Total eclipse

10am 11 August 1999

A few minutes before 10am on the morning of 11 August, the Moon will begin to take a bite out of the Sun. Just over an hour later, if you happen to be in Cornwall, the Moon will have totally swallowed our star in what will be Britain's first total solar eclipse in 72 years. It promises to be as vivid a demonstration of nature's hidden depths as it was to the ancients, who could only stand in awe of a spectacle that they could hardly predict, let alone comprehend.

Totality - the point at which the Moon completely blocks our view of the Sun - will last only a minute, but for those who witness the event it promises to be the longest minute of their lives. Few people who have seen a total solar eclipse fail to be moved by the event. How strange it must be to have an unexpected appearance of night-time during the day, when a bizarre, metallic-grey light permeates the ether.

On 29 June 1927, the last time a total eclipse occurred in Britain, a journalist for the Daily Express described how he felt as the event unfolded on the playing fields of Giggleswick Grammar School in Yorkshire. "I caught my breath as I looked up. The grey shadows near me were really men and women but they looked like ghosts. Their faces were dry and dead. This, I thought, is what it must be like to be dead - grey, colourless and quiet, all standing together in an unearthly light, waiting, rather frightened and full of unspeakable revenge."

If this is how a 20th-century mind can perceive an eclipse, just imagine the panic and trepidation the event must have inspired in the past, before people fully understood that they were the result of a rare but explicable alignment of the Sun, Earth and Moon?

The ancient Chinese thought they were caused by the Sun being devoured by a dragon, who had to be frightened away by beating drums, shooting arrows into the air and generally causing as much commotion as possible. Legend has it that two royal astronomers in the 22nd century BC failed to predict an eclipse owing to drunkenness, incurring the wrath of the Emperor, who had them beheaded for fear that the dragon might never again be scared away.

As bad omens go, eclipses are about as bad as they come. History is replete with stories of solar and lunar eclipses presaging something dire and awful (a lunar eclipse occurs when the Earth comes between the Sun and Moon and casts a shadow over the planet's natural satellite). Sometimes it has become a self-fulfilling prophecy, such as when a lunar eclipse occurred in 413BC.

Athens and Sparta were waging the Peloponnesian War and the Greeks, led by Nicias, were just about to engage their enemy at a crucial battle at Syracuse on the south coast of Sicily when it happened. Taking advice from his soothsayers - the spin doctors of the ancient world - Nicias delayed his army's departure for almost a month, giving time for the enemy to build up its forces. The Athenians lost the battle and Nicias was killed in action.

It was, according to Christopher Walker, keeper of western Asiatic antiquities at the British Museum in London, a classic example of how fears about an eclipse altered the course of history. Almost every ancient civilisation turned the awe of witnessing a solar or lunar eclipse into a dread. But eclipses were useful for one thing. They gave kings and generals an excuse for putting off an inevitable battle. "People wanted omens because they wanted to avoid trouble," Walker says.

No wonder there was a premium on prediction. There is some evidence to suggest that the ancient Chinese may have been able to do it. The Babylonians certainly seem to have learnt the trick, as did the Greeks, who probably got the idea from Babylonia.

The key to eclipse prediction is the Saros cycle, the period of 6,585.3 days (18 years, 11 days and 8 hours) that governs the recurrence of lunar and solar eclipses. The Chaldeans, the ancient Semitic people who controlled southern Babylonia from the late eighth to the late seventh century BC, knew about the Saros cycle, although they did not call it by that name; it was first coined by Edmund Halley, the great English astronomer, who "rediscovered" the cycle in the 17th century.

The Chaldeans used their knowledge of the Saros cycle to make accurate records of solar eclipses. The cycle was particular useful for predicting lunar eclipses, which are inherently easier to forecast because a lunar eclipse results in a large shadow of the Earth being cast over the Moon. This means that a lunar eclipse can be seen anywhere in the world where the Moon is above the horizon.

A solar eclipse is more difficult to forecast for a given region of the Earth because the Moon casts a smaller shadow on the Earth. This means that if you are outside the area covered by the shadow you have no idea whether or not an eclipse is taking place over the horizon. The Chaldeans did not know about this because they did not understand the motion of the planets, but at the height of their empire they did seem to understand something about the Saros cycle.

The ability to predict eclipses imbued people with enviable power. Herodotus, the Greek historian of the fifth century BC, claimed that Thales of Miletus had predicted the total solar eclipse of 585BC which occurred during a battle between the Medes and the Lydians. The eclipse, taken as a bad omen by both sides, appears to have forced the two opposing armies to agree a peace treaty after five years of war. Duncan Steel, an astronomer, whose book Eclipse is published on 22 July, doubts whether Thales really could have predicted its date and circumstances. "Thales does seem to have understood the rudiments of solar eclipses, recognising that they are due to the Moon passing in front of the Sun, although in his day the nature of orbits was unsuspected. Thales thought of the Earth as a flat disc floating on a great sea, the Sun and Moon being other discs moving above it which sometimes happened to align," Steel writes.

Modern astronomers have back-calculated the eclipse in question and tracked its path of totality - the swath of land swept by the Moon's shadow - which on the afternoon of 28 May crossed west to east from the Mediterranean to Asia Minor, where the battle took place. It blanked out the Sun for a full six minutes.

"It was an unusual event, but Herodotus wrote only that Thales gave the year, making one wonder whether it was a true prediction or just a lucky guess. Predicting partial solar eclipse is one thing, but getting a total solar eclipse right is another matter entirely," Steel says. It seems likely that Herodotus had embellished Thales' predictive capacity, and that though the Greeks used a primitive knowledge of the Saros cycle to make accurate estimates of past eclipses, they did not in fact use it for predictions. "The eclipse knowledge gathered by the Babylonians lay dormant for many centuries," Steel believes.

Nearly 2,000 years later, the power of predicting eclipses became apparent when Christopher Columbus was holed up on the Caribbean island of Jamaica with a mutinous crew, a bad case of a worm-eaten ship and a bunch of local natives who refused to provide what remained of his crew with water and rations. The cunning Italian, however, possessed a copy of the Ephemerides, published by Regiomontanus in 1474, which predicted a total lunar eclipse on the evening of 29 February 1504.

Columbus was desperate for provisions and invited the natives on board his ship. During the entertainment he explained that he believed in a powerful God who was angry at their failure to help him. As a measure of His anger, the Moon would turn blood red before disappearing from view. When the eclipse began, the natives were understandably alarmed, pleading with Columbus to intercede on their behalf.

"Columbus was too smart to agree immediately," Steel writes. "For added effect he retired to his cabin, knowing that the total phase would last for about one-and-three-quarter hours. Having timed his withdrawal with a sandglass. Columbus re-emerged at the appropriate time to tell the Jamaicans that he had consulted God, and persuaded Him to cease the shielding of the Moon, so long as they promised to behave themselves and supply the Spanish for so long as they needed to stay."

Needless to say the natives agreed, and a few moments later the Moon "miraculously" reappeared. The indigenous Jamaicans made sure that Columbus continued to be fed and watered until a rescue ship appeared some months later. It was yet another little piece of history that might have turned out differently if the eclipse had never happened.

See Saturday's newspaper for a chance to win one of 10 pairs of tickets to travel by train to Cornwall to view the eclipse on 11 August