Modern Reanalysis Of Halleys Results

In the late seventeenth and early eighteenth centuries astronomers at the Paris Observatory made micrometer measurements of the apparent solar diameter, and in the 1980s French scientists compared these with modern values. They concluded that, if the old measures were correct, then three centuries ago the Sun was about one part in 480 larger than it is now. That is an appreciable fraction from the perspective of the possible climatic effect.

The problem is this: How accurate were those observations made 300 years ago, barely a century after the telescope was first used to peruse the Sun? If they were good, then the Sun must be shrinking, which might cause it to heat up, flooding the Earth with an increased flux of sunlight, thus adding to the greenhouse effect. Alternatively, if those early measurements of the Sun's size made from Paris were imprecise, then we might be able to discount such a possibility.

What was needed to solve this question was some alternative determination of the solar diameter from a few centuries back, but of greater precision. The precision attainable from direct measure ments, however, was limited by the available technology, whether the telescopes used were French, Italian, or British. Some analogue determination of greater accuracy was required: an indirect measure.

Halley's detailed account of the reports he received of the eclipse from far-flung parts of Britain provides just such an analogue, as was realized in 1988 by Leslie Morrison and Richard Stephenson (whose work on old eclipses has already been mentioned), along with their colleague John Parkinson. If the Sun had been larger by almost 0.2 percent in 1715, then the limits of totality would have been about 6.5 miles narrower, just over 3 miles at both northern and southern edges. Coupled with our modern knowledge of the distances and orbits of the Sun and Moon, the actual observations of totality (or lack of it), which Halley preserved verbatim, enabled this team to determine the edges of the track to within a few hundred yards. As the Moon has not changed size, the small residual uncertainty implies that the Sun has not shrunk by as much as one part in 20,000 since 1715.

Halley's remarkable records of the observations of the eclipse of 1715 remain not only an exemplar of a great eighteenth-century scientist at work, but also the best evidence we have that the Sun has not greatly altered in size over the past several centuries.

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