Edmond Halley And Eclipses

The first real predictor of eclipses will come as a bit of a surprise. Edmond Halley (1656—1742) knew that the comet bearing his name would come back in 1758, long after his death, and said he hoped that when it did appear it would be recalled that it was an Englishman who had foreseen its return. But Halley has another claim to fame with respect to predictions: in the modern era it was he who recognized how to use the saros to pre-calculate eclipses. In fact, his contemporaries considered that he had discovered that cycle, not realizing that the Babylonians and Greeks had known of it so long before, the understanding having been lost. It was Halley who gave the saros its name.

From the late seventeenth century Halley was one of the lions of the Royal Society of London (see Figure 3-4). His scientific interests were many and various. In 1693 alone Halley read papers at meetings of the Society covering such disparate subjects as:

• How to determine the positions of the tropics

• The pressure within a diving bell

• How the length of the shortest day varies with latitude

• How deformed fingers are inherited within some families

• Mortality rates and annuities

• How crabs and lobsters regrow amputated claws

• A hydrographic survey of the coast of Sussex, in the south of England ecihivni». iiAi.ucrvs u..H

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FIGURE 3-4. Edmond Halley pictured in his younger days, shortly after he discovered the saros. The inscription shows that, apart from being a doctor of laws, Halley was also Savillian Professor of Geometry at Oxford University and Secretary of the Royal Society of London. Later he took up the appointment of Astronomer Royal.

Obviously he was a very busy man.

Halley's investigations of eclipses was a recurring theme, and the previous November he is recorded to have given ". . . an account of the Eclipses of the Sun and Moon to bee computed by an easy calculus, from the Consideration of the Period of 223 Months, shewing how to aequate between the extreams of the excess of the odd hours above even days, which is always between 6.20 and 8.50. He produced a Table ready calculated for this purpose, and shewed the use thereof. Which he promised to exemplify against the next Meeting." That, in effect, is the announcement of the discovery of the saros, Halley having recognized even the limits to the odd hours and minutes above any particular 18-year plus 10- or 11-day period. The following week "Halley shewed a Paper wherein he had computed the Eclipses of the Moon in severall Series, and said, that he found, that he could very well represent them all; much nearer than they were observed by the severall observers." How could one predict something more accurately than it could be observed? The answer is that Halley had found that lunar eclipses predicted using the saros provided a more precise timepiece than the mechanical clocks used by the observers, and for matters of navigation that was potentially a most valuable discovery.

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