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as well for eclipses in the past. When astronomers try to place the moon at a date in
the past when there was known to be an eclipse, their equations often say there was no
such eclipse, or place the eclipse at a different location from where it was observed.
The easiest solution to this problem is to add a small term to the equations of the moon's
motion that will place the eclipse in the right time and place. The change in motion is
called the “ secular acceleration. ”
The existence of this acceleration was first suspected by Edmund Halley in 1692,
and was confirmed by astronomers in the eighteenth century.
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An accurate value for
this acceleration is quite important, because without it, it becomes very difficult to
predict the moon's motion more than a century or two away from the present. Unfor-
tunately calculating this value is rather challenging, as one needs an accurate location
for the moon in the distant past. Reports of historical eclipses provide exactly this
data—when and where the eclipse was seen. Using historical records proved to be
problematic, however, and there was significant disagreement about how best to go
about it.
The Nineteenth Century
Astronomers had begun working with ancient eclipses to determine the secular accel-
eration in the late seventeenth and eighteenth centuries. In the nineteenth century they
became more reflective of the difficulties involved in using historical sources for precise
calculations. In the middle of the century, the Astronomer Royal George Airy argued
that Laplace's powerful lunar theory and Hansen's new tables made obsolete all previous
work, and demanded a recalculation of the secular acceleration. In this process he raised
a simple but profound problem. In a solar eclipse the moon can cover varying amounts
of the solar disk (characterized as the magnitude of the eclipse), and different magni-
tudes indicate different locations for the moon. Building on earlier work by the famous
eclipse hunter Francis Baily, Airy argued that one well-known eclipse must have been
total, thus giving even more precise data than previously available. This eclipse, known
as the eclipse of Thales, was reported by the historian Herodotus as having stilled a
great battle in Asia Minor, which Airy drew out as evidence for totality: “I have myself
seen two total eclipses (those of 1842 and of 1851), being on both occasions in the open
country; and I can fully testify to the sudden and awful effect of a total eclipse. I have
seen many large partial eclipses, and one annular eclipse concealed by clouds; and I
believe that a body of men, intent on military movements, would scarcely have remarked
on these occasions anything unusual.”
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