Popular Science Monthly/Volume 14/March 1879/The Sun's Long Streamers
|THE SUN'S LONG STREAMERS.|
PROFESSOR CLEVELAND ABBE, an American astronomer and meteorologist, who had intended to observe the eclipse of the sun last July from the summit of Pike's Peak, in Colorado, more than 14,000 feet above the sea-level, fell ill after he had reached that place, and was carried down to the Lake House (elevation 10,000 feet), there to remain while the rest of his party staid to view the eclipse from the summit. Probably if he had remained with them his observations would have differed in no very marked degree from those which other astronomers made on that occasion. He would have devoted a few seconds, perhaps, to the study of the sun's corona with the naked eye. He would probably have made some telescopic, spectroscopic, or polariscopic observations during the rest of the three minutes during which the total eclipse lasted, and possibly he might have noted some feature rather more effectively and satisfactorily than most of the other observers. But under the actual circumstances he could not hope thus to take his place among the thousands of observers who have noted the phenomena of total solar eclipses. He had no optical or other instrument. Worse than all, he is near-sighted; and, though he had a pair of spectacles, it was not quite strong enough to correct his near-sightedness.
Yet Professor Abbe succeeded in making observations far exceeding in interest any which were made by the entire force of eclipse observers in 1874 and 1875, and fairly comparable in this respect with the most remarkable discoveries effected during the great eclipses of 1868, 1869, 1870, and 1871. Debarred from instrumental researches, unable to do what most observers of eclipses seem anxious to do—namely, to see everything that can be seen—he was compelled to restrict himself to precisely that line of observation which we indicated eight years ago as likely to be most instructive. He gave his whole attention to the corona, and especially to its. outlying and feebler portions. Studying the phenomena with the naked eye, or at least with only spectacles to aid him, he could recognize faint luminosity which the telescope would inevitably have concealed from his view. He was not hurried; nor was he disturbed by the thought that such and such instruments must be attended to in turn while still totality lasted, with care also that in the darkness nothing should be disturbed or injured. As he said after the observations were completed, and as we pointed out in 1870, "a glance of a few seconds will no more suffice to do justice to the delicate phenomena" (of the corona) "than it would suffice to enable a naturalist to draw the distinguishing features of a new shell or insect, or would enable an artist to correctly sketch in a landscape."
Before describing what Professor Abbe actually saw, it may be well to indicate first the nature of the observations he proposed to make, and secondly his preconceived ideas as to what he was likely to see, for otherwise the value of his observations will not be fully appreciated.
Our readers may perhaps remember that in the year 1870 a discussion took place on the question whether the glory of light seen around the sun during total eclipse belongs to the sun or not. There were those who maintained very confidently the opinion that this glory is either a purely optical phenomenon only or else is due to the passage of the solar rays through our own atmosphere all round the place of the eclipsed sun. On the other hand, there were some (ourselves among the number) who pointed out that the corona must necessarily belong to the sun, since its features could not possibly be reconciled with any other theory. The greater number of astronomers seemed, however, to form no opinion one way or the other, but to prefer to leave the matter to be decided by fresh evidence. For too many imagine that the best way of showing how greatly they value observations is by declining to investigate the full significance of observations already made.
It will be remembered that before long the new observations devised to settle a question which had been abundantly answered by observations already made proved unmistakably the solar nature of the corona. Photographs were taken during the total eclipse of December, 1870, and in greater number during that of December, 1871. On the latter occasion photographic views of the corona taken at stations far apart agreed closely together, showing that the corona could not possibly be an atmospheric phenomenon. No one could imagine that the air above Baicull, where Mr. Davis (Lord Lindsay's photographer) took his views, could by some amazing accident produce coronal features resembling those produced by the air above Ootacamund, one station being close to the seashore, the other hundreds of miles inland and some 10,000 feet above the sea-level. On the other hand, the resemblance of the several views taken at either station showed that the coronal glory could not be due to the illumination of some matter on the hither side of the moon, but far outside our own atmosphere. For the solar rays, passing athwart the lunar disk to fall upon such matter, would shift rapidly in position as the moon moved onward, so that the features seen at the beginning of total eclipse would differ markedly from those seen toward the end. Since the six pictures taken at Baicull closely resembled each other, as did the six taken at Ootacamund, so that all twelve views represented the same corona (though of course not all to the same distance from the sun), it was manifest that the corona then seen was a solar appendage. The actual distance to which the corona can be traced in these pictures corresponds to about 900,000 miles.
But the believers in an atmospheric corona were not even yet wholly satisfied. Nay, before the recent total eclipse one among them even went so far as to say that the observations and photographs of 1870 and 1871, while demonstrating the solar nature of the glory immediately surrounding the sun, proved the long rays extending much farther from the sun to be non-solar phenomena. "The non-solar origin of the radial structure," said Mr. Lockyer as late as July 20th last, "was conclusively established" during the eclipse of December, 1871.
To say the truth, there is no possible way of interpreting the lon grays as phenomena of our own atmosphere or of matter (gaseous, meteoric, or dust-like) on the hither side of the moon. The idea is one which mathematicians may casually have thrown out. Indeed, Mädler and Airy, after the eclipse of 1860, advanced the hypothesis that the long rays belong to matter between us and the moon, while Sir John Herschel adopted in his "Familiar Lectures" the notion that these rays belong to matter at a great height in our own atmosphere. But it would be to misrepresent these eminent astronomers to assert that they ever maintained these views. The available evidence, analyzed as any one of these mathematicians could have analyzed it, had he seen fit, would have shown convincingly that the rays must come from matter lying far beyond the moon. Sir John Herschel admitted this in a letter addressed to the present writer. Whether Airy or Mädler ever examined the evidence closely we do not know. If they did they doubtless were led to the same result as Sir J. Herschel. The matter may be put in this way: Since these long rays extend from the black disk of the moon during mid totality, they occupy then a part of the sky where no sun-illuminated air lies at such a time; therefore they cannot belong to our air; but if there were some very tenuous matter, aerial or dust-like, extending as far as the moon's orbit, the whole region of the sky athwart which these rays extend would contain matter of this sort under full solar illumination; no rays then would be seen, but a nearly uniform glare, which should become brighter and brighter as the distance from the sun's place increased. If we add to this that at midnight the whole of the sky, except a round spot some four or five times the diameter of the moon, would be occupied by this cis-lunar matter under direct solar illumination, instead of that illumination from behind which such matter would receive during total eclipse, we see that the darkness of our midnight sky speaks as decisively against this theory as does the brightness of the long rays seen during total eclipse.
Notwithstanding the overwhelming evidence available to show that these rays lie far beyond the moon, Professor Abbe had adopted the opinion that the rays belong to the earth's atmosphere, or else are mere optical illusions. "I had hitherto firmly believed them," he says, "to be either in the earth's atmosphere or in the observer's eyes. . . . Such rays," he adds, "were seen by members of my eclipse party at Sioux Falls City, Dakota, August, 1869; but at that time and ever since I have doubted their existence." It is manifest that he did not begin his observations with the preconceived idea that the rays belong to matter far more distant than the moon, but with a strong opinion, if not a strong prejudice, the other way.
Next let us consider the actual circumstances under which he observed the eclipse, for they also are important in enabling us to estimate the value of his result. "Having been somewhat hastily carried," he says, "from the summit of Pike's Peak down to the Lake House (elevation 10,000 feet), I had by Monday noon recovered sufficiently to be laid on the ground upon a gentle slope facing westward, where I studied the rays visible about the sun during totality. I had no optical or other instrument, and unfortunately had only a pair of spectacles not quite sufficient even to correct my near-sightedness. By straining my eyes somewhat I was, however, able to do something. My whole attention was given to the rays that extended beyond the brilliant ring which I presume represents the true solar atmosphere. I was undisturbed by any other consideration except to get a true presentation of these rays. . . . I went over the region around the sun again and again—at least six times—leisurely during the 161 seconds of totality, and cannot doubt the truthfulness and fairness of my drawing and description. . . . Two stakes were driven down on either side of me; and between them was placed a rotable axis, on which my drawing-board and paper were fastened. . . . By slightly tipping my drawing-board I kept the sun just above it, or just hidden from view, as I wished, while I drew in such details as I wished, and that too, as it seemed to me at the time, with great ease and accuracy, especially as to the angular position of the rays."The moon or sun appeared surrounded by a narrow brilliant white ring, less than 140,000 miles broad. (We alter the technical indication of apparent breadth into the actual breadth in miles, as likely to be more intelligible to most of our readers.) This ring was as brilliant as the full moon. It was of uniform tint and light, continuous and without any break or structure visible to Professor Abbe, "Outside of this there was no other concentric coronal appearance, and no external boundary; but the immaculate blue-black sky immediately adjoined
The picture which accompanies Professor Abbe's description in the "Colorado Springs Daily Gazette" is doubtless not intended to present with any accuracy the actual tints or degrees of brightness of the various features observed. The shape of the streamers is shown with sufficient exactness in the accompanying figure. It will be understood, of course, that the rays numbered were seen on a dark background, the "immaculate blue" of Abbe's description.
The tapering ray marked No. 1 was the first seen by him. He says he saw it on his first glance at the corona. It then seemed to extend about three times the diameter of the sun; but in a minute or so, as the observer's eyes became accustomed to the sight, he was able to trace its tapering end to a distance of six diameters of the sun's disk. "Its sides were straight lines, its axis passing slightly below the sun's center. Its light was an exceedingly faint and delicate white, apparently overlaid or intermingled with the blue of the atmosphere. I saw no striation, texture, or variation of light. There was no decided increase of brightness in that part of the ray near the sun's edge, nor in the axis of the beam, the delicate light continuing uniform up to the corona, in whose glare it was lost." We must note here two points. In all probability the words "in a minute or so" are used in their colloquial sense for presently, because the whole totality did not last two minutes and a half, and in the course of that time Professor Abbe noted all the features of the corona six several times. Secondly, we find that, both in the "Daily News" and in "Nature," Professor Abbe is described as tracing the rays to a distance of six degrees from the eclipsed sun, not six diameters only; so that, as the sun's apparent diameter is little more than half a degree, these accounts would suggest that he saw the rays to double the distance described in the "Colorado Daily Gazette." But there seems little reason to doubt that the accounts given in the "Daily News" and "Nature," which constitute in reality but one account, seeing that they both came from the same source, are incorrect; for the account sent to the Colorado paper was written by Professor Abbe himself. It contains an illustration from a drawing of his own (reproduced above), which agrees with his description. Moreover, we received the paper directly from Professor Abbe; and unquestionably he would have struck out the word "diameters" and substituted "degrees" if he had really seen the ray extending to the greater distance. Note also that the word "diameter" is used throughout the descriptions of other rays.
The ray marked 2 was seen as soon as 1. Its bounding edges, diverging from each other, but not from the sun's center, produced a somewhat fan-shaped ray. "When first seen," says Abbe, "I estimated its outer limit at one diameter, but subsequently traced it to a diameter and a half from the sun. Its left-hand edge appeared somewhat sharper and brighter than the right-hand edge. With this exception the light was very uniformly distributed throughout its surface, fading away rapidly at its outer end. It also remained changeless throughout the totality."
No. 3 was also seen at the same time as No. 1. "It was narrower and shorter than No. 1: its estimated length, three diameters. It broadened at its base, like No. 1, and had the same uniform tint and intensity."
No. 4 "was not noticed at all until the totality was half over. Its length was one diameter, and it was certainly brighter at the end farthest from the sun. It remained perfectly steady," adds Professor Abbe, "after I once noticed it, and gradually I became aware of a faint light partially connecting it with No. 3, so that the final impression left on me was that these two constituted one fan-shaped projection similar to No. 2, but fading out in the central portions. The axis of No. 1 and of Nos. 3 and 4 passed nearly, if not exactly, through the sun's center.
No. 5 extended fully five diameters from the sun's limb, "and was in all respects similar to No. 1. Its base was broader than that of No. 1, which I attributed," says Abbe, "to the glare of the increasing corona" and of a mound of the ruddy prominence matter (low-lying, so as to form only an extension of the sierra). The light of No. 5 was fainter, Professor Abbe thought, than that of No. 1. "Its edges were straight, except in so far as the coronal glare appeared to unduly broaden the base. Its axis passed very nearly through the sun's center, and was in the prolongation of the axis of No. 2."
Professor Abbe's explanation of these rays or streamers occurred to him an hour or so after seeing them. He advances it as one which "will probably result in the overthrow of all previously entertained theories respecting the character and cause of these streams of light." But in reality it is not nearly so novel as he seems to imagine. It is, indeed, partly new, and in our opinion it is in great part true; but what is true in it is not new, and we question greatly whether what is new in it can possibly be true. Let astronomers judge.
"Meteor streams," says Professor Abbe, "is the key to the solution—not such meteors as some suppose to be falling into the sun daily, but the grand streams of meteors that cause the numerous shooting stars of August and November, and of the existence of which there is indubitable proof. These streams consist of fine particles or pieces, each a long way from its neighbor, but all rushing along in parallel orbits about the sun, like the falling drops of rain in a thunder-shower. The August stream is calculated to be several hundred thousand miles broad and thick, and many million miles long. Such a stream, when far beyond the sun, but still lighted up by it, would reflect to us a faint uniform light precisely like that of these rays. If one end of the stream were farther from us than the other, the effect of the perspective would be to produce a tapering or wedge-shaped appearance. In some other part of our orbit, or with the meteor stream in some other part of its orbit, the perspective might vanish and the two ends appear of the same width. In this way we shall undoubtedly be able to explain the very numerous historical and memorable occasions on which flaming coronas, swords, comets, etc., seen in the sky during a total eclipse have been regarded by the superstitious as divine omens."
We have very little doubt that the great extension of the corona in certain directions during many total eclipses, and the probably far greater extension of a fainter, not readily discerned lustre during all eclipses, is due to the existence of meteor streams. It is also undoubtedly true that several of the meteor systems encountered by our earth in her journey round the sun have the vast dimensions mentioned by Professor Abbe. Indeed, he far underrates the dimensions of the August and November meteor systems, each of which must be measured in length by hundreds of millions of miles, not by mere millions. But it is absolutely impossible that any of the meteor systems traversed by our earth, or any meteor systems of no greater degree of richness, should present the appearance of streamers surrounding the sun, like those in our figure above. So far as the two systems specially mentioned by Professor Abbe are concerned, inasmuch as we know the exact shape and position of the orbits along which the meteors forming these systems travel, we can determine the exact position which the meteoric streams occupy in the heavens at any moment; and most certainly neither of them on July 29th last occupied the position of the two beams shown across the sun in our figure. The August system was the one which at the time passed nearest to the sun's place on the sky, but it did not come within several degrees of the sun. The November system did not even cross the part of the sky where the sun was. These two systems, therefore, could not possibly be connected in any way with the two streams, of whatever nature, which produced the rays intersecting exactly at the sun.
But there is a more general objection to the theory that such meteor systems may explain coronal streamers seen during total eclipses of the sun. If such streams could be seen when situated beyond the sun, they would be seen far better when opposite the sun on the dark background of the midnight sky. Take, for instance, the November meteors. We know that the flight of meteors, some 2,000,000,000 of miles long, which the earth traversed in November, 1866, 1867, 1868, 1869, 1870, and 1871, is now nearing the remotest part of the long orbit of the November system, many millions of miles beyond the path of Uranus. We know that at midnight in winter the richest part of that system lies due south, at an elevation varying from 30° to 50° above the horizon. There, illuminated fully by the sun, though at a great distance from him, it ought to be far better seen than a similar system lying beyond the sun and visible only through the light of the brightest part of the corona. But no one has ever, on the darkest and clearest night and under the most favorable atmospheric conditions, even suspected the existence of the faintest possible light where the heart of the November system is really situated. Much less, then, could such a system be seen during total eclipse (if so situated as to lie athwart the sun). Systems less rich than the November system (the richest known to us) would have still less chance of being discerned.
If, then, we are to account for the radial streamers seen by Professor Abbe, and also seen during many other total eclipses, though to a less distance, by the meteoric theory, we must consider meteor systems very unlike those through which the earth herself passes. The meteor systems required by the theory must be much denser and much more brightly illuminated than the August and November systems. To say they must be much more brightly illuminated is equivalent to saying that they must be much nearer the sun. And in this we see an escape from another difficulty. Meteor systems very near the sun would be far more likely to appear as streamers extending radially from him than systems at a great distance from him. A distant system might, by a mere chance, so appear. For instance, if a total eclipse of the sun had occurred on or about May 10, 1865, the November meteor system (whose richest part was then crossing the earth's track at the point she occupies on November 13th) would have appeared, if discernible at all, as a streak athwart the sun's place in the sky, and therefore forming two rays on opposite sides of him, somewhat like 2 and 5 in our figure. Sixteen years or so earlier or later the November system would present a similar appearance, only very much fainter, on account of greatly increased distance, during a total eclipse occurring on or about November 13th. At no other time in the year except November 13th and May 10th, or about these dates, could the November system present such an appearance. But a system traveling close to the sun, and not far from the plane near which all the planets travel, would present at all times nearly the appearance of a pair of rays like 2 and 5 of our figure. On this account, therefore, as well as on account of the greater brightness with which such meteor systems would be illuminated, we must prefer the theory that the systems to which the coronal rays are due travel near to the sun.
Yet, even as thus presented, the meteor theory alone seems inadequate to explain the coronal streamers. There is an enormous mass of evidence showing that meteor systems are most richly strewed throughout a region around the sun, extending nearly to the distance of the planet Mercury; but there is also abundant reason for believing that these multitudinous systems would present an appearance very different from that depicted in Professor Abbe's view of the coronal streamers. We want something quite distinct from the theory of a mere aggregation of meteors to account for these rays, whether pointed or fan-shaped, extending directly from the sun. The aggregation of meteors might present the appearance of a luminous cloud around the place of the eclipsed sun. This cloud might be to some degree radiated, because each meteor system would have a course carrying it either directly athwart the sun's place on the sky, or nearly so. But there would be nothing like those sharply defined streamers extending separately from the sun to distances of ten or twelve sun-breadths. Sir George Airy, describing the appearance of the corona during the eclipse of 1851, pictures just such a cloud as we should expect to result from the aggregation of meteors. "Its color," he said, "was white, or resembling that of Venus; there was no flickering or unsteadiness; it was not separated from the moon, nor had it any annular structure: it looked like a radiated luminous cloud behind the moon." The long streamers manifestly require a different explanation.
We can not but think that the true explanation of these streamers, whatever it may be (we are not in the least prepared to say what it is), will be found whensoever astronomers have found an explanation of comets' tails. These singular appendages, like the streamers seen by Professor Abbe, extend directly to the sun, as if he exerted some repellent action on the matter forming the heads of comets. Indeed, Sir John Herschel did not hesitate to say that the existence of such a repulsive force was, to all intents and purposes, demonstrated by the phenomena of comets' tails. Now we know that meteors and comets are in some way associated, though the actual nature of the connection between them is not clear. It is certain that the November meteors, the August meteors, and other such systems, follow in the track of known comets. We know that when, in 1862, the earth passed through the region of space along which Biela's comet had recently traveled, there was a display of thousands of meteors, all radiating from just that part of the heavens from which bodies traveling parallel to the orbit of Biela's comet would have seemed to radiate. It follows from this association between comets and meteors, and from the fact that probably thousands of meteoric and cometic systems travel close to the sun, that in all probability there must exist generally, if not always, in the sun's neighborhood, enormous quantities of the substance whence comets' tails are formed by the sun's repellent action. This being so, we should expect to find generally, if not always, long streams of matter extending from the sun's immediate neighborhood, in the same way that comets' tails extend from comets' heads. Whether the repulsive force is electrical, magnetic, or otherwise, does not at present concern us; or rather it does concern us, but at present we are quite unable to answer the question. All that we know certainly is that, in the first place, the sun does in some way cause streams of luminous matter to appear beyond the heads of comets, in a direction opposite to his own, and to enormous distances; and, in the second place, that the matter forming comets' heads is probably present at all times, in large quantities, in the sun's immediate neighborhood. We can hence infer, with extreme probability, that such long streamers as Abbe saw last July, Myer in August, 1869, Feilitzsch in June, 1860, and several Swedish observers during the eclipse of 1733, are produced in the same way as comets' tails, and therefore really extend (as they seem to do) radially from the sun. It is also certain that if they did not really extend radially from the sun, their always seeming to do so would be altogether inexplicable. So that the theory to which we are led in one direction leads us also out of what would else be a very perplexing difficulty in another direction.—Cornhill Magazine.