Astronomical Society of Edinburgh
Journal 40

Leonids 1999

The Leonid meteor shower occurs every year between the 15th/20th November and is the result of the Earth passing through the trail of debris laid down by comet Temple Tuttle, the parent comet of the Leonids. Each time the comet passes near to the sun it lays down more debris for the Earth to encounter. The orbital period of comet Temple Tuttle is 33.2 years and it was last at perihelion early in 1998. A normal Leonid meteor shower would typically produce tens of meteors per hour, but roughly every 33 years there is a period of very high activity producing a meteor storm with rates of around 100,000 per hour. The last meteor storm occurred in the USA in 1966 so the 1999 Leonids were eagerly awaited. However, simply adding 33 years to the date of the last period of high activity has been shown not to be reliable and this is in no small way due to the deviant effect of the gravitational attraction on the comet's orbit by the large planets in the Solar System such as Jupiter, Saturn and Uranus.

In accordance with good observational practice, we kept checking the sky on the night before the predicted maximum which was anticipated to be the 17/18th November. The Leonids caught many people out in 1998 by peaking on the 16/17th November, but this year there was no indication of an early peak. Of course you should watch for activity after the predicted maximum too.

Despite the forecast which predicted a widespread frost with northerly winds over much of the country, the 17/18th November was almost totally cloudy with the exception of the vital time. The maximum of the Leonids was estimated to be around 02.00, at which time, the clouds dispersed leaving a 41 minute window of observation. The Moon was not going to be a problem, having set at 00.40hrs. Because of the unfavourable weather elsewhere, Edinburgh was one of the very few places in the United Kingdom to see the Leonid shower of 1999. Lorna, Douglas and Jenni McCalman from Hillside Crescent Edinburgh at 02.10 hrs. The limiting magnitude was 4.4 estimated by using the north polar sequence as described in Neil Bone's book "The Observer's Handbook - Meteors" published by Philips. (This book is currently in our library and is a very helpful guide to the beginner and the experienced observer alike and gives lots of useful information. Why not check it out before the Geminids in December?)

Our city, north facing garden was an unfavourable observing site because of light pollution from surrounding buildings which also restricted the angle of view up to 20 degrees at our imposed horizon, 1/4 of the celestial sphere. Our field of view covered Ursa Major in the east (we could not see the radiant point in Leo), a small part of Auriga overhead, Cassiopeia in the west but very little to the north beyond Ursa Minor.

The meteors came thick and fast and it became an exercise in simply keeping count. We counted 74 meteors in the first 15 minutes and 57 in the next 25 minutes. During the observing time of 41 minutes we counted 131 meteors. Of all the meteors observed, only one was sporadic. One fireball was seen in Perseus at 02.37hrs heading almost due north magnitude -4. with a thick train lasting for 1 second. At least half of the meteors were very bright, magnitude -1 to -2. These were fast moving with trains, but none of the trains was persistent as had been the case in 1998. (Many of the 1998 Leonids were fireballs with spectacular trains.) The remainder of the meteors were around magnitude +2 to +3, fast, faint and many with trains. Most of the meteors were seen in Ursa Major, Ursa Minor or overhead in Auriga, but this could be accounted for by our view restricted to the north only. At 02.40hrs we had 80% thick cloud cover through which we saw a very bright flash probably in Ursa Major, but we were unable to see the meteor. Clouds clearing at 03.55hrs showed that the meteor rate had significantly dropped and the next 10 minutes resulted in observing only 4 Leonids.

We were not the only ones to be frustrated by the clouds. Patrick Moore, on the BBC's News 24, suffered a re-run of his live TV, "cloudy eclipse commentary", only this time it was live TV, "cloudy Leonid coverage". Never mind, according to Rob McNaught, all is not lost. Rob has been working on the mathematical analysis of the orbit of comet Temple Tuttle and he thinks that there is a good chance of a Leonid meteor storm in 2001. For more information check the website of the Astronomical Society of Australia at http://www.atnf.csiro.au/asa_www/leonids.html.

Lorna McCalman