Popular Science Monthly/Volume 1/May 1872/The Recent Eclipse of the Sun
By R. A. PROCTOR, B. A., F. R. A. S.
THE eclipse of the sun which took place on December 12th last was looked forward to by astronomers with some anxiety, because many months must pass before they will have any similar opportunity of studying the sun's surroundings. Year after year, for four years in succession, there have been total eclipses of the sun—in each year one—and each eclipse has taught us much that has been worth knowing; but during the present year there will be no total solar eclipse worth observing; there will be none in 1873, only one (and not a very important one) in 1874, while during the total eclipse of 1875 the moon's shadow will traverse a path very inconveniently situated for intending observers.
Besides, the inquiries and discussions of astronomers had reached a very interesting stage before the recent eclipse occurred. A sort of contest—though, of course, a friendly and philosophic contest—had been waged over the sun's corona, the halo or glory which is seen around the black disk of the moon when the sun is totally concealed; and, though, in the opinion of most astronomers, the contest had really been decided by the observations made during the total eclipse of December, 1870, some slight doubts still existed in the minds of a few. It was hoped—and the hope would appear to have been justified—that during the late eclipse these doubts would be finally removed. A few weeks must elapse even after the present paper appears, and five or six from the present time of writing, before the sun-painted pictures, which are to decide the question, can be in the hands of the judges. But, from the description which has already reached us, we can feel very little doubt as to the nature of the decision which will be arrived at.
A brief sketch of the progress of the inquiry into the subject of the solar corona will serve to exhibit the nature of the doubts which the recent expeditions to the Indian seas were intended to remove.
From very early ages it had been known that when the sun's disk is wholly concealed by the moon, a glory of light starts into view, rendering the scene less terrible, though scarcely less striking, than it would be were total darkness to prevail.
Now, gradually, it began to be recognized that this glory around the sun consisted of several distinct portions. In the first place, quite close to the moon's black body, a very narrow ring of light had been observed, so bright that many astronomers were led to believe that the sun was not in reality totally concealed, but that a ring of sunlight remained even at the moment of central eclipse. This excessively bright ring of light is not, however, always seen, if (as many accounts suggest) it is to be distinguished from the bright inner corona of which I shall presently have to speak. During the recent eclipses we have had no clear evidence respecting this brilliant but very narrow ring; and it is just possible that the accounts derived from earlier eclipses have been a little exaggerated.
Then, secondly, a red border is seen around portions of the black disk of the moon. This border has commonly a serrated edge, and has been called the sierra, from a well-known Spanish name for a range of hills. From what thus resembles a chain of rose-colored mountains, appear to spring certain red projections which have been called the solar prominences. Their general appearance during eclipse may be inferred from the description given by those who first observed them, in 1842, who compared the moon's disk surrounded by these glowing objects to a black brooch set round with garnets. But it is now known that such names as prominences and protuberances are not properly applicable to these red objects, and that the word sierra is equally inapplicable to the rim of colored light beneath the red projections. The prominences as well as the sierra (for, however unsuitable, the names continue in use) are in reality formed of glowing gas, hydrogen being their chief constituent element, but other elements being also present in a gaseous form. Only, the reader must not run away with the notion that these great red masses, some of which are more than a hundred thousand miles in height, are of the nature of our gas-flames. They are not, properly speaking, flames at all, but masses of gas glowing with intensity of heat.
Many of the most important discoveries recently made respecting the sun relate to these wonderful objects; but in this place I shall refrain from speaking more about them than seems necessary to illustrate the subject of the corona; for, as a matter of fact, the observers during the late eclipse turned scarcely a thought to the colored prominences, nor is it likely that any thing new respecting them will ever be learned during total eclipses of the sun.
Outside the sierra and the prominences, the true corona is seen. To ordinary vision, and probably also even under the scrutiny of powerful telescopes, it appears to be divided into two distinct portions. There is in the first place an inner and brighter region, extending apparently to a distance from the sun equal to about one fifth of his diameter. The outline of this inner corona is uneven but not radiated, and, though not sharply defined, appears yet to be very definitely indicated by the rapid falling off of luster beyond its limits. The inner corona has been described as of a white, pearly luster by some observers; but under the most favorable conditions it appears, when carefully observed, to have a somewhat ruddy hue.
Extending much farther from the sun, how far is not as yet known, is the radiated corona. It is much fainter than the inner corona, and its light grows fainter and fainter with distance from the sun, until lost to view on the dark but not black background of the sky. Through this faint and softly graduated corona extend radiations of somewhat greater brightness. It is between these radiations that those dark gaps or rifts appear, which have figured so much in the narratives of recent eclipse observations. The dark gaps are, indeed, more striking features than the radiations which form them; but it must be remembered, nevertheless, that the radiations are the only positive features in this case, the gaps being merely regions where there are no radiations.
We may typically represent the corona, as it had been revealed to us during former eclipses, by the accompanying sketch from a photograph taken by Mr. Brothers at Syracuse during the eclipse of December, 1870. Only, it must be remembered that the photograph may not represent the full extent of the corona, while many details of its structure are too delicate to be shown in a figure so small as is here given. It will be understood further that the inner part, marked R, is much brighter than the whole of the outer part, marked c, and that this outer part shades off gradually into the dark background of the sky.
Now, the question which has agitated astronomy during the past few years has been simply whether the glory of light seen around the sun is in reality a solar appendage, or may not be due wholly or in part to the illumination either of our own atmosphere or of some other matter (not necessarily atmospheric) lying much nearer to us than the sun does. If we consider the figure, we can see at once that if we have here a real solar appendage—that is, matter which exists all around the sun's globe—it is an appendage of the most amazing extent. The black disk which forms the centre of the figure is of course intended to represent the moon, whose diameter we know is about 2,200 miles, and if for a moment we suppose the corona c and r surrounds the moon, we see that it must extend on one side to about 5,000 miles, and elsewhere to about 2,800 miles. But exactly behind the moon lies the sun, a little more than concealed by the moon; and the sun's diameter is about 850,000 miles. So that, if the corona is something which surrounds the sun, it extends, as the picture shows, to at least 2,000,000 miles on one side, and elsewhere to about 1,200,000 miles. Neglecting the dark rifts for the moment, and regarding the whole corona as shaped like a globe, and having a diameter four times as great as the sun's, we should have to regard its volume as exceeding his not four times, nor sixteen times, but sixty-four times. And when we are reminded that the sun's own volume exceeds that of this earth on which we live some 1,200,000 times, we see what a stupendous conclusion we must arrive at, if we regard the corona as a solar appendage. Of course, we need not imagine that the corona has a continuous substance completely filling a space some 77,000,000 times larger than the earth. It may be made up of multitudes of minute bodies, with vacant spaces between. But the conclusion remains that a region of space, exceeding our earth's volume so many millions of times, is thus occupied by matter of some sort.
Nor is the conclusion rendered a whit less surprising if we take the dark rifts into account. Nay, we obtain an enhanced idea of the wonderful nature of the corona, regarded as a solar appendage, when we consider that it possesses so remarkable a structure that, as seen from our distant stand-point, it shows well-defined gaps or rifts. For unquestionably it is not to be regarded as something flat or plane shaped, like its picture, or a decoration (which in appearance it often strikingly resembles). It must extend on all sides from the sun (if it is indeed a solar appendage), and not merely from the sides of the disk he turns toward us at the time of an eclipse; and it can easily be seen that its shape, in length and breadth and thickness, must be strange, to account for such rifts as are shown in the figure. If we take an orange to represent the sun, and, boring holes all over it, stick spills in these holes to represent the region occupied by the corona, we shall find that, in order that our spillikined orange may exhibit a rifted corona in whatever position it is placed, we must either leave several large parts of its surface without spills, or that the spills over many such parts must be very short. "When this consideration is attended to, the spillikin corona will be found to have a very complex and remarkable figure.
It is not to be wondered at that, so soon as the corona began to be thought about at all, astronomers were led to believe that it is not of the nature of a solar appendage, but either a sort of halo in our own atmosphere, or else an appendage belonging in some way to the moon. Kepler and Halley and Newton, to say nothing of a host of other astronomers who considered the question during the infancy of modern astronomy, were led to different conclusions, by the comparatively imperfect evidence available in their day. We may pass over the arguments adduced in favor of the three several theories which were in question. Suffice it that, gradually, it was admitted more and more generally that the corona must be some appendage surrounding the sun; and, in comparatively recent times—a quarter of a century ago, or thereabouts—the opinion began to prevail that the corona is in fact the sun's atmosphere.
But quite recently discoveries were made which seemed to throw great doubt upon this opinion. By means of the instrument called the spectroscope, astronomers have learned not only how to study the sun's colored prominences when the sun is shining in full splendor, but also to determine to some extent the condition of the glowing gas of which those prominences are formed. When this was done, it did not appear that the density of the glowing gas—even close by the sun's body was so great—as might be expected if the corona were an atmosphere properly so called. Some prominences are shown in the figure; and if we consider the pressure to which objects so placed must be subjected, supposing them to lie at the bottom of an atmosphere more than a million miles in height, we shall at once see that the pressure of our own air at the sea-level would be a mere nothing by comparison. It is supposed that our air may be two or three hundred miles in depth, but, even if we suppose it to be ten times as deep as this, the depth of the imagined solar atmosphere would be many times greater. And then the pressure of our air is caused by the earth's attraction, and would be greater if the earth exerted a greater attraction. But the attractive energy of the sun (at his surface) exceeds the force of the earth's gravity about twenty-seven times. "We may safely infer, then, that an atmosphere such as the corona was supposed to be, would cause a pressure exceeding the atmospheric pressure we experience some thousands of times. The gas forming the prominences would be correspondingly compressed under these circumstances. But as a matter of fact the pressure at the very base of the colored prominences appears to be a mere fraction of that which our own air exerts at the sea-level.
Accordingly, Mr. Lockyer, who had taken a prominent part in establishing this very interesting result, was led to express the opinion that the sun's atmosphere has no such extent as had been imagined, and that the corona is an appearance (only) in our own air, "an atmospheric effect merely," "due to the passage of the sun's rays through our own atmosphere."
This conclusion was, however, not very generally accepted. Several astronomers at once pointed out that the air which lies toward the place on the heavens where the corona is seen, is not illuminated at all by the sun's rays during total eclipse. I also pointed out that whatever light that particular part of the air receives during totality—not direct sunlight, but light from the prominences, and so much of the corona as might be admitted to be solar—would extend over the very place of the moon, and gradually increase thence on all sides instead of gradually diminishing, as happens with the corona. This would not be the place to exhibit the reasoning by which these results can be demonstrated; for mathematical considerations, not altogether simple, are involved in the complete discussion of the matter. Let it suffice to say, as respects the air between the observer and the moon, that, since the observer can see the colored prominences and the inner bright corona during totality, the air all around him (toward the moon as well as elsewhere) must be lit up by their light. And as respects the gradual increase of brightness on all sides of the place where the eclipsed sun is, let the reader consider that, if, at any time during totality, a bird were to fly (with enormous rapidity) from the observer's station directly toward the moon's centre, that bird would remain in the moon's shadow as he so flew; but if he flew in any other direction he would presently pass out of the shadow—that is, he would reach a place where the air is illuminated. And he would so much the more quickly reach the illuminated air, as he flew more directly from the moon's place on the sky. So that, putting the line of the observer's sight instead of the swiftly-flying bird, we see that this line will so much the sooner reach illuminated air, according as it is turned farther from the place of the moon on the heavens. Thus the air toward the place of the moon, though illuminated, is less brightly illuminated than that lying toward any other part of the sky; and the atmospheric illumination must gradually increase the farther we turn our eyes from the moon's place.
So matters stood when preparations were being made for the expeditions to view the eclipse of 1870. Evidence had, indeed, been obtained during the eclipse of 1869 in America, which seemed to show that the substance of the corona is gaseous; and singularly enough it appeared as though this substance, whatever it might be, shone with a light resembling that of the aurora borealis. But those who regarded the corona as a mere glare in our own atmosphere, rejected these results because they seemed "bizarre and perplexing in the extreme." The American astronomers, however, were not willing to have their observations rejected in this summary fashion; and they, therefore, crossed the Atlantic in great force to observe the Mediterranean eclipse of December, 1870.
It was with some little regret, I must confess, that, as the eclipse of 1870 drew near, I found many of the intending observers proposing to direct their chief attention to the question whether the corona is a solar appendage or a mere glare in our own atmosphere. It seemed to me clear that the atmospheric theory was completely disposed of by the evidence, while a host of interesting questions remained to be answered respecting the nature of the amazing solar appendage thus shown to exist. "I think I have not erred," I wrote in October, 1870, "in insisting that we have ample evidence to prove that the corona is a solar appendage; but what sort of appendage it may be, remains yet to be shown. Observations directed to show whether it is or not a solar appendage will, I apprehend, be a total waste of time; and it is for this reason that I have, at the meetings of the Astronomical Society and elsewhere, deprecated all such observations."—(Preface to second edition of "Other Worlds.") Nay, I fear I even offended one or two by the zeal with which I urged the importance of endeavoring to determine, not whether the corona is a solar appendage, but what sort of solar appendage it may be.
However, the observations were made, photographs and sketches were taken, and the general conclusion drawn from the work of 1870 was that which Sir John Herschel, only six weeks before his lamented decease, enunciated in the following terms in a letter addressed to : "The corona is certainly extra-terrestrial and ultra-lunar."
Even then, however, some doubts still remained in a few minds. The question of the corona was still mooted in essays and lectures—nay, the atmospheric theory was so successfully defended before the British Association last August, as to lead Prof. Tait to remark that, in his opinion, it was in the main true; while the president of the meeting—Prof. Thompson—even expressed the opinion that the special observations made last December proved that the greater part of the corona was a mere phenomenon of our own atmosphere. It must be pointed out, however, in justice to these eminent mathematicians, that only one side of the question had been adequately presented to them.
Thus another year had passed, and the subject of the corona stood almost exactly as in the autumn of 1870. Well-appointed expeditions were again about to set forth to view an important eclipse, and again the question which the observers had before them was the worn-out problem whether the corona is or is not a solar appendage.
But much more faith was placed in photography than had been the case in 1870. Then, men doubted whether photography could give good pictures of the corona. The colored prominences had been photographed repeatedly; but the finest telescopes had failed to bring the corona fairly on to the glass. Mr. Brothers, of Manchester, however, showed how this difficulty was to be surmounted. He discarded the telescope and employed the ordinary photographic camera. The results were most satisfactory. The eclipsed sun was indeed partially hidden by clouds during all but the last few seconds of totality; but for eight seconds the camera was fairly at work; and the result was, "the corona as it had never been seen on glass before."
the sun's corona.
R, the inner or ring formed corona; C, the outer radiated corona.
During the late eclipse, Mr. Brothers's plan was adopted at several stations, and most successfully, by all the photographing parties whose accounts have yet reached Europe. For many weeks, however, these photographs will not be available for examination. The great point which we know already respecting them is this: that they show an extensive corona, with persistent rifts—those taken at the beginning of totality differing from those taken at the end only as respects parts of the corona very far from the sun. All those doubts, which had been based on the circumstance that Mr. Brothers's best photograph was taken nearly at the close of totality, are therefore removed by the photographs taken on the present occasion.
But, the corona was so favorably seen even with the naked eye, during the recent eclipse, as to dispose of all the doubts formerly entertained. In an interesting letter in the Daily News, an eye-witness at Bekul, describing Mr. Lockyer's observations, says that so soon as the totality began the corona appeared, rigid in the heavens, like a magnificent decoration, suggesting by its fixity the idea of perfect rest in those distant regions. It was marked with radial streaks of great brilliancy, separated by relatively dark furrows, and extending all round the upper and lower parts of the moon's circumference, but less conspicuous (or altogether wanting—the account is not very clear on this point) at the sides. This observation is of great interest, because the upper and lower parts of the sun's circumference at the moment of observation corresponded to the sun's equatorial regions, while the sides corresponded to the position of the solar poles. Mr. Lockyer's account thus seems to support a theory lately urged, according to which the corona is caused by radial emanations chiefly from the neighborhood of the solar equator. It is clear, however, from the rifts (especially as shown in the figure), that such emanations cannot be continuous, but must take place locally, and, as it were, fitfully.
But the most important account which has yet reached Europe is that contained in a letter from M. Janssen, the eminent spectroscopist, to M. Faye, the president of the French Academy of Sciences. It should be noted, in the first place, that in a letter to the secretary of the Academy Janssen says: "I have just observed the eclipse, only a few moments ago, with an admirable sky; and, while still under the emotion occasioned by the splendid phenomenon which I have but now witnessed, I send you a few lines by the Bombay Courier. The result of my observations at Sholoor indicates, without any doubt, the solar region of the corona and the existence of material substances (matières) outside the sierra." Then follows his letter to the president, which runs thus: "I have seen the corona as I could not in 1868, when I gave myself wholly to the prominences. Nothing could be more beautiful or more brilliant; and there were definite forms which exclude all possibility of an origin in our own atmosphere." He proceeds to describe the coronal spectrum, confirming the American observations—with one notable exception: he recognized the solar dark lines in the spectrum of the corona, a proof that no inconsiderable portion of its light is reflected sunlight. Then he draws his letter to a conclusion with these decisive words: "I conceive that the question whether the corona is due to our own atmosphere is disposed of (tranchée), and we have before us in perspective the study of the regions lying outside the sun, which must needs be most interesting and fruitful." I could wish that the same opinion had been received when it was advocated twenty-two months ago in almost the same words.—Cassell's Magazine.