The 1999 return of the Leonid meteor shower, bringing with it the potential for a storm to rival those seen at roughly 33-year intervals in the past, was certainly one of the most eagerly-anticipated astronomical events of recent years. The shower did, indeed, deliver a spell of very high meteor rates, with most of the exceptionally strong activity compressed into a two-hour interval centred around 02h Universal Time (UT, equivalent to GMT) on the night of 17-18 November.

This spectacular image of a 1999 Leonid was taken from Dagali, Norway, at 0225 UT on November 18. The image was captured using an Olympus OM-1, with a 24mm lens @ f2.8, on Fuji Super G Plus 400 exposed for five minutes. The fireball -- estimated to have reached a magnitude of -8 -- cast shadows on the ground and left a smoke trail that hung in the air for some 15 minutes. Photo: Copyright 1999 Arne Danielsen.
 

The display, with Leonid meteors raining down at the rate of at least one every second, and often higher (some observers reported bursts in which between five and eight meteors were visible simultaneously), was best seen from the clear skies of the Mediterranean and the Sinai Desert. Unfortunately, for those of us left behind in the UK, the weather was less than cooperative, with clouds and rain moving through on the critical night.

Analysis of the 1999 reports continues, but the clear picture is of rapidly rising activity after a slow start, from 01h UT onwards, with the highest rates coming in the five-minute interval around 01h 58m UT. Calculated equivalent zenithal hourly rates (EZHRs) for this period, making allowance for the altitude of the shower's radiant and the sky transparency, come out close to 2,500. This doesn't mean that observers saw this many meteors per hour -- far from it, the activity was at this level for only a few minutes. Use of EZHRs, calculated by scaling up short-interval counts, gives us a practical compromise, enabling comparison to be made with more conventional analyses -- based on complete hours of observation -- made from results at other times.

Prediction power
Occurrence of the highest EZHRs around 1999 November 18d 01h 58m UT has been presented as a triumph of prediction for Robert McNaught and David Asher, who have developed a working model of the Leonid meteor stream based on "arcs" of debris, that are shed around the stream's orbit on successive perihelion returns of its parent comet: 55P/Tempel-Tuttle. By allowing for gravitational perturbations by the planets, Asher and McNaught have presented forecasts of which arcs should be encountered, and at what time, in the several years around the comet's most recent (1998) perihelion -- a period already marked by elevated Leonid activity. The 1999 peak came within a few minutes of Earth's expected encounter with the debris arc ejected from the comet in 1899. Other sub-peaks on the profile can be ascribed to material shed in 1933 and 1966.

McNaught and Asher have gone as far as suggesting that 1999 was not the strongest Leonid peak of the current series. According to their model, the best is yet to come -- over the Far East at around 18h UT on 2001 November 18, when we run through the debris arc from 55P/Tempel-Tuttle's 1866 return. In the meantime, there is hope for those of us who were robbed of the spectacle by clouds in 1999, in the Leonid forecast for 2000.

A second chance?
The McNaught-Asher model predicts an encounter with the strand of debris from the comet's 1733 return close to 03h 44m UT on 2000 November 17-18 (conveniently, a Friday night to Saturday morning). Returns such as this will offer a more stringent test of the model, and serious meteor observers are preparing to cover this part of the Leonids in 2000 despite rather unfavourable moonlit conditions. A further test will come after dawn has reached western Europe: observers in North America have another bite during the predicted encounter with 1866-vintage debris at 07h 51m UT on the same Saturday morning.

Leonid meteors come from a radiant in the "Sickle" of Leo, which doesn't rise until about 23h local time on a mid-November night. Most Leonids are seen when the radiant is high, during the early-morning hours.

In 2000, there is the added complication of moonlight. The Moon is at last quarter on November 18 and, at the time of the forecast Leonid peak, will lie in Cancer, fairly close to the radiant. Indeed, the Moon rises not long before the radiant, and remains in the sky for the rest of the night.

Meteor observers regard the Moon as a malign influence -- it swamps fainter events, making reliable counts difficult. During last August's Perseids, some observing was still possible with the 10-day-old waxing gibbous Moon low in the southwestern sky. Observers should find the waning, last quarter Moon somewhat less troublesome for the Leonids, but its light will still be a nuisance. The best plan would be to observe in a direction away from the Moon -- looking towards the Plough or elsewhere in the northern sky. The Moon can also be hidden behind a suitable obstruction -- buildings or trees at the observing site, for example. Being unable to see the radiant will not necessarily be a disadvantage; if rates get high, it will be pretty obvious where the meteors are coming from.

Observing the Leonids in 2000
Under normal circumstances, meteor observers carry out watches for intervals of, ideally, hours at a time, recording the appearance times, magnitude and type (shower membership or whether sporadic) for individual events. Coupled with an estimate of sky limiting magnitude (if faint stars can't be seen, neither will be faint meteors), such reports allow activity profiles to be determined. Usually, observers prefer to make watches where the limiting magnitude is +5 or better; on this occasion, with the Moon in the sky, limiting magnitude may be no better than +4.5 to +5.0.

The Leonid Radiant, in the "Sickle", does not rise until late in the night, and is highest in the early-morning hours. AN graphic by Mark McLellan.

Should activity become high, normal practice may have to be abandoned. It might well become impracticable to try and record times and magnitudes for individual meteors. Instead, observers should concentrate on simply recording Leonid numbers in 5-minute intervals. Obviously, to be useful, reports should include details of the start and end times for which the counts apply, and limiting magnitude information will still be required. If last year's storm was anything to go by, such counts will be required mainly for a two-hour period centred on the peak. Most observers will probably concentrate their efforts into the interval from about 02h UT, when the radiant is beginning to gain a respectable altitude in the eastern sky, until around 06h UT.

Photographers may wish to keep a camera handy. Moonlight will preclude long-duration exposures, but if the shower gets busy, it should be worth attempting shots lasting a couple of minutes at a stretch. Fast film -- ISO 400, preferably -- and a standard 50mm f/2 or 28mm wide-angle f/2.8 lens will give the best results. Obviously, the frame will have to be aimed away from direct moonlight. Photography will only record meteors brighter than magnitude 0, however, and it should be noted that these may not be all that abundant in the 2000 Leonids, even under high-activity conditions.

The storm peak last year delivered rates of up to 60 meteors per minute. Given that many were faint, and such events will be drowned by the moonlight, should a similar-strength display (as forecast by McNaught and Asher) materialise, then observed rates might be closer to 15-20 Leonids per minute at best.

Of course, there is no guarantee that exceptionally high rates will occur. Only observations will tell! No matter how good a model may be, the ultimate proof of its accuracy has to come from observers in the field at the critical time, and it is important that as many as possible go out on the key night, and report what they see. As in previous years, the BAA Meteor Section will be the main clearing house for UK-based observations.

An Alternative View
Robert McNaught and David Asher are not the only modellers trying to forecast Leonid activity. An alternative "isoline" method, based on a theoretical debris cloud of varying density surrounding the nucleus of Comet 55P/Tempel-Tuttle has been developed by Ignacio Ferrin, of the University of the Andes, in Venezuela. Ferrin's latest predictions, published in the current issue of the Journal of the BAA, contrast markedly with those of McNaught and Asher. Ferrin believes that 2000 will produce the strongest return of the current Leonid series, and puts the timing almost a day earlier, on 17 November at 09h 20m UT. This possibility will be open to checking by observers in North America. From 2001 onwards, Ferrin predicts that peak Leonid activity will decline markedly.

If, on the other hand, the Asher-McNaught forecast prevails, proving the reliability of the debris-arc model, their predictions for good activity in 2001 and 2002 may well encourage those who can afford the air fare to travel in the hope of seeing the respective displays over Asian and American longitudes.

Whichever model proves correct, there is still a lot of life in the Leonids, and observations of the shower will be carried out with considerable interest in 2000, despite the moonlight. Even as late as 2006, we may still be seeing activity to rival that of the Perseids. The possible strong return this November 17-18 may yet make up for the disappointment endured by many of us in 1999.