However, we were all mistaken. The comet of 1882 retreated on such a course, and with such variation of velocity, as to show that its real period must be measured, not by months, as had been supposed, nor even by years, but by centuries. Probably it will not return till 600 or 700 years have passed. Had this not been proved, we might have been not a little perplexed by the return of apparently the same comet in this present year. A comet was discovered in the south early in January, whose course, dealt with by Professor Kruger, one of the most zealous of our comet calculators, is found to be partially identical with that of the four remarkable comets we have been considering. Astronomers have not been moved by this new visitant on the well-worn track as we were by the arrival of the comet of 1882, or as we should have been if either the comet of 1882 had never been seen or its path had not been shown to be so wide ranging. Whatever the comet of the present year may be, it was not the comet of 1882 returned.
No one even supposes that it was the comet of 1880, or 1843, or 1668. Nevertheless, rightly apprehended, the appearance of a comet traveling on appreciably the same track as those four other comets is of extreme interest, and indeed practically decisive as to the interpretation we must place on these repeated coincidences.
Observe, we are absolutely certain that the five comets are associated together in some way; but we are as absolutely certain that they are not one and the same comet which had traveled along the same track and returned after a certain number of circuits. We need not trouble ourselves with the question whether two or more of the comets may not have been in reality one and the same body at different returns. It suffices that they all five were not one; since we deduce precisely the same conclusion whether we regard the five as in reality but four or three or two. But it may be mentioned in passing as appearing altogether more probable, when all the evidence is considered, that there were no fewer than five distinct comets, all traveling on what was practically the selfsame track when in the neighborhood of the sun.
There can be but one interpretation of this remarkable fact - a fact really proved, be it noticed (as I and others have maintained since the retreat of the comet of 1882), independently of the evidence supplied by the great southern comet of the present year. These comets must all originally have been one comet, though now they are distinct bodies. For there is no reasonable way (indeed, no possible way) of imagining the separate formation of two or more comets at different times which should thereafter travel in the same path.
No theory of the origin of comets ever suggested, none even which can be imagined, could account for such a peculiarity. Whereas, on the other hand, we have direct evidence showing how a comet, originally single, may be transformed into two or more comets traveling on the same, or nearly the same, track.
The comet called Biela's, which had circuited as a single comet up to the year 1846 (during a period of unknown duration in the past - probably during millions of years), divided then into two, and has since broken up into so many parts that each cometic fragment is separately undiscernible. The two comets into which Biela's divided, in 1846, were watched long enough to show that had their separate existence continued (visibly), they would have been found, in the fullness of time, traveling at distances very far apart, though on nearly the same orbit. The distance between them, which in 1846 had increased only to about a quarter of a million of miles, had in 1852 increased to five times that space.
Probably a few thousands of years would have sufficed to set these comets so far apart (owing to some slight difference of velocity, initiated at the moment of their separation) that when one would have been at its nearest to the sun, the other would have been at its farthest from him. If we could now discern the separate fragments of the comet, we should doubtless recognize a process in progress by which, in the course of many centuries, the separate cometic bodies will be disseminated all round the common orbit. We know, further, that already such a process has been at work on portions removed from the comet many centuries ago, for as our earth passes through the track of this comet she encounters millions of meteoric bodies which are traveling in the comet's orbit, and once formed part of the substance of a comet doubtless much more distinguished in appearance than Biela's.
There can be little doubt that this is the true explanation of the origin of that family of comets, five of whose members returned to the neighborhood of the sun (possibly their parent) in the years 1668, 1843, 1880, 1882, and 1887.1
But it is not merely as thus explaining what had been a most perplexing problem that I have dealt with the evidence supplied by the practical identity of these five comets' orbits. When once we recognize that this, and this only, can be the explanation of the associated group of five comets, we perceive that very interesting and important light has been thrown on the subject of comets generally. To begin with: what an amazing comet that must have been from which these five, and we know not how many more, were formed by disaggregative processes - probably by the divellent action of repulsive forces exerted by the sun! Those who remember the comets of 1843 and 1882 as they appeared when at their full splendor will be able to imagine how noble an appearance a comet would present which was formed of these combined together in one. But the comet of 1880 was described by all who saw it in the southern hemisphere as most remarkable in appearance, despite the faintness of its head. The great southern comet of the present year was a striking object in the skies, though it showed the same weakness about the head.
That of 1668 was probably as remarkable in appearance as even the comet of 1882. A comet formed by combining all these together would certainly surpass in magnificence all the comets ever observed by astronomers.
And then, what enormous periods of time must have been required to distribute the fragments of a single comet so widely that one would be found returning to its perihelion more than two centuries after another! When I spoke of one member of the Biela group being in aphelion when another would be in perihelion, I was speaking of a difference of only three and one-third years in time; and even that would require thousands of years. But the scattered cometic bodies which returned to the sun's neighborhood in 1668 and 1887 speak probably of millions of years which have passed since first this comet was formed. It would be a matter of curious inquiry to determine what may have been the condition of our sun, what even his volume, at that remote epoch in history.
It may be interesting to compare the orbital elements of the five comets above dealt with. They may be presented as follows; but it should be noticed that the determinations must be regarded as rough in the case of Comets I. and V., as the observations were insufficient for exact determination of the elements:
|Perih. Passage.||Feb. 29||Feb. 27||Jan. 27||Sep. 17||Jan. 11|
|Log. Per. Dist.||7.6721||7.8395||7.7714||7.8895||8.1644|
|Long. Per.||80° 15'||73° 30' 46"||74° 11' 13"||55° 37' 29"||89° 41'|
|Long. Node.||357° 17'||355° 46' 48"||356° 17' 4"||346° 1' 27"||359° 41'|
|Inclination.||125° 58'||143° 1' 31"||143° 7' 31"||141° 59' 40"||141° 16'|