The Story of the Solar System

by George F. Chambers

It is extremely probable that as time goes on we shall be able to say that an intimate connection subsists between particular comets which have been and particular meteoric swarms. We already possess proof that other comets which once came within our view were at that time revolving round the Sun in orbits so comparatively small that they should have reappeared at intervals of half-a-dozen or so years, yet they have not reappeared. The question therefore suggests itself, Have they been subject to some great internal disaster which has led to their disintegration? It may be said without doubt that this is in the highest degree probable; but short of this, that is short of total disintegration into small fragments, we have several cases on record of what I may, for the moment, call ordinary comets breaking up into two or three fragments. For a long while astronomers were naturally loath to believe that this was possible, and therefore they discredited the statements to that effect which had been made. Though it would occupy too much space to give the particulars of these comets in full it may yet be worth while just to mention the names of some of them, presumed to be of short period, which seemed nevertheless to have eluded our grasp. I would here specially mention Liais’s Comet of 1860 and the second comet of 1881 as seemingly having undergone some sort of disruption akin to what happened in the case of Biela’s Comet.

There is another group of periodical comets to be mentioned. These are six in number and seem to have periods of 70 years or a little more. Of these three have not yet given us the chance of seeing them again; two have paid us a second visit, and therefore their periods are not open to doubt; whilst the most famous of this group, “Halley’s,” has been recorded to have shown itself to the Earth no less than 25 times, beginning with the year 11 B. C. It was Halley’s comet which shone over Europe in April 1066, and was considered the forerunner of the conquest of England by William of Normandy. It figures in the famous Bayeux tapestry as a hairy star of strange shape.

It would seem that there exists in some inscrutable manner a connection between each of the three great exterior planets and certain groups of comets. In the case of Jupiter the association is so very pronounced as long ago to have attracted notice; but the French astronomer, Flammarion, has brought forward some suggestions that Saturn has one comet (and perhaps two) with which it is associated; Uranus, two (and perhaps three); and Neptune, six; whilst farther off than Neptune the fact that there are two comets, supposed periodical, without a known planet to run with them has inspired Flammarion to look with a friendly eye on the idea (often mooted) that outside of Neptune there exists another undiscovered planet revolving round the sun in a period of about 300 years.

The Jupiter group of comets deserves a few additional words. There are certainly nine, and perhaps twelve comets revolving round the Sun in orbits of such dimensions that they either reach up to or slightly overreach the orbit of Jupiter. The effect of this condition of things is that on occasions Jupiter and each of the comets may come into such proximity that the superior mass of Jupiter may exercise a very seriously disturbing influence over a flimsy and ethereal body like a comet. There is reason to suppose that some of the comets now belonging to the Jupiter group have not done so for any great length of time, but having been wandering about, either in elliptic orbits of great extent, or even in parabolic orbits, have accidentally come within reach of Jupiter, and so have been, as it were, captured by him. Hence, the origin of the term, the “capture theory,” as applied to these family groups of comets which I have just stated to exist, each presided over, as it were, by a great planet. It may be that at some future time this theory will help us to a clue to the fact that besides the comets of Lexell of 1770, Blainpain of 1819, and Di Vico of 1844, short period comets unaccountably missing, there are several others presumed to have been revolving in short period orbits when discovered, and as to which it is very strange that they should not have been seen before their only recorded visit to us, and equally strange that they should never have been seen since.

Is there any reason to fear the results of a collision between a comet and the Earth? None whatever. However vague may be, and in a certain sense must be, our answer to the question, “What is a comet?” certain is it that every comet is a very imponderable body—a sort of airy nothing, a mass of gas or vapour.[6] At the same time it always has been and perhaps still is difficult to persuade the public that whatever might be the effect on a comet if it were to strike the Earth, the effect on the Earth, were it to be struck by a comet, would be nil. This is not altogether a matter of speculation, for according to a calculation by Hind, on June 30, 1861, the Earth passed into and through the tail of the great comet of that year at about two-thirds of its distance from the nucleus. Assuredly there was no dynamical result; but it seems, however, not unlikely that there was an optical result; at any rate, traces of something of this sort were noted. Hind himself, in Middlesex, observed a peculiar phosphorescence or illumination of the sky which he attributed at the time to an auroral glare. Lowe, in Nottinghamshire confirmed Hind’s statement of the appearance of the heavens on the same day. The sky had a yellow auroral glare-like look, and the Sun, though shining, gave but feeble light. The comet was plainly visible at 7.45 p. m. (during sunshine), and had a much more hazy appearance than on any subsequent evening. Lowe adds that his Vicar had the pulpit candles lighted in the Parish Church at 7 o’clock (it was a Sunday), though only five days had elapsed since Midsummer day, which itself proves that some sensation of darkness was felt even while the Sun was shining.

So far as I remember there has been no such thing as a comet panic during the present generation, at any rate in civilised countries, but it is on record that there was a very considerable panic in 1832 in connection with the return of Biela’s Comet in the winter of that year. Olbers as the result of a careful study in advance of the comet’s movements found that the comet’s centre would pass only 20,000 miles within the Earth’s orbit, and that as the nebulosity of the comet had in 1805 been more than 20,000 miles in diameter, it was certain, unless its dimensions had diminished in the 27 years, that some of the comet’s matter would overlap the Earth’s orbit; in other words would envelop the Earth itself, if the Earth happened to be there. This conclusion when it became public was quite enough to create a panic and make people talk about the forthcoming destruction of our globe. It was nothing to the point (in the public mind) that astronomers were able to predict that the Earth would not reach the place where the comet would cross the Earth’s orbit until four weeks after the comet had come and gone. However, we now know that nothing happened, and I am justified in adding that even if there had been contact, Earth meeting comet face to face, nothing (serious) would have occurred so far as the Earth was concerned.

This seems a convenient place for referring to a matter which when it was first broached excited a great deal of interest, but about which one does not hear much now-a-days. The period of the small comet known as Encke’s (which, revolving as it does round the Sun in a little more than three years, has the shortest period of any of the periodical comets) was found many years ago to be diminishing at each successive return. That is to say, it always attained its nearest distance from the Sun at each apparition 2½ hours sooner than it ought to have done. In order to account for this gradual diminution in the comet’s period Encke conjectured the existence of a thin ethereal medium sufficiently dense to affect a light flimsy body like a comet, but incapable of obstructing a planet. It has been remarked by Hind that “this contraction of the orbit must be continually progressing, if we suppose the existence of such a medium; and we are naturally led to inquire, What will be the final consequence of this resistance? Though the catastrophe may be averted for many ages by the powerful attraction of the larger planets, especially Jupiter, will not the comet be at last precipitated on the Sun? The question is full of interest, though altogether open to conjecture.”

Astronomers are not altogether agreed as to the propriety of this explanation. One argument against it is that with perhaps one exception none of the other short-period comets (all of them small and presumably deficient in density) seem affected as Encke’s is. On the other hand Sir John Herschel favoured the explanation just given, as also does Hind who is the highest living authority on comets. A German mathematician, Von Asten, who devoted immense labour to the study of the orbit of Encke’s Comet, thought there should be no hesitation in accepting the idea of a resisting medium, subject to the limitation that it does not extend beyond the orbit of Mercury. Von Asten’s allusion to Mercury touches a subject which belongs more directly to the question of Mercury’s orbit and to that other very interesting question, “Are there any planets, not at present known, revolving round the Sun within the orbit of Mercury.”

Which is the largest and most magnificent comet recorded in history? It is virtually impossible to answer this question, because of the extravagant and inflated language made use of by ancient and medieval (I had almost added, and modern) writers. There is no doubt that the comet of 1680, studied by Sir I. Newton, the tail of which was curved, and from 70° to 90° long, must have been one of the finest on record, as it was also the one which came nearest to the Sun, for it almost grazed the Sun’s surface.

The comet of 1744, visible as it was in broad daylight, was, no doubt, the finest comet of the 18th century, though in size it has been surpassed; yet its six tails must have made it a most remarkable object. So far as the 19th century is concerned, our choice lies between the comets of 1811, 1843, 1858, and 1861. The comet of 1811 is spoken of by Hind as “perhaps the most famous of modern times. Independently of its great magnitude, the position of the orbit and epoch of perihelion passage, were such as to render it a very splendid circumpolar object for some months.” The tail as regards its length was not so very remarkable, for at its best, in October 1811, it was only about 25° long, its breadth, however, was very considerable; at one time 6°, the real length of the tail, about the middle of October, was more than 100,000,000 of miles, and its breadth about 15,000,000 of miles. The visibility of this comet was coincident with those events which proved to be the turning-point in the career of Napoleon I., and there were not wanting those who regarded the comet as a presage of his disastrous failure in Russia. Owing to the long period (17 months), during which this comet was visible, it was possible to determine its orbit with unusual precision. Argelander found its period to be 3065 years, with no greater uncertainty than 43 years. The great dimensions of its orbit will be realised when it is stated that this comet recedes from the Sun to a distance of 14 times that of the planet Neptune.