Association of Falkirk Astronomers

By Dr Russell Cockman

The Leonids meteor shower is a result of the Earth passing through streams of meteoroids laid down by Comet Tempel-Tuttle during its 33-year orbit of the sun. Every 33 years the Earth passes through the densest parts of these streams leading to hugely enhanced rates of meteors, even to storm levels (greater than 1000 meteors per hr). Leonid storms were observed in 1966 and 1999, but because of small changes in the Comet's orbit caused by Jupiter's gravity, the Earth was predicted to pass through dense debris streams again in 2000, 2001 and 2002.

Analysis of the Leonids in 1999 and 2000 by Robert McNaught, of the Australian National University, and David Asher of the Armagh Observatory, generated a remarkably accurate prediction (to within a few minutes) for the timing of the Earth's passage through the dust streams. Their refined model predicted that the Earth would again pass through dense meteoroid streams at 10:00 UT and 18:00 UT (approx) on the 18th of November. Three other research groups also put forward predictions of when and where various peaks of activity would occur, and how many meteors per hour would be visible at the peak time. Although timings were in general agreement, rates were varied, but it must be remembered that meteor shower forecasting is in its infancy- serious Leonid forecasts only go back to 1998!

Unfortunately for observers in this part of the world, peak activity would occur during our daytime when the radiant was below the horizon. The most favoured locations in the world would be the west coast of the US and the Pacific area, Japan, North Western Australia, the Philippines and Hawaii. Overall, predictions for North America ranged from 800 to 4,200 meteors per hour. For parts of Asia and Australia peak hourly rates of 8,000 or more were suggested; peak hourly rates were expected to occur during short bursts lasting 30 minutes or less.

And the results'? Well, true to predictions the Leonids put on a magnificent show in 2001. The event was well documented by scientists and will live forever in the memories of thousands of amateur astronomers and first-time viewers. Observers across North America described the show as "fabulous", "unbelievable" and "the most beautiful thing". Some saw as many as six meteors in the space of about a second and a few meteors lit up the sky. Others described fast-moving meteors zooming across all parts of the sky, sometimes leaving smoky green trails.

In New Mexico, western USA, a group of scientists reported an hourly rate of 800 shooting stars; other groups observing in the Southwest gave preliminary estimates in the neighbourhood of 2,000 meteors per hour for a short stretch of time with peak activity centred around 10:15 UT.

A stronger display was expected in Australia and parts of eastern Asia. A preliminary report from a group of NASA scientists in Hawaii claimed an hourly rate of 1,250. Observations made at the Learmonth Solar Observatory in NW Australia indicated that peak rates were 3,000-4,000 per hour, whilst one early report from China suggested that rates may have exceeded 2,000 per hour.

On Palau, an island east of the Philippines in the Pacific, British Astronomical Association observers had a good view despite patchy cloud. Calculations of observed hourly rates and equivalent zenithal hourly rates suggest that peak rates approached 4,000 at 18:05 UT, although the maximum observed rates were no more than 1200 per hour. It was a spectacular display, with lots of fireballs and bright meteors, some as bright as mag '9!

AFA's Director of Observations observed from the fabulously dark sky location of La Palma in the Canary Islands. Although storm activity was not expected, the favourable weather in the region offered promise of viewing a pleasing number of meteors and perhaps a few fireballs on the night of the 17th/18th. Sadly, that was the only cloudy night during our seven-night stay!

Leonid counts were conducted during the mornings either side; on the morning of the 17th no Leonids were noted between 05:00 UT and 05:30 UT, whilst there was more success on the morning of the 19th where 53 Leonids were observed between 01:00 UT and 04:00 UT when looking south. Zenithal Hourly Rates were estimated at around 100+. One meteor was around mag '3 and several were around mag '1. A satisfying result!

A storm is again predicted in 2002. However, the full moon will interfere, but any fireballs present should be visible.