Part 50 (1/2)
At Potsdam, the bright yellow line was perceived with astonishment by Vogel on May 31, and was next evening identified with Fraunhofer's ”D.”
Its character led him to infer a very considerable density in the glowing vapour emitting it.[1309] Ha.s.selberg founded an additional argument in favour of the electrical origin of cometary light on the changes in the spectrum of comet Wells.[1310] For they were closely paralleled by some earlier experiments of Wiedemann, in which the gaseous spectra of vacuum tubes were at once effaced on the introduction of metallic vapours. It seemed as if the metal had no sooner been rendered volatile by heat, than it usurped the entire office of carrying the discharge, the resulting light being thus exclusively of its production. Had simple incandescence by heat been in question, the effect would have been different; the two spectra would have been superposed without prejudice to either. Similarly, the replacement of the hydro-carbon bands in the spectrum of the comet by the sodium line proved electricity to be the exciting agent. For the increasing thermal power of the sun might, indeed, have ignited the sodium, but it could not have extinguished the hydro-carbons.
Sir William Huggins succeeded in photographing the spectrum of comet Wells by an exposure of one hour and a quarter.[1311] The result was to confirm the novelty of its character. None of the ultra-violet carbon groups were apparent; but certain bright rays, as yet unidentified, had imprinted themselves. Otherwise the spectrum was strongly continuous, uninterrupted even by the Fraunhofer lines detected in the spectrum of Tebb.u.t.t's comet. Hence it was concluded that a smaller proportion of reflected light was mingled with the native emissions of the later arrival.
All that is certainly known about the _extent_ of the orbit traversed by the first comet of 1882 is that it came from, and is now retreating towards, vastly remote depths of s.p.a.ce. An American computer[1312] found a period indicated for it of no less than 400,000 years; A. Thraen of Dingelstadt arrived at one of 3617.[1313] Both are perhaps equally insecure.
We have now to give some brief account of one of the most remarkable cometary apparitions on record, and--with the single exception of that identified with the name of Halley--the most instructive to astronomers.
The lessons learned from it were as varied and significant as its aspect was splendid; although from the circ.u.mstance of its being visible in general only before sunrise, the spectators of its splendour were comparatively few.
The discovery of a great comet at Rio Janeiro, September 11, 1882, became known in Europe through a telegram from M. Cruls, director of the observatory at that place. It had, however (as appeared subsequently), been already seen on the 8th by Mr. Finlay of the Cape Observatory, and at Auckland as early as September 3. A later, but very singularly conditioned detection, quite unconnected with any of the preceding, was effected by Dr. Common at Ealing. Since the eclipse of May 17, when a comet--named ”Tewfik” in honour of the Khedive of Egypt--was caught on Dr. Schuster's photographs, entangled, one might almost say, in the outer rays of the corona, he had scrutinized the neighbourhood of the sun on the infinitesimal chance of intercepting another such body on its rapid journey thence or thither. We record with wonder that, after an interval of exactly four months, that infinitesimal chance turned up in his favour.
On the forenoon of Sunday, September 17, he saw a great comet close to, and rapidly approaching the sun. It was, in fact, then within a few hours of perihelion. Some measures of position were promptly taken; but a cloud-veil covered the interesting spectacle before mid-day was long past. Mr. Finlay at the Cape was more completely fortunate. Divided from his fellow-observer by half the world, he unconsciously finished, under a clearer sky, his interrupted observation. The comet, of which the silvery radiance contrasted strikingly with the reddish-yellow glare of the sun's margin it drew near to, was followed ”continuously right into the boiling of the limb”--a circ.u.mstance without precedent in cometary history.[1314] Dr. Elkin, who watched the progress of the event with another instrument, thought the intrinsic brilliancy of the nucleus scarcely surpa.s.sed by that of the sun's surface. Nevertheless it had no sooner touched it than it vanished as if annihilated. So sudden was the disappearance (at 4h. 50m. 58s., Cape mean time), that the comet was at first believed to have pa.s.sed _behind_ the sun. But this proved not to have been the case. The observers at the Cape had witnessed a genuine transit. Nor could non-visibility be explained by equality of l.u.s.tre. For the gradations of light on the sun's disc are amply sufficient to bring out against the dusky background of the limb any object matching the brilliancy of the centre; while an object just equally luminous with the limb must inevitably show dark at the centre.
The only admissible view, then, is that the bulk of the comet was of too filmy a texture, and its presumably solid nucleus too small, to intercept any noticeable part of the solar rays--a piece of information worth remembering.
PLATE III.
[Ill.u.s.tration: The Great Comet of September, 1882.
Photographed at the Royal Observatory, Cape of Good Hope]
On the following morning, the object of this unique observation showed (in Sir David Gill's words) ”an astonis.h.i.+ng brilliancy as it rose behind the mountains on the east of Table Bay, and seemed in no way diminished in brightness when the sun rose a few minutes afterward. It was only necessary to shade the eye from direct sunlight with the hand at arm's length, to see the comet, with its brilliant white nucleus and dense white, sharply bordered tail of quite half a degree in length.”[1315]
All over the world, wherever the sky was clear during that day, September 18, it was obvious to ordinary vision. Since 1843 nothing had been seen like it. From Spain, Italy, Algeria, Southern France, despatches came in announcing the extraordinary appearance. At Cordoba, in South America, the ”blazing star near the sun” was the one topic of discourse.[1316] Moreover--and this is altogether extraordinary--the records of its daylight visibility to the naked eye extend over three days. At Reus, near Tarragona, it showed bright enough to be seen through a pa.s.sing cloud when only three of the sun's diameters from his limb, just before its final rush past perihelion on September 17; while at Carthagena in Spain, on September 19, it was kept in view during two hours before and two hours after noon, and was similarly visible in Algeria on the same day.[1317]
But still more surprising than the appearance of the body itself were the nature and relations of the path it moved in. The first rough elements computed for it by Mr. Tebb.u.t.t, Dr. Chandler, and Mr. White, a.s.sistant at the Melbourne Observatory, showed at once a striking resemblance to those of the twin comets of 1843 and 1880. This suggestive fact became known in this country, September 27, through the medium of a Dunecht circular. It was fully confirmed by subsequent inquiries, for which ample opportunities were luckily provided. The likeness was not, indeed, so absolutely perfect as in the previous case; it included some slight, though real differences; but it bore a strong and unmistakable stamp, broadly challenging explanation.
Two hypotheses only were really available. Either the comet of 1882 was an accelerated return of those of 1843 and 1880, or it was a fragment of an original ma.s.s to which they also had belonged. For the purposes of the first view the ”resisting medium” was brought into full play; the opinion of its efficacy was for some time both prevalent and popular, and formed the basis, moreover, of something of a sensational panic. For a comet which, at a single pa.s.sage through the sun's atmosphere, encountered sufficient resistance to shorten its period from thirty-seven to two years and eight months, must, in the immediate future, be brought to rest on his surface; and the solar conflagration thence ensuing was represented in some quarters, with more licence of imagination than countenance from science, as likely to be of catastrophic import to the inhabitants of our little planet.
But there was a test available in 1882 which it had not been possible to apply either in 1843 or in 1880. The two bodies visible in those years had been observed only after they had already pa.s.sed perihelion;[1318]
the third member of the group, on the other hand, was accurately followed for a week before that event, as well as during many months after it. Finlay's and Elkin's observation of its disappearance at the sun's edge formed, besides, a peculiarly delicate test of its motion.
The opportunity was thus afforded, by directly comparing the comet's velocity before and after its critical plunge through the solar surroundings, of ascertaining with approximate certainty whether any considerable r.e.t.a.r.dation had been experienced in the course of that plunge. The answer distinctly given was that there had not. The computed and observed places on both sides of the sun fitted harmoniously together. The effect, if any were produced, was too small to be perceptible.
This result is, in itself, a memorable one. It seems to give the _coup de grace_ to Encke's theory--discredited, in addition, by Backlund's investigation--of a resisting medium growing rapidly denser inwards. For the perihelion distance of the comet of 1882, though somewhat greater than that of its predecessors, was nevertheless extremely small. It pa.s.sed at less than 300,000 miles of the sun's surface. But the ethereal substance long supposed to obstruct the movement of Encke's comet would there be nearly 2,000 times denser than at the perihelion of the smaller body, and must have exerted a conspicuous r.e.t.a.r.ding influence. That none such could be detected seems to argue that no such medium exists.
Further evidence of a decisive kind was not wanting on the question of ident.i.ty. The ”Great September Comet” of 1882 was in no hurry to withdraw itself from curious terrestrial scrutiny. It was discerned with the naked eye at Cordoba as late as March 7, 1883, and still showed in the field of the great equatoreal on June 1 as an ”excessively faint whiteness.”[1319] It was then about 480 millions of miles from the earth--a distance to which no other comet--not even excepting the peculiar one of 1729--had been pursued.[1320] Moreover, an arc of 340 out of the entire 360 degrees of its circuit had been described under the eyes of astronomers; so that its course came to be very well known.
That its movement is in a very eccentric ellipse, traversed in several hundred years, was ascertained.[1321] The later inquiries of Dr.
Kreutz,[1322] completed in a volume published in 1901,[1323]
demonstrated the period to be of about 800 years, while that of its predecessor in 1843 might possibly agree with it, but is much more probably estimated at 512 years. The hypothesis that they, or any of the comets a.s.sociated with them, were returns of an individual body is peremptorily excluded. They may all, however, have been separated from one original ma.s.s by the divellent action of the sun at close quarters.
Each has doubtless its own period, since each has most likely suffered r.e.t.a.r.dations or accelerations special to itself, which, though trifling in amount, would avail materially to alter the length of the major axis, while leaving the remaining elements of the common orbit virtually unchanged.[1324]
A fifth member was added to the family in 1887. On the 18th of January in that year, M. Thome discovered at Cordoba a comet reproducing with curious fidelity the lineaments of that observed in the same lat.i.tudes seven years previously. The narrow ribbon of light, contracting towards the sun, and running outward from it to a distance of thirty-five degrees; the unsubstantial head--a veiled nothingness, as it appeared, since no distinct nucleus could be made out; the quick fading into invisibility, were all accordant peculiarities, and they were confirmed by some rough calculations of its...o...b..t, showing geometrical affinity to be no less unmistakable than physical likeness. The observations secured were indeed, from the nature of the apparition, neither numerous nor over-reliable; and the earliest of them dated from a week after perihelion, pa.s.sed, almost by a touch-and-go escape, January 11. On January 27, this mysterious object could barely be discerned telescopically at Cordoba.[1325] That it belonged to the series of ”southern comets” can scarcely be doubted; but the inference that it was an actual return of the comet of 1880, improbable in itself, was negatived by its non-appearance in 1894. Meyer's incorporation with this extraordinary group of the ”eclipse-comet” of 1882[1326] has been approved by Kreutz, after searching examination.
The idea of cometary systems was first suggested by Thomas Clausen in 1831.[1327] It was developed by the late M. Hoek, director of the Utrecht Observatory, in 1865 and some following years.[1328] He found that in quite a considerable number of cases, the paths of two or three comets had a common point of intersection far out in s.p.a.ce, indicating with much likelihood a community of origin. This consisted, according to his surmise, in the disruption of a parent ma.s.s during its sweep round the star latest visited. Be this as it may, the fact is undoubted that numerous comets fall into groups, in which similar conditions of motion betray a pre-existent physical connection. Never before, however, had geometrical relations.h.i.+p been so notorious as between the comets now under consideration; and never before, in a comet still, it might be said, in the prime of life, had physical peculiarities tending to account for that affinity been so obvious as in the chief member of the group.
Observation of a granular structure in cometary nuclei dates far back into the seventeenth century, when Cysatus and Hevelius described the central parts of the comets of 1618 and 1652 respectively as made up of a congeries of minute stars. a.n.a.logous symptoms of a loose state of aggregation have of late been not unfrequently detected in telescopic comets, besides the instances of actual division offered by those connected with the names of Biela and Liais. The forces concerned in producing these effects seem to have been peculiarly energetic in the great comet of 1882.
The segmentation of the nucleus was first noticed in the United States and at the Cape of Good Hope, September 30. It proceeded rapidly. At Kiel, on October 5 and 7, Professor Kruger perceived two centres of condensation. A definite and progressive separation into _three_ ma.s.ses was observed by Professor Holden, October 13 and 17.[1329] A few days later, M. Tempel found the head to consist of _four_ lucid aggregations, ranged nearly along the prolongation of the caudal axis;[1330] and Dr.
Common, January 27, 1883, saw _five_ nuclei in a line ”like pearls on a string.”[1331] This remarkable character was preserved to the last moment of the comet's distinct visibility. It was a consequence, according to Dr. Kreutz, of violent interior action in the comet itself While close to the sun.
There were, however, other curious proofs of a disaggregative tendency in this body. On October 9, Schmidt discovered at Athens a nebulous object 4 south-west of the great comet, and travelling in the same direction. It remained visible for a few days, and, from Oppenheim's and Hind's calculations, there can be little doubt that it was really the offspring by fission of the body it accompanied.[1332] This is rendered more probable by the unexampled spectacle offered, October 14, to Professor Barnard, then of Nashville, Tennessee, of _six or eight_ distinct cometary ma.s.ses within 6 south by west of the comet's head, none of which reappeared on the next opportunity for a search.[1333] A week later, however, one similar object was discerned by Mr. W. R.