Halley And Ancient Eclipses

However we spell or say his name, Halley's interest in eclipses provides a bridge between the subjects of Chapters 6 and 7. In the previous chapter we saw that ancient eclipse records have allowed scholars to investigate how Earth's rotation rate has slowed over the past few millennia, with various astronomical and geophysical ramifications. More than three centuries ago Halley was interested in this apparent slowdown—he was the first to notice it—but from a rather different perspective.

In his era, appointments to university positions in Britain were heavily influenced by religious considerations. Various factors counted against Halley when he was an applicant in 1691 for the Savilian astronomy professorship at Oxford University. He even held the heretical view that comets (such as that bearing his name) could smash randomly into the Earth, causing great devastation. This did not fit in well with ecclesiastical views on divine providence. Mostly, though, his opponents were disquieted by his notion that the world might be older than the biblical chronology would indicate.

Learning from his failed application, Halley gained the religious-bias initiative in the following years through his study of ancient eclipses. In October 1693 he read a paper to the Royal Society ". . . concerning a Demonstration of the Contraction of the year, and promising to make out thereby the necessity of the world coming to an end, and consequently that it must have had a beginning, which hitherto has not been evinced from any thing, that has been observed in Nature." What Halley showed was that the times of eclipses spread over millennia could only be explained if the number of days within a year were reducing. This must indeed be the case, because the absolute duration of the year stays constant, but the days are lengthening, as we saw in the preceding chapter.

Halley's interpretation of this apparent elongation of the year, based on Christian dogma, was that the age of the world must be finite. The universe, he said, must have been a divine creation ex nihilo a handful of millennia before. The academic selection panel—the members were not only from within the University of Oxford, the Archbishop of Canterbury for example being among them—regarded this most favorably. They looked upon religious correctitude as being of the utmost importance, and Halley's careful demeanor during the 1690s had the end result that he was successful in obtaining appointment to the Savilian Chair of Geometry in 1704.

Halley was skilled at computing the past tracks of totality over foreign lands after his earlier work. Looking forward, he recognized that in 1715 a total solar eclipse would sweep across southern England and Wales, the first time that London had been so-visited since 1140 (and 878 before that). He turned his hand and mind to computing its precise course, and organizing observations.

A detailed predictive map that Halley prepared is shown in Figure 7-1. A pamphlet that was widely disseminated at the time, showing such a map, was entitled The Black Day or a prospect of Doomsday exemplified in the great and terrible eclipse which will happen on the 22nd ofApril 1715. If the simple information that

FIGURE 7-1. The ground track over England and Wales of the solar eclipse of 1715, as computed ahead of time by Edmond Halley. In reality the track was slightly wider. This was by just a few miles at the northern extreme, but with a southeasterly displacement of about 20 miles for the southern boundary (compare this pre-eclipse prediction with the post-eclipse map, also drawn by Halley, as shown in Figure 7-2).

FIGURE 7-1. The ground track over England and Wales of the solar eclipse of 1715, as computed ahead of time by Edmond Halley. In reality the track was slightly wider. This was by just a few miles at the northern extreme, but with a southeasterly displacement of about 20 miles for the southern boundary (compare this pre-eclipse prediction with the post-eclipse map, also drawn by Halley, as shown in Figure 7-2).

an eclipse was to occur didn't rustle up public interest, that pamphlet was sure to do so.

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