Each of the
planets solicits the comet to depart from its track, and though the
amount of these attractions may be insignificant in comparison with
the supreme controlling force of the sun, yet the departure from the
ellipse is quite sufficient to produce appreciable irregularities in
the comet's movement. At the time when Halley lived, no means
existed of calculating with precision the effect of the disturbance a
comet might experience from the action of the different planets.
Halley exhibited his usual astronomical sagacity in deciding that
Jupiter would retard the return of the comet to some extent. Had it
not been for this disturbance the comet would apparently have been
due in 1757 or early in 1758. But the attraction of the great planet
would cause delay, so that Halley assigned, for the date of its
re-appearance, either the end of 1758 or the beginning of 1759.
Halley knew that he could not himself live to witness the fulfilment
of his prediction, but he says: "If it should return, according to
our predictions, about the year 1758, impartial posterity will not
refuse to acknowledge that this was first discovered by an
Englishman." This was, indeed, a remarkable prediction of an event
to occur fifty-three years after it had been uttered.
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