A Popular History of Astronomy During the Nineteenth Century / Fourth Edition

by Agnes M. Clerke

With the comet of 1668 the case seemed more dubious. The circumstances of its appearance are barely reconcilable with the identity attributed to it, although too vaguely known to render certainty one way or the other attainable. It might however, be expected that recent observations would at least decide the questions whether the comet of 1843 could have returned in less than thirty-seven, and whether the comet of 1880 was to be looked for at the end of 17-1/2 years. But the truth is that both these objects were observed over so small an arc—8° and 3° respectively—that their periods remained virtually undetermined. For while the shape and position of their orbits could be and were fixed with a very close approach to accuracy, the length of those orbits might vary enormously without any very sensible difference being produced in the small part of the curves traced out near the sun. Dr. Wilhelm Meyer, however, arrived, by an elaborate discussion, at a period of thirty-seven years for the comet of 1880,[1289] while the observations of 1843 were admittedly best fitted by Hubbard's ellipse of 533 years; but these Dr. Meyer supposed to be affected by some constant source of error, such as would be produced by a mistaken estimate of the position of the comet's centre of gravity. He inferred finally that, in spite of previous non-appearances, the two comets represented a single regular denizen of our system, returning once in thirty-seven years along an orbit of such extreme eccentricity that its movement might be described as one of precipitation towards and rapid escape from the sun, rather than of sedate circulation round it.

The geometrical test of identity has hitherto been the only one which it was possible to apply to comets, and in the case before us it may fairly be said to have broken down. We may, then, tentatively, and with much hesitation, try a physical test, though scarcely yet, properly speaking, available. We have seen that the comets of 1843 and 1880 were strikingly alike in general appearance, though the absence of a formed nucleus in the latter, and its inferior brilliancy, detracted from the convincing effect of the resemblance. Nor was it maintained when tried by exact methods of inquiry. M. Brédikhine found that the gigantic ray emitted in 1843 belonged to his type No. 1; that of 1880 to type No. 2.[1290] The particles forming the one were actuated by a repulsive force ten times as powerful as those forming the other. It is true that a second noticeably curved tail was seen in Chili, March 1, and at Madras, March 11, 1843; and the conjecture was accordingly hazarded that the materials composing on that occasion the principal appendage having become exhausted, those of the secondary one remained predominant, and reappeared alone in the "hydro-carbon" train of 1880. But the one known instance in point is against such a supposition. Halley's comet, the only great comet of which the returns have been securely authenticated and carefully observed, has preserved its "type" unchanged through many successive revolutions. The dilemma presented to astronomers by the Great Southern Comet of 1880 was unexpectedly renewed in the following year.

On the 22nd of May, 1881, Mr. John Tebbutt of Windsor, New South Wales, scanning the western sky, discerned a hazy-looking object which he felt sure was a strange one. A marine telescope at once resolved it into two small stars and a comet, the latter of which quickly attracted the keen attention of astronomers; for Dr. Gould, computing its orbit from his first observations at Cordoba, found it to agree so closely with that arrived at by Bessel for the comet of 1807 that he telegraphed to Europe, June 1, announcing the unexpected return of that body. So unexpected that theoretically it was not possible before the year 3346; and Bessel's investigation was one which inspired and eminently deserved confidence. Here, then, once more the perplexing choice had to be made between a premature and unaccountable reappearance and the admission of a plurality of comets moving nearly in the same path. But in this case facts proved decisive.

Tebbutt's comet passed the sun, June 16, at a distance of sixty-eight millions of miles, and became visible in Europe six days later. It was, in the opinion of some, the finest object of the kind since 1861. In traversing the constellation Auriga on its début in these latitudes, it outshone Capella. On June 24 and some subsequent nights, it was unmatched in brilliancy by any star in the heavens. In the telescope, the "two interlacing arcs of light" which had adorned the head of Coggia's comet were reproduced; while a curious dorsal spine of strong illumination formed the axis of the tail, which extended in clear skies over an arc of 20°. It belonged to the same "type" as Donati's great plume; the particles composing it being driven from the sun by a force twice as powerful as that urging them towards it.[1291] But the appendage was, for a few nights, and by two observers perceived to be double. Tempel, on June 27, and Lewis Boss, at Albany (N.Y.), June 26 and 28, saw a long straight ray corresponding to a far higher rate of emission than the curved train, and shown by Brédikhine to be a member of the (so-called) hydrogen class. It had vanished by July 1, but made a temporary reappearance July 22.[1292]

The appendages of this comet were of remarkable transparency. Small stars shone wholly undimmed across the tail, and a very nearly central transit of the head over one of the seventh magnitude on the night of June 29, produced—if any change—an increase of brilliancy in the object of this spontaneous experiment.[1293] Dr. Meyer, indeed, at the Geneva Observatory, detected apparent signs of refractive action upon rays thus transmitted;[1294] but his observations remain isolated, and were presumably illusory.

The track pursued by this comet gave peculiar advantages for its observation. Ascending from Auriga through Camelopardus, it stood, July 19, on a line between the Pointers and the Pole, within 8° of the latter, and thus remained for a lengthened period constantly above the horizon of northern observers. Its brightness, too, was no transient blaze, but had a lasting quality which enabled it to be kept steadily in view during nearly nine months. Visible to the naked eye until the end of August, the last telescopic observation of it was made February 14, 1882, when its distance from the earth considerably exceeded 300 million miles. Under these circumstances, the knowledge acquired of its orbit was of more than usual accuracy, and showed conclusively that the comet was not a simple return of Bessel's; for this would involve a period of seventy-four years, whereas Tebbutt's comet cannot revisit the sun until after the lapse of two and a half millenniums.[1295] Nevertheless, the twin bodies move so nearly in the same path that an original connection of some kind is obvious; and the recent example of Biela readily suggested a conjecture as to what the nature of that connection might have been. The comets of 1807 and 1881 are, then, regarded with much probability as fragments of a primitive disrupted body, one following in the wake of the other at an interval of seventy-four years.

Imperfect photographs were taken of Donati's comet both in England and America;[1296] but Tebbutt's comet was the first to which the process was satisfactorily applied. The difficulties to be overcome were very great. The chemical intensity of cometary light is, to begin with, extraordinarily small. Janssen estimated it at 1/300000 of moonlight.[1297] Hence, if the ordinary process by which lunar photographs are taken had been applied to the comet of 1881, an exposure of at least three days would have been required in order to get an impression of the head with about a tenth part of the tail. But by that time a new method of vastly increased sensitiveness had been rendered available, by which dry gelatine-plates were substituted for the wet collodion-plates hitherto in use; and this improvement alone reduced the necessary time of exposure to two hours. It was brought down to half an hour by Janssen's employment of a reflector specially adapted to give an image illuminated eight or ten times as strongly as that produced in the focus of an ordinary telescope.[1298]

The photographic feebleness of cometary rays was not the only obstacle in the way of success. The proper motion of these bodies is so rapid as to render the usual devices for keeping a heavenly body steadily in view quite inapplicable. The machinery by which the diurnal movement of the sphere is followed, must be especially modified to suit each eccentric career. This, too, was done, and on June 30, 1881, Janssen secured a perfect photograph of the brilliant object then visible, showing the structure of the tail with beautiful distinctness to a distance of 2-1/2° from the head. An impression to nearly 10° was obtained about the same time by Dr. Henry Draper at New York, with an exposure of 162 minutes.[1299]

Tebbutt's (or comet 1881 iii.) was also the first comet of which the spectrum was so much as attempted to be chemically recorded. Both Huggins and Draper were successful in this respect, but Huggins was more completely so.[1300] The importance of the feat consisted in its throwing open to investigation a part of the spectrum invisible to the eye, and so affording an additional test of cometary constitution. The result was fully to confirm the origin from carbon-compounds assigned to the visible rays, by disclosing additional bands belonging to the same series in the ultra-violet; as well as to establish unmistakably the presence of a not inconsiderable proportion of reflected solar light by the clear impression of some of the principal Fraunhofer lines. Thus the polariscope was found to have told the truth, though not the whole truth.

The photograph so satisfactorily communicative was taken by Sir William Huggins on the night of June 24; and on the 29th, at Greenwich, the tell-tale Fraunhofer lines were perceived to interrupt the visible range of the spectrum. This was at first so vividly continuous, that the characteristic cometary bands could scarcely be detached from their bright background. But as the nucleus faded towards the end of June, they came out strongly, and were more and more clearly seen, both at Greenwich and at Princeton, to agree, not with the spectrum of hydro-carbons glowing in a vacuum tube, but with that of the same substances burning in a Bunsen flame.[1301] It need not, however, be inferred that cometary materials are really in a state of combustion. This, from all that we know, may be called an impossibility. The additional clue furnished was rather to the manner of their electrical illumination.[1302]