1843.  Each was remarkable for its extremely long tail and for the comparative insignificance of its head.  In the northern skies, indeed, the comet of 1843 showed a very straight tail, and it is usually depicted in that way, whereas the comet of 1668 had a tail showing curvature.  But pictures of the comet of 1843, as seen in the southern hemisphere, show it with a curved tail, and also the tail appeared forked toward the end during that part of the comet’s career.

However, the best observations, and the calculations based on them, seemed to show that the period of the comet of 1843 could not be less than 500 years.

Astronomers were rather startled, therefore, when, in 1880, a comet appeared in the southern skies which traversed appreciably the same course as the comets of 1668 and 1843.  When I was in Australia, in 1880, a few months after the great comet had passed out of view, I met several persons who had seen both the comet of that year and the comet of 1843.  They all agreed in saying that the resemblance between the two comets was very close.  Like the comet of 1843, that of 1880 had a singularly long tail, and both comets were remarkable for the smallness and dimness of their heads.  One observer told me that at times the head of the comet of 1880 could barely be discerned.

Like the comets of 1668 and 1843, the comet of 1880 grazed close past the sun’s surface.  Like them, it was but about two hours and a half north of the earth’s orbit place.  Had it only resembled the other two in these remarkable characteristics, the coincidence would have been remarkable.  But of course the real evidence by which the association between the comets was shown was of a more decisive kind.  It was not in general character only, but in details, that the path of the comet of 1880 resembled those on which the other two comets had traveled.  Its path had almost exactly the same slant to the earth’s orbit plane as theirs, crossed that plane ascendingly and descendingly at almost exactly the same points, and made its nearest approach to the sun at very nearly the same place.  To the astronomer such evidence is decisive.  Mr. Hind, the superintendent of the “Nautical Almanac,” and as sound and cautious a student of cometic astronomy as any man living, remarked, so soon as the resemblance of these comets’ paths had been ascertained, that if it were merely accidental, the case was most unusual; nay, it might be described as unique.  And, be it noticed, he was referring only to the resemblance between the comets of 1880 and 1843.  Had he recalled at the time the comet of 1668, and its closely similar orbit, he would have admitted that the double coincidence could not possibly be merely casual.

But this was by no means the end of the matter.  Indeed, thus far, although the circumstances were striking, there was nothing to prevent astronomers from interpreting them as other cases of coincident, or nearly coincident, cometic paths had been interpreted.  Hind and others, myself included, inferred that the comets of 1880, 1843, and 1668 were simply one and the same comet, whose return in 1880 probably followed the return in 1843 after a single revolution.