Unique Eclipses

THE morning of August 11, 1999, was one of unprecedented excitement and chaos in the West Country. It was the scene of the last total eclipse of the Sun to be seen from anywhere in Britain for another 82 years. A total eclipse of the Sun is arguably the most tremendous sight that nature can offer. Only 73 of them will have been seen over British skies between the year l AD and 3000, an average of one every four centuries. I owe this statistic to a splendid short book, UK Solar Eclipses from Year 1, by Sheridan Williams (Clock Tower Press, Leighton Buzzard, £11.95), which meticulously reports and predicts every such event from the birth of Christ until far into the future.

An average of one every four centuries.

What is fascinating is how people reacted to past eclipses in epochs before their causes were known. They always assumed that they sprung from political events. In 733, the Anglo Saxon Chronicle reported in a matter of fact tone: ``In this year Aethelbald captured Somerton; and the Sun was eclipsed, and all the Sun's disc was like a black shield; and Acca was driven from his bishopric." One can imagine the indignation Acca must have felt that the heavens should have connived at his expulsion. The Chronic der Seuchen said of the same eclipse: ``One year after the Arabs had been driven back across the Pyrenees [into Spain] after the Battle of Tours, the Sun was so much darkened on the 19th of August as to excite universal terror." This presumably meant that people saw the eclipse as a sign that the Arabs were returning for reconquest. They didn't––which must have left the forecasters of doom feeling foolish. One reason why total solar eclipses might have appeared to have a supernatural cause is that they happen with such randomness. Some places get many of them, while other places hardly any. They seem to shun Britain's southern metropolitan areas. Mr Williams has discovered that, in the 3,000 years of his survey, London, Dover and Dublin will have an average of eight, while Penzance, the Scilly Islands, York, the Orkneys and Shetlands each are due to average 19. In 1339, the Orkneys and Shetlands had the strangest total eclipse on record. It passed between the two groups of islands without touching either. The path of totality was less than a mile wide, and it lasted only a second! Every habitable planet in the galaxy will have its own distinct style of solar eclipse––if indeed they have moons as large as ours to eclipse their suns. Because the Sun and Moon have almost the same apparent size (because the Sun is about 400 times larger than the Moon, and at the same time 400 times further away) a terrestrial solar eclipse, surrounded by its thin "ring of fire", may be unique to Earth.

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