Fig. 25.—Sawerthal’s Comet, June 4, 1888 (Charlois).Fig. 25.—Sawerthal’s Comet, June 4, 1888 (Charlois).
Fig. 25.—Sawerthal’s Comet, June 4, 1888 (Charlois).
What is a comet made of? Men of Science equally with the general public would like to be able to answer this question, but they cannot do so with satisfactory certainty. A great many years ago Sir John Herschel wrote thus:—“It seems impossible to avoid the following conclusion, that the matter of the nucleus of a comet is powerfully excited and dilated into a vaporous state by the action of the Sun’s rays escaping in streams and jets at those points of its surface which oppose the least resistance, and in all probability throwing that surface or the nucleus itself into irregular motions by its reaction in theact of so escaping, and thus altering its direction.” This passage was written of course before the spectroscope had been brought to bear on the observations of comets, but so far as Sir John Herschel’s remark implies the presence of vapour, that is gas, in a comet, the surmise has been amply borne out by later discoveries. The fact that as a comet approaches the Sun some forces, no doubt of solar origin, come into operation to vaporise and therefore expand the matter composing the comet is sufficiently shown by the great developement which takes place as we have seen in the tails of comets, but in regard to the heads of comets we are face to face with a strange enigma. Though the tails expand the heads contract as the comet approaches its position of greatest proximity to the Sun. Having passed this point the head expands again. This curious circumstance, first pointed out by Kepler in 1618, has often been noticed since, and noticed indeed not as the result of mere eye impressions, but after careful micrometrical measurement with suitable instruments. I think the confession must be made that we are hopelessly ignorant of the nature of comet’s except that gases are largely concerned in their constitution.
It seems impossible to doubt that some tails of comets are hollow cylinders or hollow cones. Such a theory would account for the fact, so often noticed, that single tails are usually much brighter at their two edges than at the centre. This is the natural effect of looking transversely at any translucent cylinder of measureable thickness.
It was long a moot point whether comets are self-luminous, or whether they shine by reflectedlight; but it is now generally admitted that whilst a part of the light of a comet may be derived by reflection from the Sun yet as a rule they must be regarded as shining by their own intrinsic light.
It should be stated here by way of caution that the observations on this subject are not so consistent as one could wish, and it seems necessary to assume that all comets are not constituted alike, and that therefore what is true of one does not necessarily apply to another.
To those who possess telescopes (not necessarily large ones) opportunities for the study of comets have much multiplied during the last few years, for we are now acquainted with a group of small comets which are constantly coming into view at short intervals of time. The comets have now become so numerous that seldom a year passes without one or more of them coming into view. Whilst that known as Encke’s revolves round the Sun in 3¼ years, Tuttle’s doing the same in 13½ years, there are four others whose periods average about 5½ years, 5 which average 6½ years, together with one of 7½ years and one of 8 years. It is thus evident that there is a constant succession of these objects available for study, and that very few months can ever elapse that some one or more of them are not on view. They bear the names of the astronomers who either discovered them originally, or who, by studying their orbits, discovered their periodicity. The names run as follows, beginning with the shortest in period and ending with the longest:—
I cannot stay to dwell upon either the history or description of these comets separately, but must content myself by saying generally that whilst as a rule they are not visible to the naked eye, yet several of them may occasionally become so visible when they return to perihelion under circumstances which bring them more near than usual to the earth.
Several other comets are on record which it was supposed at one time would certainly have been entitled to a place in the above list, but three of them in particular have, under very mysterious circumstances, entirely disappeared from the Heavens.
Chief amongst the mysterious comets must be ranked that which goes by the name of Biela. This comet, first seen in 1772, was afterwards found to have a period of about 6¾ years, and on numerous occasions it reappeared at intervals of that length down to 1845, when the mysterious part of its career seems to have commenced. In December of that year this comet threw off a fragment of nearly the same shape as itself, and the two portions travelled together side by side for four months, the distance between the fragments slowly increasing. At the end of the four months in question the comet passed out of sight owing to the distance from the earth to which it had attained. The comet returned again to perihelion in 1852, remaining visible for three weeks. The two portions of the comet noticed in 1846retained their individuality in 1852, but the distance between them had increased to about eight times the greatest distance noticed in 1846. As a comet Biela’s Comet has never been seen since 1852, and it must now be regarded as having permanently disappeared. But what seems to have happened is this, that Biela’s Comet has become broken up into a mass of meteors. On November 27, 1872, and again in November 1885, when the earth in travelling along its own orbit reached a certain point where its orbit intersected the former orbit of Biela’s Comet the Earth encountered, instead of the comet which ought to have been there, a wonderful mass of meteors; and it is now generally accepted that these meteors, which apparently are keeping more or less together as a fairly compact swarm, are nought else than the disintegrated materials of what once was Biela’s Comet.
Fig. 26.—Biela’s Comet, February 19, 1846.Fig. 26.—Biela’s Comet, February 19, 1846.
Fig. 26.—Biela’s Comet, February 19, 1846.
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 cometsto 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 11B. 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 ofJupiter. 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 thesame 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 benil. 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 considerablepanic 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 soonerthan 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 withperhapsone 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.
Fig. 27.—The Great Comet of 1811.Fig. 27.—The Great Comet of 1811.
Fig. 27.—The Great Comet of 1811.
Donati’s comet of 1858, has already received a good deal of notice at my hands, but the question remains, what are its claims, to be regarded as the comet of the century, compared with that of 1843? It is not a little strange that though there must have been many persons who saw both, yet it was only quite recently that I came across, for the first time, a description of both these comets from the same pen. It ought, however, to be mentioned by way of explanation, that the inhabitants of Europe only saw the comet of 1843, when its brilliancy and the extent of its tail had materially diminished, about a fortnight after it was at its best.
The description of these two comets to which I have alluded, will be found in General J. A. Ewart’s “Story of a Soldier’s Life,” published in 1881. Writing first of all of the comet of 1843, General Ewart says:—
“It was during our passage from the Cape of Good Hope to the Equator, and when not far from St. Helena, that we first came in sight of the great comet of 1843. In the first instance a small portion of the tail only was visible, at right angles to the horizon; but night after night as we sailed along, it gradually became larger and larger, till at last up came the head, or nucleus, as I ought properly to call it. It was a grand and wonderful sight, for the comet now extended the extraordinary distance of one-third of the heavens, the nucleus being, perhaps, about the size of the planet Venus.”—(Vol. i., p. 75.)
Fig. 28.—The Great Comet of 1882, on October 19 (Artus).Fig. 28.—The Great Comet of 1882, on October 19 (Artus).
Fig. 28.—The Great Comet of 1882, on October 19 (Artus).
As regards Donati’s comet of 1858, what the General says is:—
“A very large comet made its appearance about this time, and continued for several weeks to be a magnificent object at night; it was, however,nothing to the one I had seen in the year 1843, when on the other side of the equator.”—(Vol. ii., p. 205.)
Passing over the great comet of 1861, on which I have already said a good deal, I must quit the subject of famous comets by a few words about that of 1882, which, though by no means one of the largest, was, in some respects, one of the most remarkable of modern times. It was visible for the long period of nine months, and was conspicuously prominent to the naked eye during September, but these facts, though note-worthy, would not have called for particular remark, had not the comet exhibited some special peculiarities which distinguished it from all others. In the first place, it seems to have undergone certain disruptive changes, in virtue of which the nucleus became split up into four independent nuclei. Then the tail may have been tubular, its extremity being not only bifid, but totally unsymmetrical with respect to the main part. The tubular character of the tail was suggested by Tempel. To other observers, this feature gave the idea of the comet, properly so-called, being enclosed in a cylindrical envelope, which completely surrounded the comet, and overlapped it for a considerable distance at both ends. Finally (and in this resembling Biela’s comet) the comet of 1882 seemsto have thrown off a fragment which became an independent body.
What has gone before, will, I think, have served abundantly to establish the position with which I started, namely, that comets occupy (and deservedly so) the front rank amongst those astronomical objects in which, on occasions, the general public takes an interest.
I have thus completed, so far as the space at my disposal has permitted, a popular descriptive Survey of the Solar System. Those who have perused the preceding pages, however slight may have been their previous acquaintance with the Science of Astronomy taken as a whole, will have no difficulty in realising that what I have said bears but a small proportion to what I have left unsaid. They will equally, I hope, be able to see, without indeed the necessity of a suggestion, that all those wondrous orbs which we call the planets could neither have come into existence nor have been maintained in their allotted places for so many thousands of years, except as the result of Design emanating from an All-powerful Creator.
[1]The remark in the text applies to all the major planets and to a large number of the minor planets, but certain of the minor planets travel in orbits which are considerably inclined to the ecliptic, and therefore to all the other planets.[2]Given in full in myHandbook of Astronomy, 4th ed., vol. i., p. 26.[3]“Recollections of Past Life,” 2nd ed., p. 305.[4]For some information respecting these Secchi “Types” of Stars, see my “Story of the Stars,” 2nd ed., p. 140.[5]The circle and the ellipse are what are called “closed” curves.[6]It is not a little singular that the Chinese in bygone centuries have often alluded to comets under the name of vapours;e.g., the comet of 1618 is recorded as having been “a white vapour 20 cubits long.”
[1]The remark in the text applies to all the major planets and to a large number of the minor planets, but certain of the minor planets travel in orbits which are considerably inclined to the ecliptic, and therefore to all the other planets.
[2]Given in full in myHandbook of Astronomy, 4th ed., vol. i., p. 26.
[3]“Recollections of Past Life,” 2nd ed., p. 305.
[4]For some information respecting these Secchi “Types” of Stars, see my “Story of the Stars,” 2nd ed., p. 140.
[5]The circle and the ellipse are what are called “closed” curves.
[6]It is not a little singular that the Chinese in bygone centuries have often alluded to comets under the name of vapours;e.g., the comet of 1618 is recorded as having been “a white vapour 20 cubits long.”
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