Chapter XI: RECENT COMETS (continued) (2)

A group of five comets, including Halley's, own a sort of cliental dependence upon the planet Neptune. They travel out from the sun just to about his distance from it, as if to pay homage to a powerful protector, who gets the credit of their establishment as periodical visitors to the solar system. The second of these bodies to affect a looked-for return was a comet--the sixteenth within ten years--discovered by Pons, July 20, 1812, and found by Encke to revolve in an elliptic orbit, with a period of nearly 71 years. It was not, however, until September 1, 1883, that Mr. Brooks caught its reappearance; it passed perihelion January 25, and was last seen June 2, 1884. At its brightest, it had the appearance of a second magnitude star, furnished with a poorly developed double tail, and was fairly conspicuous to the naked eye in Southern Europe, from December to March. One exceptional feature distinguished it. Its fluctuations in form and luminosity were unprecedented in rapidity and extent. On September 21, Dr. Chandler[1339] observed it at Harvard as a very faint, diffused nebulosity, with slight central condensation. On the next night, there was found in its place a bright star of the eighth magnitude, scarcely marked out, by a bare trace of environing haze, from the genuine stars it counterfeited. The change was attended by an eight-fold augmentation of light, and was proved by Schiaparelli's confirmatory observations[1340] to have been accomplished within a few hours. The stellar disguise was quickly cast aside. The comet appeared on September 23 as a wide nebulous disc, and soon after faded down to its original dimness. Its distance from the sun was then no less than 200 million miles, and its spectrum showed nothing unusual. These strange variations recurred slightly on October 15, and with marked emphasis on January 1, when they were witnessed with amazement, and photometrically studied by Müller of Potsdam.[1341] The entire cycle this time was run through in less than four hours--the comet having, in that brief space, condensed, with a vivid outburst of light, into a seeming star, and the seeming star having expanded back again into a comet. Scarcely less transient, though not altogether similar, changes of aspect were noted by M. Perrotin,[1342] January 13 and 19, 1884. On the latter date, the continuous spectrum given by a reddish-yellow disc surrounding the true nucleus seemed intensified by bright knots corresponding to the rays of sodium.

A comet discovered by Mr. Sawerthal at the Royal Observatory, Cape of Good Hope, February 19, 1888, distinguished itself by blazing up, on May 19, to four or five times its normal brilliancy, at the same time throwing out from the head two lustrous lateral branches.[1343] These had, on June 1, spread backward so as to join the tail, with an effect like the playing of a fountain; ten or eleven days later, they had completely disappeared, leaving the comet in its former shape and insignificance. Its abrupt display of vitality occurred two full months after perihelion.

On the morning of July 7, 1889, Mr. W. R. Brooks, of Geneva, New York, eminent as a successful comet-hunter, secured one of his customary trophies. The faint object in question was moving through the constellation Cetus, and turned out to be a member of Jupiter's numerous family of comets, revolving round the sun in a period of seven years. Its past history came then, to a certain extent, within the scope of investigation, and proved to have been singularly eventful; nor had the body escaped scatheless from the vicissitudes to which it had been exposed. Observing from Mount Hamilton, August 2 and 5, Professor Barnard noticed this comet (1889, v.) to be attended in its progress through space by four _outriders_, "The two brighter companions" (the fainter pair survived a very short time) "were perfect miniatures," Professor Barnard tells us,[1344] "of the larger comet, each having a small, fairly defined head and nucleus, with a faint, hazy tail, the more distant one being the larger and less developed. The three comets were in a straight line, nearly east and west, their tails lying along this line. There was no connecting nebulosity between these objects, the tails of the two smaller not reaching each other, or the large comet. To all appearance they were absolutely independent comets." Nevertheless, Spitaler, at Vienna, in the early days of August, perceived, as it were, a thin cocoon of nebulosity woven round the entire trio.[1345] One of them faded from view September 5; the other actually outshone the original comet on August 31, but was plainly of inferior vitality. It was last seen by Barnard on November 25, with the thirty-six inch refractor, while its primary afforded an observation for position with the twelve-inch, March 20, 1890.[1346] A cause for the disruption it had presumably undergone had, before then, been plausibly assigned.

The adventures of Lexell's comet have long served to exemplify the effects of Jupiter's despotic sway over such bodies. Although bright enough in 1770 to be seen with the naked eye, and ascertained to be circulating in five and a half years, it had never previously been seen, and failed subsequently to present itself. The explanation of this anomaly, suggested by Lexell, and fully confirmed by the analytical inquiries both of Laplace and Leverrier,[1347] was that a very close approach to Jupiter in 1767 had completely changed the character of its orbit, and brought it within the range of terrestrial observation; while in 1779, after having only twice traversed its new path (at its second return it was so circumstanced as to be invisible from the earth), it was, by a fresh encounter, diverted into one entirely different. Yet the possibility was not lost sight of that the great planet, by inverting its mode of action, might undo its own work, and fling the comet once more into the inner part of the solar system. This possibility seemed to be realized by Chandler's identification of Brooks's and Lexell's comet.[1348] An exceedingly close approach to Jupiter in 1886 had, he found reason to believe, produced such extensive alterations in the elements of its motion as to bring the errant body back to our neighbourhood in 1889. But his inference, though ratified by Mr. Charles Lane Poor's preliminary calculations, proved dubious on closer inquiry, and was rendered wholly inadmissible by the circumstances attending the return of Brooks's comet in 1896.[1349] The companion-objects watched by Barnard in 1889 had by that time, perhaps, become dissipated in space, for they were not redetected. They represented, in all likelihood, wreckage from a collision with Jupiter, dating, perhaps, so far back as 1791, when Mr. Lane Poor found that one of the fateful meetings to which short-period comets are especially subject had taken place.

The Lexell-Brooks case was almost duplicated by the resemblance to De Vico's lost comet of 1844[1350] of one detected November 20, 1894, by Edward, son of Lewis Swift. Schulhof[1351] announced the identity, and Chandler,[1352] under reserve, vouched for it. Had the comet continued to pursue the track laboriously laid down for it at Boston, and shown itself at the due epoch in 1900, its individuality might have been considered assured; but the formidable vicegerent of the sun once more interposed, and, in 1897, swept it out of the terrestrial range of view. Hence the recognition remains ambiguous.

On the morning of March 7, 1892, Professor Lewis Swift discovered the brightest comet that had been seen by northern observers since 1882. About the time of perihelion, which occurred on April 6, it was conspicuous, as it crossed the celestial equator from Aquarius towards Pegasus, with a nucleus equal to a third magnitude star, and a tail twenty degrees long. This tail was multiple, and multiple in a most curiously variable manner. It divided up into many thin nebulous streaks, the number and relative lustre of which underwent rapid and marked changes. Their permanent record on Barnard's and W. H. Pickering's plates marked a noteworthy advance in cometary photography. Plate IV. reproduces two of the Lick pictures, taken with a six-inch camera, on April 5 and 7 respectively, with, in each case, an exposure of about one hour. The tail is in the first composed of three main branches, the middle one having sprung out since the previous morning, and the branches are, in their turn, split up into finer rays, to the number of perhaps a dozen in all. In the second a very different state of things is exhibited. "The southern component," Professor Barnard remarked, "which was the brightest on the 5th, had become diffused and fainter, while the middle tail was very bright and broad. Its southern side, which was the best defined, was wavy in numerous places, the tail appearing as if disturbing currents were flowing at right angles to it. At 42° from the head the tail made an abrupt bend towards the south, as if its current was deflected by some obstacle. In the densest portion of the tail, at the point of deflection, are a couple of dark holes, similar to those seen in some of the nebulæ. The middle portion of the tail is brighter, and looks like crumpled silk in places."[1353] Next morning the southern was the prominent branch, and it was loaded, at 1° 42' from the head, with a strange excrescence, suggesting the budding-out of a fresh comet in that incongruous situation.[1354] Some of these changes, Professor Barnard thought, might possibly be explained by a rotation of the tail on an axis passing through the nucleus, and Pickering, who formed a similar opinion on independent grounds, assigned about 94 hours as the period of the gyrating movement.[1355] He, moreover, determined accelerative velocities outward from the sun of definite condensations in the tail, indicating for its materials, on Brédikhine's theory, a density less than one half that of hydrogen.[1356] This conclusion applied also to Rordame's comet, which exhibited a year later phenomena analogous to those remarked in Swift's. Their photographic study led Professor Hussey[1357] to significant inferences as to the structure and rapid changes of cometary appendages.

PLATE IV.

Seven comets were detected in 1892, and all, strange to say, were visible together towards the close of the year.[1358] Among them was a faint object, which unexpectedly left a trail on a plate exposed by Professor Barnard to the stars in Aquila[1359] on October 12. This was the first comet actually discovered by photography, the Sohag comet having been simultaneously seen and pictured. It has a period of about six years. Holmes's comet is likewise periodical, in rather less than seven years. Its path, which is wholly comprised between the orbits of Mars and Jupiter, is less eccentric than that of any other known comet. Subsequently to its discovery, on November 6, it underwent some curious vicissitudes. At first bright and condensed, it expanded rapidly with increasing distance from the sun (to which it had made its nearest approach on June 13), until, by the middle of December, it was barely discernible with powerful telescopes as "a feebly luminous mist on the face of the sky."[1360] But on January 16, 1893, observers in Europe and America were bewildered to find, as if substituted for it, a yellow star of the seventh magnitude, enveloped in a thin nebulous husk, which enclosed a faint miniature tail.[1361] This condensation and recovery of light lasted in its full intensity only a couple of days. The almost evanescent faintness of Holmes's comet at its next return accounted for its invisibility previous to 1892, when it was evidently in a state of peculiar excitement. Mr. Perrine was barely able, with the Lick 36-inch, to find the vague nebulous patch which occupied its predicted place on June 10, 1899.

The origin of comets has been long and eagerly inquired into, not altogether apart from the cheering guidance of ascertained facts. Sir William Herschel regarded them as fragments of nebulæ[1362]--scattered débris of embryo worlds; and Laplace approved of and adopted the idea.[1363] But there was a difficulty. No comet has yet been observed to travel in a decided hyperbola. The typical cometary orbit, apart from disturbance, is parabolic--that is to say, it is indistinguishable from an enormously long ellipse. But this circumstance could only be reconciled with the view that the bodies thus moving were casual visitors from outer space, by making, as Laplace did, the tacit assumption that the solar system was at rest. His reasoning was, indeed, thereby completely vitiated, as Gauss pointed out in 1815;[1364] and the objections then urged were reiterated by Schiaparelli,[1365] who demonstrated in 1871 that a large preponderance of well-marked hyperbolic orbits should result if comets were picked up _en route_ by a swiftly-advancing sun. The fact that their native movement is practically parabolic shows it to have been wholly imparted from without. They passively obeyed the pull exerted upon them. In other words, their condition previous to being attracted by the sun was one very nearly of relative repose.[1366] They shared, accordingly, the movement of translation through space of the solar system.

This significant conclusion had been indicated, on other grounds, as the upshot of researches undertaken independently by Carrington[1367] and Mohn[1368] in 1860, with a view to ascertaining the anticipated existence of a relationship between the general _lie_ of the paths of comets and the direction of the sun's journey. It is tolerably obvious that if they wander at haphazard through interstellar regions their apparitions should markedly aggregate towards the vicinity of the constellation Lyra; that is to say, we should meet considerably more comets than would overtake us, for the very same reason that falling stars are more numerous after than before midnight. Moreover, the comets met by us should be, apparently, swifter-moving objects than those coming up with us from behind; because, in the one case, our own real movement would be added to, in the other subtracted from, theirs. But nothing of all this can be detected. Comets approach the sun indifferently from all quarters, and with velocities quite independent of direction.

We conclude, then, that the "cosmical current" which bears the solar system towards its unknown goal carries also with it nebulous masses of undefined extent, and at an undefined remoteness, fragments detached from which, continually entering the sphere of the sun's attraction, flit across our skies under the form of comets. These are, however, almost certainly so far strangers to our system that they had no part in the long processes of development by which its present condition was attained. They are, perhaps, survivals of an earlier, and by us scarcely and dimly conceivable state of things, when the swirling chaos from which sun and planets were, by a supreme edict, to emerge, had not as yet separately begun to be.

FOOTNOTES:

[Footnote 1267: _Astr. Nach._, Nos. 1,172-4.]

[Footnote 1268: _Berichte Sächs. Ges._, 1871, p. 174.]

[Footnote 1269: _Natur der Cometen_, p. 124; _Astr. Nach._, No. 2,086.]

[Footnote 1270: _Annales de l'Obs. de Moscou_, t. iii., pt. i., p. 37.]

[Footnote 1271: _Bull. Astr._, t. iii., p. 598. The value of the repellent force for the comet of 1811 (which offered peculiar facilities for its determination) was found = 17·5.]

[Footnote 1272: Faye, _Comptes Rendus_, t. xciii., p. 13.]

[Footnote 1273: _Annales_, t. v., pt. ii., p. 137.]

[Footnote 1274: _Am. Jour. of Sc._, vol. xxxii. (2nd ser.), p. 57.]

[Footnote 1275: _Astr. Nach._, No. 2,082.]

[Footnote 1276: _Annales de l'Obs. de Moscou_, t. vi., pt. i., p. 60.]

[Footnote 1277: _Astr. Register_, March, 1883.]

[Footnote 1278: _Astr. Nach._, No. 3,018.]

[Footnote 1279: _Ibid._, No. 3,093.]

[Footnote 1280: _Constitution de l'Espace Céleste_, p. 224.]

[Footnote 1281: _Astroph. Jour._, vol. iii., p. 36.]

[Footnote 1282: _Physikalische Zeitschrift_, November 10 and 17, 1900; _Astroph. Jour._, vol. xiii., p. 344. _Cf._ Schwarzschild, _Sitzungsb._, München, 1901, Heft iii.; J. Hahn, _Nature_, vols. lxv., p. 415; lxvi., p. 55.]

[Footnote 1283: _Astr. Nach._, No. 2,307.]

[Footnote 1284: _Ibid._, No. 2,304.]

[Footnote 1285: _Observatory_, vol. iii., p. 390.]

[Footnote 1286: _Astr. Nach._, No. 2,319.]

[Footnote 1287: _Ueber die Kometen von 371 v. Chr._, 1668, 1843, I. und 1880 I. Göttingen, 1880.]

[Footnote 1288: _Meteor._, lib. i., cap. 6.]

[Footnote 1289: _Mém. Soc. Phys. de Genève_, t. xxviii., p. 23.]

[Footnote 1290: _Annales de l'Obs. de Moscou_, t. vii., pt. i., p. 60.]

[Footnote 1291: Brédikhine, _Annales_, t. viii., p. 68.]

[Footnote 1292: _Am. Jour. of Sc._, vol. xxii., p. 305.]

[Footnote 1293: Messrs. Burton and Green observed a dilatation of the stellar image into a nebulous patch by the transmission of its rays through a nuclear jet of the comet. _Am. Jour. of Sc._, vol. xxii., p. 163.]

[Footnote 1294: _Archives des Sciences_, t. viii., p. 535. _Cf._ Perrine's negative results for Swift's comet in 1899, _Astr. Nach._, No. 3,602.]

[Footnote 1295: Riem concluded in 1896 for a definitive period of 2,429 years; _Observatory_, vol. xix., p. 282.]

[Footnote 1296: Holden, _Publ. Astr. Pac. Soc._, vol. ix., p. 89.]

[Footnote 1297: _Annuaire_, Paris, 1882, p. 781.]

[Footnote 1298: _Annuaire_, 1882, p. 766.]

[Footnote 1299: _Am. Jour. of Sc._, vol. xxii., p. 134.]

[Footnote 1300: _Report Brit. Assoc._, 1881, p. 520.]

[Footnote 1301: _Month. Not._, vol. xlii., p. 14; _Am. Jour. of Sc._, vol. xxii., p. 136.]

[Footnote 1302: Piazzi Smyth, _Nature_, vol. xxiv., p. 430.]

[Footnote 1303: _Astr. Nach._, No. 2,395.]

[Footnote 1304: _Ibid._]

[Footnote 1305: _Astr. Nach._, No. 2,411.]

[Footnote 1306: _Month. Not._, vol. xlii., p. 49.]

[Footnote 1307: _Astr. Nach._, No. 2,414.]

[Footnote 1308: _Copernicus_, vol. ii., p. 229.]

[Footnote 1309: _Astr. Nach._, Nos. 2,434, 2,437.]

[Footnote 1310: _Ibid._, No. 2,441.]

[Footnote 1311: _Report Brit. Assoc._, 1882, p. 442.]

[Footnote 1312: J. J. Parsons, _Am. Jour. of Science_, vol. xxvii., p. 34.]

[Footnote 1313: _Astr. Nach._, No. 2,441.]

[Footnote 1314: _Observatory_, vol. v., p. 355. The transit had been foreseen by Mr. Tebbutt, but it occurred after sunset in New South Wales.]

[Footnote 1315: _Observatory_, vol. v., p. 354.]

[Footnote 1316: Gould, _Astr. Nach._, No. 2,481.]

[Footnote 1317: Flammarion, _Comptes Rendus_, t. xcv., p. 558.]

[Footnote 1318: Captain Ray's sextant observation of the comet of 1843, a few hours before perihelion, was too rough to be of use.]

[Footnote 1319: _Astr. Nach._, No. 2,538.]

[Footnote 1320: _Nature_, vol. xxix., p. 135.]

[Footnote 1321: _Astr. Nach._, No. 2,482.]

[Footnote 1322: _Vierteljahrsschrift Astr. Ges._, Jahrg. xxiv., p. 308; _Bull. Astr._, t. vii., p. 513.]

[Footnote 1323: _Observatory_, vol. xxiv., p. 167.]

[Footnote 1324: The attention of the author was kindly directed to this point by Professor Young of Princeton (N. J.). _Cf._ Rebeur-Paschwitz, _Sirius_, Bd. xvi., p. 233.]

[Footnote 1325: Oppenheim, _Astr. Nach._, No. 2,902.]

[Footnote 1326: _Astr. Nach._, No. 2,717.]

[Footnote 1327: Gruithuisen's _Analekten_, Heft 7, p. 48.]

[Footnote 1328: _Month. Not._, vols. xxv., xxvi., xxviii. _Cf._ Plummer, _Observatory_, vol. xiii., p. 263.]

[Footnote 1329: _Nature_, vol. xxvii., p. 246.]

[Footnote 1330: _Astr. Nach._, No. 2,468.]

[Footnote 1331: _Athenæum_, February 3, 1883.]

[Footnote 1332: _Astr. Nach._, Nos. 2,462, 2,466.]

[Footnote 1333: _Ibid._, No. 2,489.]

[Footnote 1334: _Annales_, Moscow, t. ix., pt. ii., p. 52.]

[Footnote 1335: _Comptes Rendus_, t. xcvii., p. 797.]

[Footnote 1336: _Astr. Nach._, No. 2,966.]

[Footnote 1337: _Copernicus_, vol. ii., p. 235.]

[Footnote 1338: _Comptes Rendus_, t. xcvi., p. 371.]

[Footnote 1339: _Astr. Nach._, No. 2,553.]

[Footnote 1340: _Ibid._]

[Footnote 1341: _Astr. Nach._, No. 2,568.]

[Footnote 1342: _Annales de l'Observatoire de Nice_, t. ii., c. 53.]

[Footnote 1343: Fényi, _Astr. Nach._, No. 2,844; Kammermann, _Ibid._, No. 2,849.]

[Footnote 1344: _Publ. Astr. Pac. Soc._, vol. i., p. 72.]

[Footnote 1345: _Annuaire_, Paris, 1891, p. 301.]

[Footnote 1346: _Astr. Nach._, No. 2,989.]

[Footnote 1347: _Comptes Rendus_, t. xxv., p. 564.]

[Footnote 1348: _Astr. Journ._, Nos. 205, 231.]

[Footnote 1349: _Ibid._, Nos. 228, 244, 380.]

[Footnote 1350: _Observatory_, vol. xviii., pp. 60, 163 (Denning and Lynn).]

[Footnote 1351: _Astr. Nach._, No. 3,267; Plummer, _Knowledge_, vol. xix., p. 156.]

[Footnote 1352: _Astr. Jour._, Nos. 333, 338.]

[Footnote 1353: _Astr. and Astroph._, vol. xi., p. 387.]

[Footnote 1354: _Knowledge_, vol. xv., p. 299.]

[Footnote 1355: _Harvard Annals_, vol. xxxii., pt. ii., p. 272.]

[Footnote 1356: _Ibid._, p. 287.]

[Footnote 1357: _Publ. Astr. Pac. Soc._, vol. vii., p. 161.]

[Footnote 1358: H. C. Wilson, _Astr. and Astroph._, vol. xii., p. 121.]

[Footnote 1359: _Observatory_, vol. xvi., p. 92.]

[Footnote 1360: Barnard, _Astr. and Astroph._, vol. xii., p. 180; _Astroph. Jour._, vol. iii., p. 41.]

[Footnote 1361: Palisa, _Astr. Nach._, No. 3,147; Denning, _Observatory_, vol. xvi., p. 142.]

[Footnote 1362: _Phil. Trans._, vol. ci., p. 306.]

[Footnote 1363: _Conn. des Temps_, 1816, p. 213.]

[Footnote 1364: _OEuvres_, t. vi., p. 581.]

[Footnote 1365: _Mem. dell' Istit. Lombardo_, t. xii., p. 164; _Rendiconti_, t. vii., p. 77, 1874.]

[Footnote 1366: W. Förster, _Pop. Mitth._, 1879, p. 7; Fabry, _Étude sur la Probabilité des Comètes Hyperboliques_, Marseille, 1893, p. 158.]

[Footnote 1367: _Mem. R. A. Soc._, vol. xxix., p. 335.]

[Footnote 1368: _Month. Not._, vol. xxiii., p. 203.]