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