The Eclipse - 11.11am, 11.8.99: History and Fiction; From Babylonia to Tintin

THE ABILITY to predict an eclipse has got some people out of tricky situations - and failing to predict them has landed others in serious trouble.

Tintin, the cartoon hero, escaped being sacrificed by a tribe of South American sun worshippers by forecasting an approaching eclipse - courtesy of an old newspaper cutting. A convenient pocket almanac of future eclipses enabled Rider Haggard to rescue one of his heroes in King Solomon's Mines, who was threatened by a bunch of bloodthirsty natives. And Mark Twain's hero in A Connecticut Yankee in King Arthur's Court escaped from being burnt at the stake by, you've guessed it, forecasting an eclipse.

Myth has it that two Chinese astronomers, the brothers Hsi and Ho, failed to predict a solar eclipse because of a prior appointment with a bottle or two of wine and lost their heads as a result. The Chinese, in common with many ancient cultures, took eclipses very seriously, believing they were caused by the Sun being devoured by a dragon, which had to be frightened away by making as much noise as it was possible to create.

But observing and cataloguing eclipses, which the Chinese were rather good at, is not quite the same as being able to calculate when and where they are expected to appear. The ancient Babylonians, or more specifically the semitic people known as the Chaldeans, seem to have come closest to understanding the complicated cycle that determines both lunar and solar eclipses.

But the man who really knew about forecasting eclipses was Edmund Halley, the great 18th-century English astronomer who, among other things, gave his name to Halley's Comet - that other astronomical omen of doom.

Halley worked out the basis of eclipse prediction mathematically and called it the Saros cycle, a word he took from ancient Babylonian texts. He discovered that the Sun, the Moon and the Earth moved in such a way that after 6,585.32 days (18 years, 11 days and 8 hours) they ended up back in almost the same relative positions.

Once the Saros cycle was understood, scientists could calculate down to minutes and seconds when, where and how long the shadow of a total solar eclipse would fall. It is not easy because, for one thing, the orbit of the Earth around the Sun is slightly elliptical, meaning that it travels faster around the star in January than in July.

Even minor errors in the positions of the Sun, Moon and Earth can have dramatic effects on the accuracy of an eclipse prediction. Scientists will be monitoring the exact position and timing of totality to compare the actual events, as they happen, with predictions of when they should have occurred. This should not make any difference to the people gathering in Cornwall this Wednesday, because the differences between what is predicted to happen and what actually happens can be measured only with the most precise instruments.

Eventually, in many thousands of years from now, total eclipses will be a thing of the past. As the Moon moves further away from the Earth its disc in the sky will become too small to cover the Sun completely. Catch it while you can.