had at some time formed one body, or had come from the same source of origin. It is conceivable that the 1843 Comet might be identical with the 1668 Comet, but the 1880 and 1882 Comets can by no possibility be either identical with one another or identical with either of the 2 earlier ones, for all the computers who investigated the orbit of the 1882 comet assigned to it periods varying from 600 to 900 years.[1]
These comets are not only kindred in regard to their orbits but physically very much alike as regards size and brilliancy. Moreover they came to the Sun from the direction of the star Sirius (α Canis Majoris), that is, from the direction from which the Sun is moving with respect to the stars, and escaped notice in the Northern Hemisphere until near perihelion; and passed nearly half-way round the Sun in a few hours at very short absolute distances from the Sun.
On the point of jumping at conclusions as to the identification of comets, Young has remarked that caution must be observed, for:— "Even if the result of this investigation appears to show that the comets are probably identical, we are not yet absolutely safe in the conclusion, for we have what are known as 'cometary groups'. These are groups of comets which pursue nearly the same orbits, following along one after the other at a greater or smaller interval, as if they had once been united, or had come from some common source. The existence of such groups was first pointed out by Hoek of Utrecht in 1865. The most remarkable group of this sort is the one composed of the great comets of 1668, 1843, 1880, and 1882; and there is some reason to suspect that the little comet visible on the picture of the Corona of the Egyptian Eclipse [of 1882][2] also belongs to it. The bodies of this group have orbits very peculiar in their extremely small perihelion distance (they actually go within 12 a million miles of the Sun's surface), and yet, although their elements are almost identical they cannot possibly all be different appearances of
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