The doctrine of Gravitation seemed to
show that just as the planets revolved around the sun in ellipses, so
also must the comets. The orbit, however, in the case of the comet,
is so extremely elongated that the very small part of the elliptic
path within which the comet is both near enough and bright enough to
be seen from the earth, is indistinguishable from a parabola.
Applying these principles, Halley thought it would be instructive to
study the movements of certain bright comets, concerning which
reliable observations could be obtained. At the expense of much
labour, he laid down the paths pursued by twenty-four of these
bodies, which had appeared between the years 1337 and 1698. Amongst
them he noticed three, which followed tracks so closely resembling
each other, that he was led to conclude the so called three comets
could only have been three different appearances of the same body.
The first of these occurred in 1531, the second was seen by Kepler in
1607, and the third by Halley himself in 1682. These dates suggested
that the observed phenomena might be due to the successive returns of
one and the same comet after intervals of seventy-five or seventy-six
years. On the further examination of ancient records, Halley found
that a comet had been seen in the year 1456, a date, it will be
observed, seventy-five years before 1531.
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