Popular Science Monthly/Volume 22/January 1883/The Great Comet of 1882
|THE GREAT COMET OF 1882.|
By Professor C. A. YOUNG.
THE comet which is fading in the morning sky is one of the most interesting that has ever appeared. Few, if any, have ever been more brilliant, and though others have been larger, and have continued visible for a longer time, none of them have presented more remarkable phenomena.
Of late we have been much favored in the matter of bright comets. According to the list given by Humboldt in his "Cosmos," it appears that the average interval between such apparitions for the last five centuries has been something like eight years. During the last fifty years the frequency has been about the same, conspicuous comets having appeared in 1835, 1843, 1858, 1861, 1862, and 1874. But since the beginning of 1880 we have already had five which were visible to the naked eye, and three of them comets of the highest rank. The comet of 1880 was indeed visible only in the southern hemisphere; but we all remember the fine comet which appeared in June, 1881, and was not much, if at all, inferior to the present one. Schäberle's comet, which followed in August, would have been regarded as very satisfactory had its predecessor been less brilliant; and Wells's comet of last summer, though not well seen in the United States, was a very respectable comet in South Africa.
It is not yet certain when or where the present comet was first seen, but, so far as now appears, the priority belongs to Dr. Gould, or one of his assistants, at the observatory of Cordoba in South America. In a private letter to Mr. Chandler, of Cambridge, mainly occupied with other matters, Dr. Gould, under date of September 15th, mentions that a brilliant comet had been visible there near the celestial equator for "more than a week": he had already two observations, and was waiting for clear weather again in hopes of being able to catch it on the meridian. This would put its discovery on or before September 7th. It was seen on the 8th by Mr. Finlay, an assistant in the Royal Observatory at the Cape of Good Hope; and on the 12th it was observed at Rio Janeiro, by Cruls, who telegraphed the news to Europe, announcing it (erroneously) as the expected comet of 1812 on its return. We have not yet sufficiently full accounts from the southern observatories to know whether it was lost sight of at all after its discovery, but we have the account of a most interesting and unprecedented observation made at the Cape Town Observatory, on the 17th. Mr. Gill, the director of the observatory, writes: "The comet was followed by two observers with separate instruments right up to the sun's limb, where it suddenly disappeared at 4h 50m 58s local mean time." This was about an hour and a half before its perihelion passage.
A few hours previously it had been independently discovered by Mr. Common in England, in the full blaze of sunlight, and clouds alone prevented him from making the same observation as Mr. Gill.
It is evident that the comet must have been most intensely brilliant to be visible under such circumstances. When it passed on to the sun's disk (it was between us and the sun at the time), it disappeared, being either transparent, or else practically as bright as a portion of the sun's own surface. If this comet had been in the place of the little "Tewfik" which was seen close to the sun at the time of the Egyptian eclipse last May, it would have been something to remember.
On September 18th the comet had reached a greater distance from the sun (about 3°), and had become so conspicuous that it was simultaneously rediscovered by a multitude of observers in all parts of the world, and accurate determinations of its position were made at several observatories. On the next day every one had heard of it, and people interested in astronomy thought and talked of nothing else.
On the 19th and 20th the comet was still easily seen by the naked eye. On the 21st it was visible only in places when the air was very clear, and the sky darkly blue. On the 22d a curious observation of it was made at Paris by M. Mallet, who, at the request of M. de Fonvielle, ascended for the purpose in a balloon provided by the latter, thus getting above the clouds with which the city was thickly covered. Of course, it was not possible in this manner to make any precise determination of position, but the aeronaut obtained a fine view of the celestial visitor.
For a few days after this the comet does not appear to have been observed until it had receded far enough from the sun to become visible before sunrise. Then, for a while, during the early days of October, it was a most magnificent object, with a head at first rivaling Jupiter in brightness, and a tail which, though not of unusual dimensions, never much exceeding 60,000,000 miles, was remarkably well defined, dense, and luminous. It moved slowly toward the south and west, and when, once in a while, a clear morning permitted the view, it was seen to be growing fainter and more diffuse, though not smaller.
To the naked eye or opera-glass it has perhaps presented fewer phenomena of interest than some other comets—that of 1858, for instance; it has not exhibited any of the peculiar secondary tails or straight streamers which were so characteristic of that comet, nor has the striation of the tail been marked, though evident enough on close inspection.
From September 27th to October 1st, however, the tail was "rifted." There was one obscure streak extending from the nucleus through its whole length, described both by Ricco, of Palermo, and Dr. Hastings, of Baltimore, and the latter mentions another fainter one parallel to the first, and shorter. On October 2d the tail, as seen at Princeton, was about 14° long, exceedingly bright and sharp in its outlines, slightly curved and convex to the horizon. It was especially well defined near the head, and almost equally so on both sides. On the 4th the upper edge was veiled and rendered indefinite by a faint nebulosity which appeared to have emanated from the head. Ricco's drawing of it, as seen at this date in the clear Italian sky, shows something resembling a bright comet, enveloped in a fainter one; but the smaller one is eccentric, and south of the middle of the hazy envelope.
On the 10th this external nebulosity had considerably increased. Professor Smith, of Kansas University, noticed on the 9th a pale stream of light with parallel edges, and nearly as wide as the tail of the comet, extending toward the sun. On the 15th the phenomenon had become much more conspicuous. The streamer was now over half a degree in width, well defined at both edges, of nearly uniform brightness throughout, though nowhere as bright as even the faintest portions of the tail, and extended from its origin, a degree or two above the nucleus, to a distance of two or three decrees below the head, where it faded out. The dotted lines in Fig. 1 indicate its form and dimensions.
This streamer, which remained visible only a few days, may have originated in the enveloping comet of Ricco's figure just spoken of, but no other comet is known to have shown anything of the kind. It is not to be confounded with the sunward jets sometimes ejected by cometary nuclei, nor did it at all resemble the anomalous tail, directed toward the sun, shown by Pechüle's comet (in December, 1880), in addition to its ordinary tail.
On October 9th, Schmidt, of Athens, announced the discovery of a small companion comet, 4° southwest of the large one, and moving parallel with it. So far as we know, no one else has observed this companion, though it was carefully looked for at Washington, Princeton, and elsewhere. On October 21st, however, Mr. Brooks, of Phelps, New York, observed either the same or another one, some 8° south and east from the large comet. Like Schmidt's companion it was very faint (though large), and we have seen no observations of it from other sources. We have no means of ascertaining whether these attendants accompanied the comet on its way to the sun as separate objects, or whether they are fragments detached from the main body.
Mr. Brooks seems to think that the nebulous mass observed by him was in some way connected with the faint envelope and streamer just spoken of, which is not unlikely.
When the writer first saw the comet, on September 19th, it was impossible, with the great twenty-three-inch equatorial, to make out much except the nucleus itself. The comet was so near the sun that the object-glass could not be screened from the direct sunshine, which filled the whole field with glaring light. The finder of the instrument is itself, however, a powerful telescope of five inches aperture, and this was perfectly screened by the great tube, so that it furnished an admirable view of the beautiful object. To the naked eye the comet looked like some white-winged bird in swift flight toward the sun. The telescope showed the wings to be long, curved streams flowing backward from each side of the head—backward, that is, with reference to the sun; but they were, of course, really in advance of the comet, which at this time was receding from the sun. The head of the comet had for its center a small round and brilliant nucleus, not well defined, but rather a nebulous star, some 4" in diameter; in front of this at a distance of perhaps 30" was an "envelope," and there was a second one at a distance of 2' or 3'. They were connected by a pair of eccentric circular arcs, and these arcs, coalescing with the outer envelope and prolonged, formed the skeleton of the "wings." Back of the nucleus, traces of the usual dark stripe could be detected. Fig. 2 presents the main features in outline, and every one will notice its close resemblance to Brodie's picture of Coggia's comet as seen on July 13, 1874. (The picture alluded to forms the frontispiece of Chambers's "Descriptive Astronomy," third edition.)
On the next day the comet was seen at Princeton for a few moments through clouds, just long enough to get imperfect observations for position, but nothing more. It was noticed, however, that the eccentric arcs had disappeared. On October 2d the comet was observed for more than an hour before daybreak with the great telescope. The most notable features were a single bright cap or envelope at a distance of about half a minute from the nucleus, and the nucleus itself, which, instead of being round, was considerably elongated. There were, however, no jets, or starfish-like projections such as the comet of 1881 presented so often. There was not much of structural detail to be made out in the head of the comet, but the dark stripe behind the nucleus was very conspicuous. This dark stripe, by-the-way, is a very remarkable phenomenon, not yet explained, so far as we know, though observed in most large comets. The common impression is, that it is merely a space behind the nucleus, screened as it were by the nucleus itself, from the rush of luminous matter which is being driven backward by the sun's repulsion. But if this be so, then, as Mr. Proctor has pointed out, in a recent article, there is no reason why it should appear so well defined and so dark. The cross-section of the tail, a little way behind the nucleus, was, in the present case at least, 100,000 miles in diameter: now, merely taking away the luminous matter from a tunnel 6,000 or 8,000 miles in diameter along the axis of the tail, could make but little difference with the amount of light received by the eye at a distance. If there were no tunnel, we should get from the central line of the tail the brightness corresponding to a thickness of 100,000 miles of luminous matter. Boring the tunnel would only reduce it to some 90,000 miles, and the difference would be hardly perceptible.
It seems more likely, if the writer may venture the suggestion, that the stripe is a stream or beam of non-luminous, cooler vapor or gas, which is nearly opaque to the radiation emitted by the same kind of gas when luminous, and therefore cuts off all the light from whatever portions of the comet's luminous drapery is behind it; in the same way that cool sodium-vapor, for instance, is relatively opaque to the light of a sodium-flame. If this is correct, the dark stripe ought not to be black, but just about half as bright as the neighboring nebulosity; which corresponds to the actual fact. If one could catch a star passing behind the stripe, it would perhaps be easy to settle the question. At any rate, if the star shone more brightly when in the stripe, we might be sure that the hypothesis is wrong. The star should be dimmed a little, if anything, though, of course, star-light would not be so much affected as the light from cometary matter. Mr. Proctor has suggested a different hypothesis, which seems to the writer rather less probable, but there is no time to discuss it here.
On October 4th the nucleus had become much more elongated, so as to be shaped something like an Indian club. The envelope, which was conspicuous on the 2d, had disappeared, or degenerated into an indefinite nebulosity, and the dark stripe had become much fainter.
Continued bad weather prevented observation until the 10th, and on this date the nine-and-a-half-inch telescope of the School of Science Observatory was used. A great change had taken place. The nucleus had become an irregular, spindle-shaped streak some 40" long, made up of six or eight star-like knots of luminosity connected and veiled by shining haze. One of these knots, about a third of the way from the sunward extremity, was considerably larger and brighter than any of the others, and should, perhaps, be considered as the true nucleus. The next one beyond it (reckoning from the direction of the sun) was second in size, and separated by an interval of 2" or 3", the space being filled, however, with nebulosity. The dark stripe was still visible, but directed, not along the prolongation of the nuclear streak, but inclined at an angle of 8° or 10°, while a bright jet from the nucleus, two or three minutes in length, touched one side of the dark stripe, and kept nearly in the axis of the tail. Fig. 3 is an attempt to illustrate the appearance and relation of things by a mere outline sketch, which, of course, can not be considered in any sense a representation, since it fails entirely to give an idea of the shading and gradation of light. The head of the comet presented no definite outline what-ever, and the nucleus very little. The knots were mere condensations of brightness in the midst of diffuse light. When the dawn came on, the fainter parts successively disappeared, so that at a certain stage the nucleus seemed to be divided into two portions. A small telescope would probably show things in the same way even before dawn, and this is undoubtedly the origin of the reports that the comet had split in two.
This great and unprecedented elongation of the nucleus is a most remarkable phenomenon. If it had occurred at or near the time of perihelion passage, it might have been naturally attributed to the divellent action of the sun's attraction; but it is a little difficult to see why the thing should have pulled out and come to pieces in such a way after getting safely by the crisis. It is worth noting that this peculiarity of the comet adds greatly to the difficulty of making accurate observations of its position: one does not know just upon what point to direct his instrument.
Continuous cloudy weather prevented any observation of the comet until the 15th. On that date the appearance of things as seen in the great equatorial was very much what it had been on the 10th with the smaller telescope. There were no envelopes, and the only "jet" was the bright streak following the nucleus. The dark stripe had wholly disappeared, as if obliterated and replaced by the bright one. The "knots" in the nucleus were seen to be irregular in form, and were arranged not in a straight line, but in a somewhat broken curve, conforming to the curvature of the tail, which at this time extended 18°, and was fully 60,000,000 miles in length. The bright stream originated not at the extremity of the nucleus, but came out tangentially from the convex side, and perhaps had its source in the largest of the knots, which was now the third from the sunward extremity. The whole length of the nucleus measured 4812", corresponding to a length of more than 40,000 miles, the diameter of the largest single mass being about 5,000 or 6,000 miles. The only other observation we have been able to make at Princeton was nine days later, on October 24th. No material changes were noticed, though the comet was very much fainter. The same lengthened granular nucleus continued, and seems likely to persist until the comet disappears.
The spectroscopic observations have been very interesting. On September 18th the French physicist Thollon was an independent discoverer of the comet, coming upon it accidentally in sweeping around the sun. His spectroscopic apparatus consists of a so-called siderostat, the mirror of which throws the rays from the object to be examined upon the lens of a horizontal telescope nine and a half inches in diameter and about twenty feet long. At the focus of this telescope in a darkened room is placed a spectroscope, and, of course, this may be of any form and power best suited to the occasion. In the present case he used an instrument with a single prism of high dispersive power. The most marked feature of the spectrum was the presence of the lines of sodium in the spectrum of the nucleus. They were very bright, and were displaced toward the red by an amount equal to about one fourth of the interval between them, thus indicating that the comet was rapidly receding from the earth. A very narrow, bright, continuous spectrum was also shown by the nucleus. In this the dark lines of Fraunhofer were not conspicuous if visible at all, showing that the principal brilliancy of the comet was not reflected sunlight. The usual carbon bands of the cometary spectrum were not visible through the sky illumination, and no other bright lines except those of sodium were seen by Thollon. On the 22d the comet's spectrum was observed in the early morning just before sunrise by Ricco, of Palermo. He reports his observation thus: "The spectrum was formed of the narrow continuous spectrum of the nucleus, traversed by a large and strong line of sodium (D); by enlarging the slit I saw a globular, monochromatic image of the nucleus and coma. Besides the line of sodium many others were present, but, my spectroscope not having a micrometer, I did not determine them. I observed a band in the red, a line in the yellow near and after D, two others in the green, and an enlargement of the continuous spectrum in the green and blue." It is exceedingly unfortunate that the position of these lines could not have been determined, at least approximately. No one can predict when such an opportunity will occur again.
The weather in this part of the country was abominable up to November. The writer attempted to get spectroscopic observations on September 20th, but was foiled by clouds, and has since succeeded only on October 2d, 4th, 10th, 15th, and 24th. On the first of these dates the sodium-lines were still easily visible, though not conspicuous. The carbon bands were magnificent, especially the brightest one (in the green), in which could be clearly seen the three fine lines observed in the spectrum of Coggia's comet. The band in the violet was very faint. The nucleus gave a strong continuous spectrum, on which the carbon bands were superposed; and in the tail the proportion of white light (continuous spectrum) to carbon light appeared to be about the same as in the nucleus. The bands could be followed far out into the tail by widening the slit, but were lost before the continuous spectrum quite vanished. No dark lines were made out. On the 4th the results were the same, except that the sodium-lines were very hard to see, and they disappeared entirely before the next date. The later observations added nothing more. It is much to be hoped that, when the different results of all observers come to be collected and published, something will be found to supply what is so unfortunately wanting in Ricco's most interesting but incomplete observation—hiatus valde deflendus.
The highest interest of the present comet lies in its orbit, however, its relation to preceding comets, and its possible speedy destruction by the sun. Almost as soon as it appeared, Professor Boss in this country and Hind in England proposed the hypothesis that it is identical with the great comet of 1880, the period of the latter comet having been shortened by some resistance. If so, this comet will be back again in a few months, and before long must fall upon the sun. They have weighty arguments on their side, but on the whole a different conclusion is more probable.
On September 17th the comet passed its perihelion at a distance of about 750,000 miles from the sun's center, and within 300,000 of its surface, rushing through the coronal regions with a velocity exceeding 300 miles per second: it swept over 180° of its orbit in three hours and a half. Thus far we find in our lists of cometary orbits only four with so small a perihelion distance, viz., the comets of 1668, 1680, 1843, and 1880. (As to the comet of 1008 there is some doubt, because it was only observed for about three weeks, and its motion during that time was such that it answers almost equally well to either of two quite different orbits.) There are half a dozen others with perihelion distances between one and a half and five million miles, viz., comets of 1767, 1816, 1826, 1847, 1865, and 1870; and Wells's comet, which disappeared only a few weeks ago, is just outside that limit, with a perihelion distance of 5,675,000 miles. Now, as to the comets of the first class, we find that, excepting that of 1680, their orbits are extremely similar; their plane and direction of motion are almost exactly the same; the perihelion distances are nearly the same for all; and the axes of the orbits all point to the same part of space; they have all come toward the sun from the same region of the heavens, in the immediate neighborhood of the great star Sirius. In the little table below are given what are called the elements of their orbits: Ω, is the longitude of the node, i the inclination of the orbit to the ecliptic, π the longitude of the perihelion, and q the perihelion distance, expressed as a decimal fraction of the earth's distance from the sun; e is the eccentricity of the orbit; and the—in the last line denotes that the motion is retrograde. The orbits of the first two are
|Ω||357° 17’||361° 12’||356° 17’||345° 50’|
|i||35° 58’||35° 41’||36° 53’||38° 05’|
|π||277° 2’||278° 39’||278° 23’||276° 28’|
from the catalogue in Chambers's "Descriptive Astronomy"; that of 1880 is the orbit computed by Meyer, of Geneva, from the whole assemblage of observations, and that of 1882 is the last orbit computed by Mr. Chandler, of Cambridge, and may be found to need some correction when later observations come to hand. Fig. 4 shows in a rough way how these orbits lie in relation to the orbit of the earth, and how very long and narrow the comet's orbit is as compared with the circle described by the earth.
Now, the similarity between these orbits may be explained in two different ways. It might be accounted for by supposing that we have to do with different visits to the sun of a single comet, or that we have here a group or family of comets, very likely of common origin, but separate, and following each other. Hoek, of Utrecht, showed some years ago that such comet-families exist. When we compare the orbits of the comets of 1843 and 1668 there is nothing that forbids the idea of their identity. The differences are no greater than probable perturbations might account for. Then, again, the comets of
1843 and 1880 may easily be identical. Indeed, the orbit given for the latter comet corresponds to a period of almost thirty-seven years, and Meyer has shown that the observations can not be reconciled with a period less than thirty or greater than fifty years. Now, thirty-seven years would take us back just to 1843, so that it is very likely that these two comets are really one and the same. So far the "identifiers" have matters their own way. But, now, as to the comet of 1882. Can it be identical with the comet of 1880? We think not. The orbit of the latter was computed exclusively from observations taken after its perihelion passage, so that no action of the sun depending upon its close approach at perihelion can account for its return in less than three years, and the inclination of its orbit is such that ever since it went out of sight it has been out of harm's way as to perturbations by the planets. Then, again, the orbit of the comet of 1882 does not agree with the idea of identity. Whatever other effects may have been produced by the resistance of the solar atmosphere at perihelion, this resistance must have tended to shorten its period, if it changed it at all. Now, the observations thus far taken, though perhaps not sufficient to settle the orbit definitively, seem to be absolutely inconsistent with a period of anything like three years (corresponding to an eccentricity of 0·9963). The period can not well be less than ten or twelve years, according to the last results, and may be several thousand. It is to be noted, further, that, as regards Ω, and q, the two orbits differ more than can well be consistent with the theory of identity. It seems to be an almost necessary consequence that these two comets can not be identical with each other, though they may, perhaps, both be fragments of the comets of 1668 or 1843, or of some comet more ancient than either.
It is an interesting fact that Mr. Chandler finds that his orbit, computed entirely from post-perihelion observations, satisfies almost exactly the observation of Mr. Finlay, taken on September 8th, as well as the observation of the comet's disappearance at the sun's edge. If the observations of Dr. Gould, when they come to hand, agree as well, it will be proof positive that no sensible resistance or disturbance of any kind was suffered by the comet in passing within 300,000 miles of the sun's surface at the rate of 300 miles a second.
Of course, if the view we have taken is correct, there is no possibility that our comet can return in six months and fall into the sun. Not that there is any absurdity in the idea by itself considered. If the comet of 1880, when receding from the sun, had moved in an orbit corresponding to a three years' period, and if the present comet were found to have a period of three years or less as it is now receding from the sun, it would be almost impossible to refuse to admit their identity, and probable speedy absorption in the sun.
We close with a single word as to the probable consequences of a comet's fall upon the sun. Unquestionably, the energy of the comet's motion would be transformed into heat, and if the comet had any considerable mass, say 1100 the mass of the earth, the heat produced would be enough to supply the sun's heat-expenditure for months. Probably, however, no comet has a mass anything like so great as that; more likely the present comet even, huge as it is, has a mass less than 1100000 of the earth's, so that its collision with the sun would produce as much heat only as the sun would expend in eight hours.
Now, if the sun were a cool, solid, or even liquid mass, the sudden accession of merely this quantity of heat would undoubtedly produce an enormous rise of temperature and a great increase of radiation. But, constituted as the sun is—mainly a mass of gas and vapor—the effect would be entirely different, the energy being principally expended in producing expansion and evaporation, with comparatively little increase of temperature or radiation. If one stirs up the fire under an open kettle, the water gets no hotter—it only boils faster. Probably the effect of the fall of a body, even as large as the earth, upon the sun, would be hardly anything more than to restore the sun to the condition it was in a century ago. The energy lost in the course of a century would be replaced—that is about all. During the few moments while the body was passing through the sun's atmosphere, there might be, and probably would be, phenomena of great interest and beauty to those who were on the watch; but it is very doubtful whether people generally would know anything about the occurrence until they read of it in the papers.
- ↑ Later observations, on November 4th, show the same general characteristics. The nucleus, if it can be so called, was now 93" in length, or more than 90,000 miles. Three stellar points could be detected in the forward portion of the nucleus, but only two in the other. The separation between the two brightest points was about 10". The spectrum showed no new developments. To the naked eye the comet was unexpectedly bright, although now distant from both sun and earth nearly 140,000,000 miles. The head looked like a fourth-magnitude star, and the tail was 16° long and 4° wide at the extremity. On November 20th the nucleus had almost vanished, appearing merely as a brighter streak in the structureless nebulosity of the head. The tail was still nearly as large as ever, and easily visible without telescopic aid, though of course much fainter than on the 4th. The comet has held out remarkably, and, so far as now appears, it may be observable for a long time yet, especially in the southern hemisphere.