I am not aware,
indeed, that any anticipations were ever suggested in regard to the
return of the comet of 1668 to our neighborhood. It was not till the
time of Halley's comet, 1682, that modern astronomy began to consider
the question of the possibly periodic character of cometic motions with
attention. (For my own part, I reject as altogether improbable the
statement of Seneca that the ancient Chaldean astronomers could
calculate the return of comets.) The comet of 1680, called Newton's, was
the very first whose orbital motions were dealt with on the principles
of Newtonian astronomy, and Halley's was the first whose periodic
character was recognized.
In 1843 another comet came up from the south, and presently returned
thither. It was, indeed, only seen during its return, having, like the
comet of 1668, been only discovered a day or two after perihelion
passage. Astronomers soon began to notice a curious resemblance between
the orbits of the two comets. Remembering the comparative roughness of
the observations made in 1668, it may be said that the two comets moved
in the same orbit, so far as could be judged from observation. The comet
of 1843 came along a path inclined at apparently the same angle to the
earth's orbit plane, crossed that plane ascendingly at appreciably the
same point, swept round in about two hours and a half that part of its
angular circuit which lay north of the earth's orbit plane, and,
crossing that plane descendingly at the same point as the comet of 1668,
passed along appreciably the same course toward the southern stellar
regions! The close resemblance of two paths, each so strikingly
remarkable in itself, could not well be regarded as a mere accidental
coincidence.
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