The Story of the Solar System :: CHAPTER XIII. COMETS.

CHAPTER XIII. COMETS.

I suppose that it is the experience of all those who happen to be in any sense, however humble, specialists in a certain branch of science, that from time to time, they are beset with questions on the part of their friends respecting those particular matters which it is known that they have specially studied. There is no fault to be found with this thirst for information, always supposing that it is kept within due bounds; but my motive for alluding to it here, is to see whether any well-marked conclusion can be drawn from it, within my own knowledge as regards astronomical facts or events. Now in the case of the science of astronomy (for which in this connection I, for the moment, will venture to speak), there is certainly no one department which so unfailingly, at all times and in all places, seems to evoke such popular sympathy and interest as the department which deals with Comets.

Sun-spots may come and go; bright planets may shine more brightly; the Sun or Moon may be obscured by eclipses; temporary stars may burst forth,—all these things are within the ken of the general public by means of newspapers or almanacs, but it is a comet which evokes more questionings and conversations than all the other matters just referred to put together. When a new and bright comet appears, or even when any comet not very bright gets talked about, the old question is still fresh and verdant—“Is there any danger to the Earth to be apprehended from collision with a Comet?” followed by “What is a Comet?” “What is it made of?” “Has it ever appeared before?” “Will it come back again?” and so on. Questions in this strain have more often than I can tell of been put to me. They seem the stock questions of all who will condescend to replace for five minutes in the day the newest novel or the pending parliamentary election.

It may be taken as a fact (though in no proper sense a rule) that a bright and conspicuous comet comes about once in 10 years, and a very remarkable comet every 30 years. Thus we have had during the present century bright comets in 1811, 1825, 1835, 1843, 1858, 1861, 1874 and 1882, whereof those of 1811, 1843, and 1858 were specially celebrated. Tested then by either standard of words “bright and conspicuous,” or “specially celebrated,” it may be affirmed that a good comet is now due, so let us prepare for it by getting up the subject in advance.

I will not attempt to answer in regular order or in any set form the questions which I have just mentioned as being stock questions, but they will be answered in substance as we go along. There is one matter in connection with comets which has deeply impressed itself upon the public mind, and that is the presence or absence of a “tail.” It is not too much to say that the generality of people regard the tail of a comet as the comet; and that though an object may be a true comet from an astronomer’s point of view, yet if it has no tail its claims go for nought with the mass of mankind. We have here probably a remnant of ancient thought, especially of that line of thought which in bygone times associated Comets universally with the idea that they were especially sent to be portents of national disasters of one kind or another. This is brought out by numberless ancient authors, and by none more forcibly than Shakespeare. Hence we have such passages as the following in Julius CÆsar (Act ii., sc. 2):—

“When beggars die there are no comets seen,

The Heavens themselves blaze forth the death of princes.”

In Henry VI. (Part I., Act i., sc. 1) we find the well-known passage:—

“Comets importing change of times and states

Brandish your crystal tresses in the sky,

And with them scourge the bad revolting stars

That have consented unto Henry’s death.”

There are in point of fact two distinct ideas evolved here: (1) that comets are prophetic of evil, and (2) stars potential for evil.

There is another passage in Henry VI. (Part I., Act iii., sc. 3) even more pronounced:—

“Now shine it like a Comet of revenge,

A prophet to the fall of all our foes.”

Again; in Hamlet (Act i., sc. 1) we find:—

“As stars with trains of fire, and dews of blood,

Disasters in the Sun.”

Once more; in the Taming of the Shrew (Act iii., sc. 2) we have the more general, but still emphatic enough, idea expressed by the simple words of reference to—

“Some Comet or unusual prodigy.”

Shakespeare may be said to have lived at the epoch when astrology was in high favour, and it may be that he only gave utterance to the current opinion prevalent among all classes in those still somewhat “Dark Ages” (so called). This, however, can hardly be said of the author of my next quotation—John Milton (Paradise Lost, bk. II.):—

“Satan stood

Unterrified, and like a Comet burned,

That fires the length of Ophiuchus huge

In th’ Arctic sky, and from its horrid hair

Shakes pestilence and war.”

Jumping over a century we find the ancient theory still in vogue, or Thomson (Seasons, Summer) would never have written:—

“Amid the radiant orbs

That more than deck, that animate the sky,

The life-infusing suns of other worlds;

Lo! from the dread immensity of space,

Returning with accelerated course,

The rushing comet to the sun descends;

And, as he sinks below the shading earth,

With awful train projected o’er the heavens,

The guilty nations tremble.”

Even Napoleon I. had servile flatterers who, as late as 1808, tried to extract astrological influence out of a comet by way of bolstering up “Old Bony.” But enough of poetry and fiction, let us hasten back to prosaic fact.

Fig. 19.—Telescope Comet with a nucleus.

Comets as objects to look at may be classed under three forms, though the same comet may undergo such changes as will at different epochs in its career cause it to put on each variety of form in succession. Thus the comet of 1825 seen during that year as a brilliant naked-eye object, after being lost in the sun’s rays, was again found on April 2, 1826 by Pons. Lamentable were his cries at the miserable plight it was in. He described it as totally destroyed: without tail, beard, coma or nucleus, a mere spectre. The simplest form of comet is a mere nebulous mass, almost always circular, or perhaps a little oval, in outline. It may maintain this appearance throughout its visibility; or, growing brighter may become a comet of the second class, with a central condensation, which developing becomes a “nucleus” or head. It may retain this feature for the rest of its career, or may pass into the third class and throw out a “coma” or beard, which will perhaps develop into a tail or tails. Doing this it will not unfrequently grow bright enough and large enough to become visible to the naked eye. In exceptional cases the nucleus will become as bright as a 2nd or even 1st magnitude star, and the tail may acquire a length of several or many degrees. In the last named case of all the comet becomes, par excellence according to the popular sentiment, “a comet.” It will now be readily inferred that the astronomer in his observatory has to do with many comets which the public at large never hear of, or if they do hear of, treat with contempt, because they are destitute of tails.

Fig. 20.—Wells’s Comet of 1882, seen in full daylight near the Sun on Sept. 18.

Fig. 21.—Quenisset’s Comet, July 9, 1893 (Quenisset).

The tails of comets exhibit very great varieties not only of size but of form; some are long and slender; some are long and much spread out towards their ends, like quill pens, for instance; some are short and stumpy, mere tufts or excrescences rather than tails. Not unfrequently a tail seems to consist of two parallel rays with no cometary matter, or it may be only a very slight amount of cometary matter traceable in the interspace; some have one main tail consisting of a pair of rays such as just described, together with one or more subsidiary or off-shoot tails. The comet of 1825 had five tails and the comet of 1744 had six tails. It might be inferred from all this that the tails of comets are so exceedingly irregular, uncertain and casual as to be amenable to no laws. This was long considered to be the case; but a Russian observer named Bredichin, as the result of much study and research, has arrived at the conclusion that all comet tails may be brought under one or other of three types; and that each type is indicative of certain distinct differences of origin and condition which he considers himself able to define. The first type comprises tails which are long and straight; “they are formed” (to quote Young’s statement of Bredichin’s views) “of matter upon which the Sun’s repulsive action is from twelve to fifteen times as great as the gravitational attraction, so that the particles leave the comet with a relative velocity of at least four or five miles a second; and this velocity is continually increased as they recede, until at last it becomes enormous, the particles travelling several millions of miles in a day. The straight rays which are seen in the figure of the tail of Donati’s Comet, tangential to the tail, are streamers of this first type; as also was the enormous tail of the comet of 1861. The second type is the curved plume-like train, like the principal tail of Donati’s Comet. In this type the repulsive force varies from 2.2 times gravity (for the particles on the convex edge of the tail) to half that amount for those which form the inner edge. This is by far the most common type of cometary train. A few comets show tails of the third type—short, stubby, brushes violently curved, and due to matter of which the repulsive force is only a fraction of gravity—from ¹/₁₀ to ½.”

Bredichin wishes it to be inferred that the tails of the 1st type are probably composed of hydrogen; those of the 2nd type of some hydro-carbon gas; and those of the 3rd of the vapour of iron, probably with some admixture of sodium and other substances. Bredichin, as a reason for these conclusions, supposes that the force which generates the tails of comets is a repulsive force, with a surface action the same for equal surfaces of any kind of matter; the effective accelerating force therefore measured by the velocity which it would produce would depend upon the ratio of surface to mass in the particles acted upon, and so, in his view, should be inversely proportional to their molecular weights. Now it happens that the molecular weights of hydrogen, of hydro-carbon gases, and of the vapour of iron bear to each other just about the required proportion.

I am here stating the views and opinions of others without definitely professing to be satisfied with them, but as they have met with some acceptance, it is proper to chronicle them, though we know nothing of the nature of the repulsive force here talked about. It might be electric, it might be anything. The spectroscope certainly lends some countenance to Bredichin’s views, but we need far more knowledge and study of comets before we shall be justly entitled to dogmatise on the subject.

Fig. 22.—Holmes’s Comet, Nov. 9, 1892 (Denning).

Fig. 23.—Holmes’s Comet, Nov. 16, 1892 (Denning).

This has been rather a digression. I go back now to prosaic matters of fact, of which a vast and interesting array present themselves for consideration in connection with comets. Let us consider a little in detail what they are, to look at. We have seen that a well-developed comet of the normal type usually comprises a nucleus, a head or coma, and a tail. Comets which have no tails generally exhibit heads of very simple structure; and if there is a nucleus, the nucleus is little else than a stellar point of light. But in the case of the larger comets, which are almost or quite visible to the naked eye, the head often exhibits a very complex structure, which in not a few cases seems to convey very definite indications of the operations going on at the time. Figs. 22 and 23 may be taken as samples of a complex cometary head, though no two comets resemble one another exactly in details. Fig. 24 forcibly conveys the idea that we are looking at a process of development analogous to an uprush of water from a fountain, or perhaps I might better say, from a burst waterpipe. There is a distinct idea of a jet. This self-same idea, in another form, presents itself in the case of those comets which exhibit what astronomers are in the habit of calling “luminous envelopes.” The jet in this case is not strictly a jet because it is not a continuous outflow, or overflow, of matter; the idea rather suggests itself of an intermittent overflow resulting in accumulated layers, or strata, of matter becoming visible. But with this we come to a standstill; we cannot tell where the matter comes from, and still less, where it goes to; we can only record what our eyes, assisted by telescopes, tell us. There can, however, I think, be no doubt that the matter of a comet becomes displayed to our senses as the result of a process of expulsion, or repulsion, from the nucleus; and then, having become launched into space, it comes under the influence, also repulsive, of the Sun. All these things are visible facts. As to causes, we suggest little, because we know so little. Anyone who has seen a comet and has watched the displays of jets and luminous envelopes, such as I have endeavoured to set forth, will realise at once how impossible it is to describe these things in words. They must be seen either in actual being or in picture. Some further allusions to this branch of the subject may perhaps be more advantageously made after we have considered the movements and orbits of comets.

Fig. 24.—Comet III. of 1862, on Aug 22, showing jet of luminous matter (Challis).

There is often a slight general resemblance between a planet and a comet, as regards the path which each class of body pursues. Probably the least reflective person likely to be following me here understands the bare fact, that all the planets revolve round the Sun, and are held to defined orbits by the Sun’s influence, or attraction, as it is called. Perhaps, it is not equally realised, that in a somewhat similar, but not quite the same way, comets are influenced and controlled by the Sun.

Comets must be considered as regards their motions to be divisible into two classes:—(1) Those which belong to the Solar System; and (2) those which do not. Each of these two classes must again be sub-divided, if we would really obtain a just conception of how things stand.

By the Comets which belong to the Sun, I mean those which revolve round the Sun in closed orbits;[5] and are, or may be, seen again and again at recurring intervals. There are 2 or 3 dozen comets which present themselves to our gaze at stated intervals, varying from about 3 to 70 years. There are again other comets which without any doubt (mathematically) are revolving round the Sun in closed orbits, but in orbits so large and with periods of revolution so long (often many centuries), that though they will return again to the sight of the inhabitants of the earth some day, yet no second return having been actually recorded, the astronomer’s prediction that they will return, remains at present a prediction based on mathematics but nothing more.

There is another class of Comet of which we see examples from time to time, and having seen them once shall never see again. This is because these Comets move in orbits which are not closed, and which are known as parabolic or hyperbolic orbits respectively, because derived from those sections of a cone which are called the Parabola and the Hyperbola. It must be understood that what I am now referring to is purely a matter of orbit, and that no relationship subsists between the size and physical features of a Comet and the path it pursues in space. The only sort of reservation, perhaps, to be made to this statement is, that the comets celebrated for their size and brilliancy, are often found to be revolving in elliptic orbits of great eccentricity, which means that their periods may amount to many centuries.

It may be well to say something now as to what is the ordinary career of a comet, so far as visibility to us, the inhabitants of the Earth, is concerned. Though this might be illustrated by reference to the history of many comets, perhaps there is no one more suitable for the purpose than Donati’s Comet of 1858. In former times, when telescopes were few or non-existent, brilliant comets often appeared very suddenly, just as a carriage or a man does, as you turn the corner of a street. Such things even happen still: for instance, the great comet of 1861 burst upon us all at once at a day’s notice. Usually, however, now in consequence of the large size of the telescopes in use, and the great number of observers who are incessantly on the watch, comets are discovered when they are very small, because remote both from the Earth and Sun, and many weeks, or even months, it may be, before they shine forth in their ultimate splendour. Now, let us see how these statements are supported by the history of Donati’s comet in 1858. On June 2 in that year, it was first seen by Donati at Florence, as a faint nebulosity, slowly journeying northwards. June passed away, and July, and August, the comet all the while remaining invisible to the naked eye; that is to say, it first became perceptible to the naked eye on August 29, having put forth a faint tail about August 20. After the beginning of September its brilliancy rapidly increased. On September 17, the head equalled in brightness a 2nd magnitude star, the tail being 4° long. Passing its point of nearest approach to the Sun on September 29, it came nearest to the Earth on October 10; though, perhaps, its appearance a few days previously, namely on October 5, is the thing best remembered by those who saw it, because it was on that night that the comet passed over the 1st magnitude star Arcturus. For several days about this time, the comet was an object of striking beauty in the Western Heavens, during the hours immediately after sun-set. After October 10, it rapidly passed away to the Southern hemisphere, diminishing in brightness, as it did so, because receding from the Earth and the Sun. It continued its career through the winter; became invisible to the naked eye; and finally invisible altogether in March 1859. It remained in view, therefore, for more than nine months, not to return again till about the year 3158 A.D., for its period of revolution was found to be about 2000 years.

I have been particular in sketching somewhat fully the history of this comet so far as we are concerned, because, as I have already said, it is typical of the visible career of many comets. Halley’s comet in 1835 and 1836, went through a somewhat similar series of changes. This comet—a well-known periodical one of great historic interest and brilliancy—may be commended to the younger members of the rising generation, because it is due to return again to these parts of space a few years hence, or in 1910.

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 the act 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 reflected light; 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:—

Encke’s.	Winnecke’s.
Temple’s Second (1873, II.)	Brorsen’s.
	Temple’s First (1867, II.)
Swift’s (1880, V.)	Wolf’s (1884, III.)
Barnard’s (1884, II.)	Faye’s.
D’Arrest’s.	Denning’s.
Finlay’s.	Tuttle’s.

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 1846 retained 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.

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.

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).

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 seems to 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.

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