Storm of stars

On November 17, whales swimming in mid-Atlantic just before dawn were treated to one of the sky's most compelling sights - the Leonid meteors in full spate. On a typical night at a dark spot, one might hope to see maybe 20 meteors an hour. But these meteors at their peak reach hundreds of thousands an hour.

The Leonids appear in our sky by an astronomical happenstance. They are caused by particles emitted by a comet, Tempel-Tuttle, whose orbit runs deep into the solar system - out to the orbit of Uranus, ten times as far from the Sun as we are. On its return to the inner solar system every 33.25 years, its orbit grazes the Earth's, and it leaves behind the tiny particles that produce the meteor storms as the Earth ploughs through them.

The Leonid meteor storms seem to have begun in 902 AD, when they were well seen during the siege of Cosenza in Italy. No records can be found of earlier sightings, so it seems that before then the orbit of Tempel-Tuttle had not intercepted ours. But it was in 1833 that the Leonids first entered scientific thinking, with a spectacular display across the eastern half of North America that was seen by millions.

It was an agreeably multicultural event, Mark Littmann points out in The Heavens on Fire . For the Plains Indians, 1833-34 became the Storm of Stars Winter, while slaves on the plantations of the south were, we are told, deeply affected. Many believed that the meteors meant Judgement Day was at hand - and plenty of whites thought the same thing.

But the most significant observer was one Denison Olmsted, professor of mathematics and natural philosophy at Yale. He was the first man in the United States to suggest that teachers might need training, and had a fascinating life despite the ravages of consumption among his family. And on November 13 1833, his was the prepared mind that met its subject. First and vitally, he realised the importance of the way in which the meteors seem to radiate out from a point in the constellation Leo (hence the name). This means that they are on parallel tracks around the Sun and allows their orbit to be calculated, although Olmsted had trouble distinguishing between various possible interpretations of their path through space. He also estimated their velocity and height and used the persistent trails that some Leonids produce to surmise the existence of high winds in the upper atmosphere. And he realised that these meteors sounded just like another mighty shower seen, by the explorer Alexander von Humboldt amongst others, in 1799.

Olmsted was not right about everything, but he proved, in the teeth of some mockery, that meteors belong in the world of astronomy and are not volcanic by-products or atmospheric effects. The name has the same root as meteorology, but that is where the connection ends.

Even after Olmsted's discovery, nobody knew exactly what meteors were. As Seneca pointed out in about AD 20, they cannot be falling stars - after they have fallen all the stars are still there. But luckily for Olmsted, the answer was coming along soon. Our hero now is Giovanni Schiaparelli, the Italian astronomer best remembered for seeing "canali" on Mars. He saw that the August meteors we call the Perseids and the comet Swift-Tuttle, discovered in 1862 by US astronomers Lewis Swift and Horace P. Tuttle, have orbits that are almost identical.

The proof that these two apparently unrelated types of astronomical phenomenon are connected must have been a revelation, like Edmond Halley's realisation that the great comets seen at 76-year intervals through history were in fact the same object on successive orbits. And this discovery was followed by the finding that Comet Tempel-Tuttle of 1861 (found by Tuttle again and Wilhelm Tempel in France) was the parent of the Leonids. Since then the cometary sources of almost all major meteor showers have been uncovered. Halley's comet has two.

To get a spectacular meteor shower, a comet does not need to be prominent in its own right - only in 1366, it seems, has Tempel-Tuttle been visible to the naked eye. But it does have to cross the Earth's orbit. Hale-Bopp looked terrific but has left no lasting debris on a collision course with the Earth.

Littmann tells this story well, aided by much colourful biography (Tuttle was a naval hero of the US civil war, but he was later sacked by the US Navy as a paymaster for embezzlement only to be reemployed by them as an astronomer). It is also enhanced by eyewitness accounts of great Leonid storms (Europe in 1866 and the US in 1966 notable among them) and details of how to observe Leonids and other meteors. And the images, especially of the 1833 Leonids, are astounding.

On a note of criticism, the book contains one chapter, on meteor impact hazards, that has nothing to do with the Leonids, which are harmless specks that burn out more than 100km above our heads. The existence of Hollywood blockbusters on the subject does not justify dragging it in here.

Despite this reservation, this book is clearly a labour of love for Littmann, who is as comfortable describing advances in modern meteor science as he is with ancient Leonid sightings. It will be read with pleasure by everyone from amateur astronomers to meteor professionals. The author will be glad if you buy the book, but even more pleased if it sends you out next November to look for the 1999 Leonids.

Martin Ince is deputy editor, The THES .