The Capital Eclipse Of 1715

A total solar eclipse has not crossed England's capital city since that day in 1715. Nor will London see such an event again for some time to come. Three centuries ago, the eclipse ran from about eight until ten in the morning, with totality lasting for a few minutes at around ten past nine. At least, that was the time in London. Not only did the shadow reach other locations at different absolute instants of time, but also in those days there was no standard time in Britain, each town keeping its own clock time according to the Sun's position, making the nationwide comparison of observations difficult. Regarding the date of the eclipse, we will come to that at the close of this chapter.

Nowadays any eclipse is gazetted well in advance, so that amateur and professional observers alike are well prepared, but that was not the case in Halley's era. He wrote to a wide variety of potential observers. From the rectors of village churches and the like he received a flood of useful information, allowing him to determine the path of totality with admirable accuracy. Not only that, but the comparative timings for the duration of the eclipse were very useful check readings, as these would be longest near the central line, dropping to zero at the edges of the path, and the duration would also vary along the track due to the Earth's curvature.

The amateur observers did well then—but what about the professionals? In Cambridge, the Plumian Professor of Mathematics, Roger Cotes, tried to time the eclipse but was distracted by what he termed "too great Company," and so he did not obtain the necessary data. Halley himself was in London for the eclipse, gathered with various other fellows of the Royal Society. That is just as well, because Oxford was clouded out. Under Halley's guidance, his group obtained useful timings.

If central London were clear, as it was, then one would anticipate that the astronomers at the Greenwich Observatory must also have made detailed observations. They may well have done so, but in a spirit of fine scientific collaboration the Astronomer Royal, John Flamsteed, refused to allow Halley direct access to the Greenwich data, which were never published. Halley succeeded Flamsteed as Astronomer Royal in 1720, and it is surprising that he did not himself dig out the 1715 eclipse observations thereafter, although to be fair he was always busy with new scientific tasks.

Be that as it may, what Halley really needed was not lots of observations from just one place, but rather information from a wide geographical scatter. That way he would be able to determine the width of the ground track. If some curate standing in his churchyard saw a brief instant of totality, and yet the verger sent to the crossroads in the village a mile to the east did not, then Halley would know that the edge of the shadow had passed between them. Thus the precise positions of the observers, plotted onto a map, were important.

This is just what Halley got. Some dozens of reports were supplied by correspondents scattered over England and Wales, enabling him to determine the northern and southern extremities of the track to within a mile or so. For example Halley was soon writing: "From these observations we may conclude that this Limit came upon the coast of England, about the middle between Newhaven and Brighthelmston [Brighton] in Sussex." Similarly he found that in Wales the northern limit ". . . entred on Pembrokeshire about the middle of St Brides Bay."

Comparing these points with Halley's pre-eclipse prediction (Figure 7-1) we see that he was inaccurate, by only 3 miles for the northerly limit, but by 20 miles for the southerly. The ground track that Halley determined from the observations is shown in Figure 7-2; it was about 183 miles wide, 23 more than his prior estimate.

That might initially seem peculiar. One could understand the track being uniformly displaced in one direction or another due to slight timing errors, but how could its width be wrong? The answer lies with the lack of precise evaluations of astronomical distances in that era. Later in this book we will discuss how James Cook was sent to the South Pacific in 1769 specifically to watch the transit of Venus across the face of the Sun, as part of an attempt to measure more accurately the mean solar distance from the Earth. Halley was one of those who invented the technique employed in that episode. Back in 1715, Halley could not be sure of the distances and sizes of either the Sun or the Moon, and in consequence he substantially underestimated the width of the eclipse track.

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