Cutting edge

November 19, 1999

A great Leonid display comes but once every 33 years, lighting up the lives of amateur astronomers everywhere

This week's Leonid meteor shower was one of the most eagerly awaited astronomical events of recent years.

The Leonids recur annually, as Earth sweeps through a stream of millimetre-sized debris shed from comet Tempel-Tuttle. Usually, they are weak, producing a handful of "shooting stars" in an hour. But periodically activity can strengthen and 1799, 1833, 1866 and 1966 brought veritable "storms", in which meteors rained down in their thousands during peaks lasting about an hour.

Leonid activity has been increasing each year since 1994, and with their parent comet having rounded the Sun 21 months ago, another strong display was likely in the early hours of yesterday. The 33-year recurrence of great Leonid displays is tied to the orbital period of the comet and its surrounding enriched debris cloud.

Last year, UK observers were pleasantly surprised by a substantial display of bright Leonids between midnight and dawn on November 17, about 16 hours before Earth's closest approach to the stream's orbit.

Meteor studies are one astronomical field where the naked eye can still be used to great effect. Most data collected by the British Astronomical Association this month will come from observers carrying out watches in which they count Leonids and background sporadic meteors, estimating their brightness and noting the presence of lingering ionisation trains. Pooled results can reveal the fine structure of meteor streams.

My role is to advise and motivate observers and collect and analyse their results. The aim is to cover as many showers as possible, year after year, weather and moonlight permitting. Examination of the BAA archives can help in studies of the long-term behaviour of meteor streams.

For example, computer modelling of December's Geminid shower by Iwan Williams and colleagues at Queen Mary and Westfield College in the early 1980s suggested we should see increased activity and a broadening of the peak as the century ended. This has been confirmed by amateur observations. On the other hand, unusually high August Perseid activity has persisted several years longer than expected following the 1992 return of their parent, Comet Swift-Tuttle.

More recently, modelling of the Leonid stream, bringing in historical observations, by David Asher at Armagh Observatory and Robert McNaught in Australia, has predicted a good display - though not a storm - this month: Asher and McNaught suggest 2001 or 2002 as likelier storm years, with the strongest displays over the United States. Only observations will tell.

Meteor observing, like many areas in amateur astronomy, is undergoing a quiet revolution as a result of the increasing availability and affordability of electronic imaging equipment and computing power.

Among the most sophisticated efforts being deployed this year will be mulitiple-station photographic systems, carefully synchronised to image meteors from locations separated by several tens of kilometres. Over such distances, meteors show a shift against the star background from one station to another. Measurements of this parallax can be used to determine orbital data for individual stream particles. Gathering these in large numbers, particularly, affords the opportunity to refine models of how the Leonids are being pushed and pulled gravitationally by the planets.

Neil Bone is director of the BAA meteor section. His most recent book, Observing Meteors, Comets, Supernovae and Other Transient Phenomena, was published by Springer last year.

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