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

by Agnes M. Clerke

In the first of these, the repellent energy of the sun is fourteen times stronger than his attractive energy;[1271] the particles forming the enormously long straight rays projected outward from this kind of comet leave the nucleus with a mean velocity of just seven kilometres per second, which, becoming constantly accelerated, carries them in a few days to the limit of visibility. The great comets of 1811, 1843, and 1861, that of 1744 (so far as its principal tail was concerned), and Halley's comet at its various apparitions, belonged to this class. Less narrow limits were assigned to the values of the repulsive force employed to produce the second type. For the axis of the tail, it exceeds by one-tenth (= 1·1) the power of solar gravity; for the anterior edge, it is more than twice (2·2), for the posterior only half as strong. The corresponding initial velocity (for the axis) is 1,500 metres a second, and the resulting appendage a scimitar-like or plumy tail, such as Donati's and Coggia's comets furnished splendid examples of. Tails of the third type are constructed with forces of repulsion from the sun ranging from one-tenth to three-tenths that of his gravity, producing an accelerated movement of attenuated matter from the nucleus, beginning at the leisurely rate of 300 to 600 metres a second. They are short, strongly bent, brush-like emanations, and in bright comets seem to be only found in combination with tails of the higher classes. Multiple tails, indeed—that is, tails of different types emitted simultaneously by one comet—are perceived, as experience advances and observation becomes closer, to be rather the rule than the exception.[1272]

Now what is the meaning of these three types? Is any translation of them into physical fact possible? To this question Brédikhine supplied, in 1879, a plausible answer.[1273] It was already a current surmise that multiple tails are composed of different kinds of matter, differently acted on by the sun. Both Olbers and Bessel had suggested this explanation of the straight and curved emanations from the comet of 1807; Norton had applied it to the faint light tracks proceeding from that of Donati;[1274] Winnecke to the varying deviations of its more brilliant plumage. Brédikhine defined and ratified the conjecture. He undertook to determine (provisionally as yet) the several kinds of matter appropriated severally to the three classes of tails. These he found to be hydrogen for the first, hydro-carbons for the second, and iron for the third. The ground of this apportionment is that the atomic weights of these substances bear to each other the same inverse proportion as the repulsive forces employed in producing the appendages they are supposed to form; and Zöllner had pointed out in 1875 that the "heliofugal" power by which comets' tails are developed would, in fact, be effective just in that ratio.[1275] Hydrogen, as the lightest known element—that is, the least under the influence of gravity—was naturally selected as that which yielded most readily to the counter-persuasions of electricity. Hydro-carbons had been shown by the spectroscope to be present in comets, and were fitted by their specific weight, as compared with that of hydrogen, to form tails of the second type; while the atoms of iron were just heavy enough to compose those of the third, and, from the plentifulness of their presence in meteorites, might be presumed to enter, in no inconsiderable proportion, into the mass of comets. These three substances, however, were by no means supposed to be the sole constituents of the appendages in question. On the contrary, the great breadth of what, for the present, were taken to be characteristically "iron" tails was attributed to the presence of many kinds of matter of high and slightly different specific weights;[1276] while the expanded plume of Donati was shown to be, in reality, a whole system of tails, made up of many substances, each spreading into a separate hollow cone, more or less deviating from, and partially superposed upon the others.

Yet these felicities of explanation must not make us forget that the chemical composition attributed to the first type of cometary trains has, so far, received no countenance from the spectroscope. The emission lines of free, incandescent hydrogen have never been derived from any part of these bodies. Dissentient opinions, accordingly, were expressed as to the cause of their structural peculiarities. Ranyard,[1277] Zenker, and others advocated the agency of heat repulsion in producing them; Kiaer somewhat obscurely explains them through the evolution of gases by colliding particles;[1278] Herz of Vienna concludes tails to be mere illusory appendages produced by electrical discharges through the rare medium assumed to fill space.[1279] But Hirn[1280] conclusively showed that no such medium could possibly exist without promptly bringing ruin upon our "dædal earth" and its revolving companions.

On the whole, modern researches tend to render superfluous the chemical diversities postulated by Brédikhine. Electricity alone seems competent to produce the varieties of cometary emanation they were designed to account for. The distinction of types rests on a solid basis of fact, but probably depends upon differences rather in the mode of action than in the kind of substance acted upon. Suggestive sketches of electrical and "light-pressure" theories of comets have been published respectively by Mr. Fessenden of Alleghany,[1281] and by M. Arrhenius at Stockholm.[1282] Although evidently of a tentative character, they possess great interest.

Brédikhine's hypothesis was promptly and profusely illustrated. Within three years of its promulgation, five bright comets made their appearance, each presenting some distinctive peculiarity by which knowledge of these curious objects was materially helped forward. The first of these is remembered as the "Great Southern Comet." It was never visible in these latitudes, but made a short though stately progress through southern skies. Its earliest detection was at Cordoba on the last evening of January, 1880; and it was seen on February 1, as a luminous streak, extending just after sunset from the south-west horizon towards the pole, in New South Wales, at Monte Video, and the Cape of Good Hope. The head was lost in the solar rays until February 4, when Dr. Gould, then director of the National Observatory of the Argentine Republic at Cordoba, caught a glimpse of it very low in the west; and on the following evening, Mr. Eddie, at Graham's Town, discovered a faint nucleus, of a straw-coloured tinge, about the size of the annular nebula in Lyra. Its condensation, however, was very imperfect, and the whole apparition showed an exceedingly filmy texture. The tail was enormously long. On February 5 it extended—large perspective retrenchment notwithstanding—over an arc of 50°; but its brightness nowhere exceeded that of the Milky Way in Taurus. There was little curvature perceptible; the edges of the appendage ran parallel, forming a nebulous causeway from star to star; and the comparison to an auroral beam was appropriately used. The aspect of the famous comet of 1843 was forcibly recalled to the memory of Mr. Janisch, Governor of St. Helena; and the resemblance proved not merely superficial. But the comet of 1880 was less brilliant, and even more evanescent. After only eight days of visibility, it had faded so much as no longer to strike, though still discoverable by the unaided eye; and on February 20 it was invisible with the great Cordoba equatoreal pointed to its known place.

But the most astonishing circumstance connected with this body is the identity of its path with that of its predecessor in 1843. This is undeniable. Dr. Gould,[1283] Mr. Hind, and Dr. Copeland,[1284] each computed a separate set of elements from the first rough observations, and each was struck with an agreement between the two orbits so close as to render them virtually indistinguishable. "Can it be possible," Mr. Hind wrote to Sir George Airy, "that there is such a comet in the system, almost grazing the sun's surface in perihelion, and revolving in less than thirty-seven years. I confess I feel a difficulty in admitting it, notwithstanding the above extraordinary resemblance of orbits."[1285]

Mr. Hind's difficulty was shared by other astronomers. It would, indeed, be a violation of common-sense to suppose that a celestial visitant so striking in appearance had been for centuries back an unnoticed frequenter of our skies. Various expedients, accordingly, were resorted to for getting rid of the anomaly. The most promising at first sight was that of the resisting medium. It was hard to believe that a body, largely vaporous, shooting past the sun at a distance of less than a hundred thousand miles from his surface, should have escaped powerful retardation. It must have passed through the very midst of the corona. It might easily have had an actual encounter with a prominence. Escape from such proximity might, indeed, very well have been judged beforehand to be impossible. Even admitting no other kind of opposition than that dubiously supposed to have affected Encke's comet, the result in shortening the period ought to be of the most marked kind. It was proved by Oppolzer[1286] that if the comet of 1843 had entered our system from stellar space with parabolic velocity it would, by the action of a medium such as Encke postulated (varying in density inversely as the square of the distance from the sun), have been brought down, by its first perihelion passage, to elliptic movement in a period of twenty-four years, with such rapid diminution that its next return would be in about ten. But such restricted observations as were available on either occasion of its visibility gave no sign of such a rapid progress towards engulfment.

Another form of the theory was advocated by Klinkerfues.[1287] He supposed that four returns of the same body had been witnessed within historical memory—the first in 371 b.c., the next in 1668, besides those of 1843 and 1880; an original period of 2,039 years being successively reduced by the withdrawal at each perihelion passage of 1/1320 of the velocity acquired by falling from the far extremity of its orbit towards the sun, to 175 and 37 years. A continuance of the process would bring the comet of 1880 back in 1897.

Unfortunately, the earliest of these apparitions cannot be identified with the recent ones unless by doing violence to the plain meaning of Aristotle's words in describing it. He states that the comet was first seen "during the frosts and in the clear skies of winter," setting due west nearly at the same time as the sun.[1288] This implies some considerable north latitude. But the objects lately observed had practically no north latitude. They accomplished their entire course above the ecliptic in two hours and a quarter, during which space they were barely separated a hand's-breadth (one might say) from the sun's surface. For the purposes of the desired assimilation, Aristotle's comet should have appeared in March. It is not credible, however, that even a native of Thrace should have termed March "winter."