Part 11 (1/2)
Encke's comet, above all, has served as an index to much curious information, and it may be hoped that its function in that respect is by no means at an end. The great extent of the solar system traversed by its eccentric path makes it peculiarly useful for the determination of the planetary ma.s.ses. At perihelion it penetrates within the orbit of Mercury; it considerably transcends at aphelion the farthest excursion of Pallas. Its vicinity to the former planet in August, 1835, offered the first convenient opportunity of placing that body in the astronomical balance. Its weight or ma.s.s had previously been a.s.sumed, not ascertained; and the comparatively slight deviation from its regular course impressed upon the comet by its attractive power showed that it had been a.s.sumed nearly twice too great.[244] That fundamental datum of planetary astronomy--the ma.s.s of Jupiter--was corrected by similar means; and it was rea.s.suring to find the correction in satisfactory accord with that already introduced from observations of the asteroidal movements.
The fact that comets contract in approaching the sun had been noticed by Hevelius; Pingre admitted it with hesitating perplexity;[245] the example of Encke's comet rendered it conspicuous and undeniable. On the 28th of October, 1828, the diameter of the nebulous matter composing this body was estimated at 312,000 miles. It was then about one and a half times further from the sun than the earth is at the time of the equinox. On the 24th of December following, its distance being reduced by nearly two-thirds, it was found to be only 14,000 miles across.[246]
That is to say, it had shrunk during those two months of approach to 1/11000th part of its original volume! Yet it had still seventeen days'
journey to make before reaching perihelion. The same curious circ.u.mstance was even more markedly apparent at its return in 1838. Its bulk, or the actual s.p.a.ce occupied by it, appeared to be reduced, as it drew near the hearth of our system, in the enormous proportion of 800,000 to 1. A corresponding expansion accompanied on each occasion its retirement from the sphere of observation. Similar changes of volume, though rarely to the same astounding extent, have been perceived in other comets. They still remain unexplained; but it can scarcely be doubted that they are due to the action of the same energetic internal forces which reveal themselves in so many splendid and surprising cometary phenomena.
Another question of singular interest was raised by Encke's acute inquiries into the movements and disturbances of the first known ”comet of short period.” He found from the first that its revolutions were subject to some influence besides that of gravity. After every possible allowance had been made for the pulls, now backward, now forward, exerted upon it by the several planets, there was still a surplus of acceleration left unaccounted for. Each return to perihelion took place about two and a half hours sooner than received theories warranted.
Here, then, was a ”residual phenomenon” of the utmost promise for the disclosure of novel truths. Encke (in accordance with the opinion of Olbers) explained it as due to the presence in s.p.a.ce of some such ”subtle matter” as was long ago invoked by Euler[247] to be the agent of eventual destruction for the fair scheme of planetary creation. The apparent anomaly of accounting for an accelerative effect by a r.e.t.a.r.ding cause disappears when it is considered that any check to the motion of bodies revolving round a centre of attraction causes them to draw closer to it, thus shortening their periods and quickening their circulation.
If s.p.a.ce were filled with a resisting medium capable of impeding, even in the most infinitesimal degree, the swift course of the planets, their orbits should necessarily be, not ellipses, but very close elliptical spirals along which they would slowly, but inevitably, descend into the burning lap of the sun. The circ.u.mstance that no such tendency can be traced in their revolutions by no means sets the question at rest. For it might well be that an effect totally imperceptible until after the lapse of countless ages, as regards the solid orbs of our system, might be obvious in the movements of bodies like comets of small ma.s.s and great bulk; just as a feather or a gauze veil at once yields its motion to the resistance of the air, while a cannon-ball cuts its way through with comparatively slight loss of velocity.
It will thus be seen that issues of the most momentous character hang on the _time-keeping_ of comets; for plainly all must in some degree suffer the same kind of hindrance as Encke's, if the cause of that hindrance be the one suggested. None of its congeners, however, show any trace of similar symptoms. True, the late Professor Oppolzer announced,[248] in 1880, that a comet, first seen by Pons in 1819, and rediscovered by Winnecke in 1858, having a period of 2,052 days (56 years), was accelerated at each revolution precisely in the manner required by Encke's theory. But M. von Haerdtl's subsequent investigation, the materials for which included numerous observations of the body in question at its return to the sun in 1886, decisively negatived the presence of any such effect.[249] Moreover, the researches of Von Asten and Backlund[250] into the movements of Encke's comet revealed a perplexing circ.u.mstance. They confirmed Encke's results for the period covered by them, but exhibited the acceleration as having _suddenly diminished_ by nearly one-half in 1868. The reality and permanence of this change were fully established by observations of the ensuing return in March, 1885. Some physical alteration of the r.e.t.a.r.ded body seems indicated; but visual evidence countenances no such a.s.sumption. In aspect the comet is no less thin and diffuse than in 1795 or in 1848.
The character of the supposed resistance in inter-planetary s.p.a.ce has, it may be remarked, been often misapprehended. What Encke stipulated for was not a medium equally diffused throughout the visible universe, such as the ethereal vehicle of the vibrations of light, but a rare fluid, rapidly increasing in density towards the sun.[251] This cannot be a solar atmosphere, since it is mathematically certain, as Laplace has shown,[252] that no envelope partaking of the sun's axial rotation can extend farther from his surface than nine-tenths of the mean distance of Mercury; while physical evidence a.s.sures us that the _actual_ depth of the solar atmosphere bears a very minute proportion to the _possible_ depth theoretically a.s.signed to it. That matter, however, not atmospheric in its nature--that is, neither forming one body with the sun nor altogether aeriform--exists in its neighbourhood, can admit of no reasonable doubt. The great lens-shaped ma.s.s of the zodiacal light, stretching out at times far beyond the earth's...o...b..t, may indeed be regarded as an extension of the corona, the streamers of which themselves mark the wide diffusion, all round the solar globe, of granular or gaseous materials. Yet comets have been known to penetrate the sphere occupied by them without perceptible loss of velocity. The hypothesis, then, of a resisting medium receives at present no countenance from the movements of comets, whether of short or of long periods.
Although Encke's comet has made thirty-five complete rounds of its...o...b..t since its first detection in 1786, it shows no certain signs of decay.
Variations in its brightness are, it is true, conspicuous, but they do not proceed continuously.[253]
The history of the next known planet-like comet has proved of even more curious interest than that of the first. It was discovered by an Austrian officer named Wilhelm von Biela at Josephstadt in Bohemia, February 27, 1826, and ten days later by the French astronomer Gambart at Ma.r.s.eilles. Both observers computed its...o...b..t, showed its remarkable similarity to that traversed by comets visible in 1772 and 1805, and connected them together as previous appearances of the body just detected by a.s.signing to its revolutions a period of between six and seven years. The two brief letters conveying these strikingly similar inferences were printed side by side in the same number of the _Astronomische Nachrichten_ (No. 94); but Biela's priority in the discovery of the comet was justly recognised by the bestowal upon it of his name.
The object in question was at no time, subsequently to 1805, visible to the naked eye. Its aspect in Sir John Herschel's great reflector on the 23rd of September, 1832, was described by him as that of a ”conspicuous nebula,” nearly 3 minutes in diameter. No trace of a tail was discernible. While he was engaged in watching it, a small knot of minute stars was directly traversed by it, ”and when on the cl.u.s.ter,” he tells us,[254] it ”presented the appearance of a nebula resolvable and partly resolved into stars, the stars of the cl.u.s.ter being visible through the comet.” Yet the depth of cometary matter through which such faint stellar rays penetrated undimmed, was, near the central parts of the globe, not less than 50,000 miles.
It is curious to find that this seemingly harmless, and we may perhaps add effete body, gave occasion to the first (and not the last) cometary ”scare” of an enlightened century. Its...o...b..t, at the descending node, may be said to have intersected that of the earth; since, according as it _bulged in or out_ under the disturbing influence of the planets, the pa.s.sage of the comet was affected _inside_ or _outside_ the terrestrial track. Now, certain calculations published by Olbers in 1828[255] showed that, on October 29, 1832, a considerable portion of its nebulous surroundings would actually sweep over the spot which, a month later, would be occupied by our planet. It needed no more to set the popular imagination in a ferment. Astronomers, after all, could not, by an alarmed public, be held to be infallible. Their computations, it was averred, which a trifling oversight would suffice to vitiate, exhibited clearly enough the danger, but afforded no guarantee of safety from a collision, with all the terrific consequences frigidly enumerated by Laplace. Nor did the panic subside until Arago formally demonstrated that the earth and the comet could by no possibility approach within less than fifty millions of miles.[256]
The return of the same body in 1845-46 was marked by an extraordinary circ.u.mstance. When first seen, November 28, it wore its usual aspect of a faint round patch of cosmical fog; but on December 19, Mr. Hind noticed that it had become distorted somewhat into the form of a pear; and ten days later, it had divided into two separate objects. This singular duplication was first perceived at New Haven in America, December 29,[257] by Messrs. Herrick and Bradley, and by Lieutenant Maury at Was.h.i.+ngton, January 13, 1846. The earliest British observer of the phenomenon (noticed by Wichmann the same evening at Konigsberg) was Professor Challis. ”I see _two_ comets!” he exclaimed, putting his eye to the great equatoreal of the Cambridge Observatory on the night of January 15; then, distrustful of what his senses had told him, he called in his judgment to correct their improbable report by resolving one of the dubious objects into a hazy star.[258] On the 23rd, however, both were again seen by him in unmistakable cometary shape, and until far on in March (Otto Struve caught a final glimpse of the pair on the 16th of April),[259] continued to be watched with equal curiosity and amazement by astronomers in every part of the northern hemisphere. What Seneca reproved Ephorus for supposing to have taken place in 373 b.c.--what Pingre blamed Kepler for conjecturing in 1618--had then actually occurred under the attentive eyes of science in the middle of the nineteenth century!
At a distance from each other of about two-thirds the distance of the moon from the earth, the twin comets meantime moved on tranquilly, so far, at least, as their course through the heaven was concerned. Their extreme _lightness_, or the small amount of matter contained in each, could not have received a more signal ill.u.s.tration than by the fact that their revolutions round the sun were performed independently; that is to say, they travelled side by side without experiencing any appreciable mutual disturbance, thus plainly showing that at an interval of only 157,250 miles their attractive power was virtually inoperative. Signs of internal agitation, however, were not wanting. Each fragment threw out a short tail in a direction perpendicular to the line joining their centres, and each developed a bright nucleus, although the original comet had exhibited neither of these signs of cometary vitality. A singular interchange of brilliancy was, besides, observed to take place between the coupled objects, each of which alternately outshone and was outshone by the other, while an arc of light, apparently proceeding from the more l.u.s.trous, at times bridged the intervening s.p.a.ce. Obviously, the gravitational tie, rendered powerless by exiguity of matter, was here replaced by some other form of mutual action, the nature of which can as yet be dealt with only by conjecture.
Once more, in August, 1852, the double comet returned to the neighbourhood of the sun, but under circ.u.mstances not the most advantageous for observation. Indeed, the companion was not detected until September 16, when Father Secchi at Rome perceived it to have increased its distance from the originating body to a million and a quarter of miles, or about eight times the average interval at the former appearance. Both vanished shortly afterwards, and have never since been seen, notwithstanding the eager watch kept for objects of such singular interest, and the accurate knowledge of their track supplied by Santini's investigations. A dangerously near approach to Jupiter in 1841 is believed to have occasioned their disruption, and the disaggregating process thus started was likely to continue. We can scarcely doubt that the fate has overtaken them which Newton a.s.signed as the end of all cometary existence. _Diffundi tandem et spargi per coelos universos._[260]
Biela's is not the only vanished comet. Brorsen's, discovered at Kiel in 1846, and observed at four subsequent returns, failed unaccountably to become visible in 1890.[261] Yet numerous sentinels were on the alert to surprise its approach along a well-ascertained track, traversed in five and a half years. The object presented from the first a somewhat time-worn aspect. It was devoid of tail, or any other kind of appendage; and the rapid loss of the light acquired during perihelion pa.s.sage was accompanied by inordinate expansion of an already tenuous globular ma.s.s.
Another lost or mislaid comet is one found by De Vico at Rome, August 22, 1844. It was expected to return early in 1850, but did not, and has never since been seen; unless its re-appearance as E. Swift's comet of 1894 should be ratified by closer inquiry.[262]
A telescopic comet with a period of 7-1/2 years, discovered November 22, 1843, by M. Faye of the Paris Observatory, formed the subject of a characteristically patient and profound inquiry on the part of Leverrier, designed to test its suggested ident.i.ty with Lexell's comet of 1770. The result was decisive against the hypothesis of Valz, the divergences between the orbits of the two bodies being found to increase instead of to diminish, as the history of the new-comer was traced backward into the previous century.[263] Faye's comet pursues the most nearly circular path of any similar known object; even at its nearest approach to the sun it remains farther off than Mars when he is most distant from it; and it was proved by the admirable researches of Professor Axel Moller,[264] director of the Swedish observatory of Lund, to exhibit no trace of the action of a resisting medium.
Periodical comets are evidently bodies which have each lived through a chapter of accidents, and a significant hint as to the nature of their adventures can be gathered from the fact that their aphelia are pretty closely grouped about the tracks of the major planets. Halley's, and five other comets are thus related to Neptune; three connect themselves with Ura.n.u.s, two with Saturn, above a score with Jupiter. Some form of dependence is plainly indicated, and the researches of Tisserand,[265]
Callandreau,[266] and Newton[267] of Yale College, leave scarcely a doubt that the ”capture-theory” represents the essential truth in the matter. The original parabolic paths of these comets were then changed into ellipses by the backward pull of a planet, whose sphere of attraction they chanced to enter when approaching the sun from outer s.p.a.ce. Moreover, since a body thus affected should necessarily return at each revolution to the scene of encounter, the same process of r.e.t.a.r.dation may, in some cases, have been repeated many times, until the more restricted cometary orbits were reduced to their present dimensions. The prevalence, too, among periodical comets, of direct motion, is shown to be inevitable by M. Callandreau's demonstration that those travelling in a retrograde direction would, by planetary action, be thrown outside the probable range of terrestrial observation. The scarcity of hyperbolic comets can be similarly explained. They would be created whenever the attractive influence of the disturbing planet was exerted in a forward or accelerative sense, but could come only by a rare exception to our notice. The inner planets, including the earth, have also unquestionably played their parts in modifying cometary orbits; and Mr. Plummer suggests, with some show of reason, that the capture of Encke's comet may be a feat due to Mercury.[268]
No _great_ comet appeared between the ”star” which presided at the birth of Napoleon and the ”vintage” comet of 1811. The latter was first described by Flaugergues at Viviers, March 26, 1811; Wisniewski, at Neu-Tscherkask in Southern Russia, caught a final glimpse of it, August 17, 1812. Two disappearances in the solar rays as the earth moved round in its...o...b..t, and two reappearances after conjunction, were included in this unprecedentedly long period of visibility of 510 days. This relative permanence (so far as the inhabitants of Europe were concerned) was due to the high northern lat.i.tude attained near perihelion, combined with a certain leisureliness of movement along a path everywhere external to that of the earth. The magnificent luminous train of this body, on October 15, the day of its nearest terrestrial approach, covered an arc of the heavens 23-1/2 degrees in length, corresponding to a real extension of one hundred millions of miles. Its form was described by Sir William Herschel as that of ”an inverted hollow cone,”
and its colour as yellowish, strongly contrasted with the bluish-green tint of the ”head,” round which it was flung like a transparent veil.
The planetary disc of the head, 127,000 miles across, appeared to be composed of strongly-condensed nebulous matter; but somewhat eccentrically situated within it was a star-like nucleus of a reddish tinge, which Herschel presumed to be solid, and ascertained, with his usual care, to have a diameter of 428 miles. From the total absence of phases, as well as from the vivacity of its radiance, he confidently inferred that its light was not borrowed, but inherent.[269]
This remarkable apparition formed the subject of a memoir by Olbers,[270] the striking yet steadily reasoned out suggestions contained in which there was at that time no means of following up with profit. Only of late has the ”electrical theory,” of which Zollner[271]
regarded Olbers as the founder, a.s.sumed a definite and measurable form, capable of being tested by the touchstone of fact, as knowledge makes its slow inroads on the fundamental mystery of the physical universe.
The paraboloidal shape of the bright envelope separated by a dark interval from the head of the great comet of 1811, and const.i.tuting, as it were, the _root_ of its tail, seemed to the astronomer of Bremen to reveal the presence of a double repulsion; the expelled vapours acc.u.mulating where the two forces, solar and cometary, balanced each other, and being then swept backwards in a huge train. He accordingly distinguished three cla.s.ses of these bodies:--First, comets which develop _no_ matter subject to solar repulsion. These have no tails, and are probably mere nebulosities, without solid nuclei. Secondly, comets which are acted upon by solar repulsion _only_, and consequently throw out no emanations _towards_ the sun. Of this kind was a bright comet visible in 1807.[272] Thirdly, comets like that of 1811, giving evidence of action of both kinds. These are distinguished by a dark _hoop_ encompa.s.sing the head and dividing it from the luminous envelope, as well as by an obscure caudal axis, resulting from the hollow, cone-like structure of the tail.
Again, the ingenious view subsequently propounded by M. Bredikhin as to the connection between the _form_ of these appendages and the _kind_ of matter composing them, was very clearly antic.i.p.ated by Olbers. The amount of tail-curvature, he pointed out, depends in each case upon the proportion borne by the velocity of the ascending particles to that of the comet in its...o...b..t; the swifter the outrush, the straighter the resulting tail. But the velocity of the ascending particles varies with the energy of their repulsion by the sun, and this again, it may be presumed, with their quality. Thus multiple tails are developed when the same comet throws off, as it approaches perihelion, specifically distinct substances. The long, straight ray which proceeded from the comet of 1807, for example, was doubtless made up of particles subject to a much more vigorous solar repulsion than those formed into the shorter curved emanation issuing from it nearly in the same direction.
In the comet of 1811 he calculated that the particles expelled from the head travelled to the remote extremity of the tail in eleven minutes, indicating by this enormous rapidity of movement (comparable to that of the transmission of light) the action of a force much more powerful than the opposing one of gravity. The not uncommon phenomena of multiple envelopes, on the other hand, he explained as due to the varying amounts of repulsion exercised by the nucleus itself on the different kinds of matter developed from it.
The movements and perturbations of the comet of 1811 were no less profoundly studied by Argelander than its physical const.i.tution by Olbers. The orbit which he a.s.signed to it is of such vast dimensions as to require no less that 3,065 years for the completion of its circuit; and to carry the body describing it at each revolution to fourteen times the distance from the sun of the frigid Neptune. Thus, when it last visited our neighbourhood, Achilles may have gazed on its imposing train as he lay on the sands all night bewailing the loss of Patroclus; and when it returns, it will perhaps be to s.h.i.+ne upon the ruins of empires and civilizations still deep buried among the secrets of the coming time.[273]
On the 26th of June, 1819, while the head of a comet pa.s.sed across the face of the sun, the earth was in all probability involved in its tail.