THE SUN'S CROWN.
L1 CIRCUMSTANCE has just been brought to light through the careful study of the photographs of the recent total solar eclipse which is full of interest and significance. When the sun is totally eclipsed, there springs suddenly into view a glory of white light, resembling the nimbus with which painters surround the head of a saint. Astronomers have agreed to call this appearance the Corona; but hitherto they have been perplexed by doubts whether this crown of glory belongs to the sun or to the moon, or whether, in fine, it is formed by our own atmosphere.
If we briefly consider what is commonly seen, we shall be the better able to appreciate the interest and importance of the discovery which has just been made respecting the corona.
As the moon is about to hide the last narrow streak of the sun's disc, the first signs of the corona make their appearance. But only when totality has commenced does the phenomenon present itself in full splendour. It is no faint gleam, like the light of a twilight sky. "I had imagined," says Mr. Bally, speaking of the eclipse of 1842, "that the corona, as to its luminous appearance, would not be brighter than the faint crepuscular light which sometimes takes place on a summer evening, I was, however, as- tonished at the splendid scene which suddenly burst upon my view." All round the eclipsed sun, to a distance equal to about a tenth of his apparent diameter, there is a brilliant ring of light, which appears under favourable circumstances of vision to have a well- defined edge. But this is not the complete corona. Beyond the edge of this ring of light extends a fainter ring, sometimes spread- ing out into rays or streamers, which extend some eight or nine times farther from the eclipsed sun than the bright inner circle of light. The colour of the corona is commonly described as white; but there can be no doubt that when seen through a pure atmo- sphere it presents tints of red, yellow, and blue.
Such is the corona as seen by astronomers. But the question will at once arise, what is the real position and what are the true dimensions of this beautiful object ? Of course, if we regard it as a mere optical phenomenon produced by our own atmosphere, we need not try to find an answer to these questions. The appearance of the corona, its apparent figure, and its variations of figure would then have merely a meteorological interest, apart, of course, from the optical questions they involve. If, on the other hand, we regard the corona as a real solar appendage, we are forced to consider it as one of the most important and striking features of the solar system. The ring of brighter light around the sun is then seen to represent a globular shell about 90,000 miles in depth, and sur- rounding the whole mass of the central luminary of the planetary system. The fainter part of the corona becomes an even more astounding phenomenon, since looked on as a solar appendage it represents a shell of matter fully 800,000 miles deep in every part, and forming with the sun, which it encloses, a sphere some two- and-a-half millions of miles in diameter,—the largest sphere of matter which the science of astronomy presents with any certainty to our consideration. But if the corona belongs to the moon, its dimensions shrink into relative insignificance,—in fact, our OWTh earth is a larger globe than the coronal sphere so understood.
The question of the corona has long been seen to rest between. the two former solutions. Halley rather favoured the notion that the corona is a lunar phenomenon ; but he admitted that one whose judgments he "must always revere '' (he referred, doubt- less, to his illustrious friend Newton) held a contrary opinion._ We now know very certainly that the moon has no atmosphere- whose extent we can measure,—certainly no atmosphere approach- ing in extent the dimensions of the coronet rings.
During the great solar eclipse of 1868 very little attention was given to the corona, because astronomers were very anxious to determine the nature of the rose-coloured prominences. But from, the few observations which were then made, the question whether the corona belongs to the sun or is a phenomenon of our own- atmosphere was left an open one. It was hoped that the problem of the corona might be solved during the total eclipse which occurred last August in North America. At first, however, the results of the observations seemed more perplexing than any which had yet been presented to the notice of astronomers. As Mr. Lockyer remarked, they were "bizarre and puzzling in the extreme." They seemed to point to the corona as a permanent solar aurora, since some of the observers found in the spectrum of the aurora the same bright lines which belong to the spectrum of the aurora borealis.
So perplexing did this result appear, that Mr. Lockyer was disposed to doubt whether some mistake had not been made. The results of his own observations had led him to the conclusion that the solar atmosphere in which the red prominences are formed is by no means so dense as the enormous dimensions of the corona would imply, if the corona really were a solar atmosphere. It will be known to many of our readers that Dr. Frankland and Mr. Lockyer have worked together in this matter, and they have found' that the appearance of the bright lines belonging to the promin- ences can be taken as a means of estimating the pressure of the atmosphere in which those prominences appear ; and the result of their observations pointed, as we have said, to a relatively rare atmosphere. But now it would seem that little further doubt can be entertained respecting the fact that the brighter coronal ring, at least, belongs to the sun. For on a careful comparison of the photographs taken during the recent total eclipse, it has been found that the disc of the moon travelled over the corona; and further, that the corona presented the same appearance as seen from widely separated places. It will be remembered that photography gave in the same way the first evidence of the true- nature of the coloured prominences. It was discovered during the eclipse of 1860 that the moon travelled over the prominences, and so astronomers pronounced decisively that these objects belong to the sun. It would appear quite as certain, now, that the corona is also a solar appendage.
But how are we to get over the difficulties suggested by Mr. Lockyer's observations? It seems perplexing in the extreme to regard the corona as a solar atmosphere, because, were it really of this nature, the pressure at the surface of the sun would be incon- ceivably great. And again, there are mechanical reasons for doubting whether an atmosphere of such a nature could exist around a body rotating so rapidly as the sun. Is it not conceiv- able that the corona may consist, as Mr. Baxendell recently sug- gested, of cosmical bodies travelling around the sun? We know that such an explanation has long been given of the zodiacal light, and that light has been found to be similar in character to the aurora borealis. May not the corona be simply the denser part of the zodiacal light? Only then there is this difficulty. A number of cesmical bodies, if shining by their own light, could only give such a spectrum as the corona if they consisted of glowing gas. Now we had no reason to believe that the aurora borealis, the zodiacal light, or the corona, consists of such a material until recent observation showed that they give a bright-line spectrum. There is, apparently, only one escape from the difficulties thus involved. We know that the aurora is in part an electric phenomenon, and we may now therefore pro- ceed safely to the conclusion that the zodiacal light and the corona are also in part electrical phenomena. We know further that the auroral action is associated with solar action, therefore we may conclude that so also is the action which produces the appearance of the zodiacal light and the corona. Now an electric flash, when examined by the spectroscope, gives a bright-line spectrum corre- sponding to the nature of the substances between which the flash passes and the character of the medium through which the discharge takes place. May we not conclude with some confidence that we see in the aurora, the zodiacal light, and the solar corona, a light due simply to such electrical discharges excited by the sun's action? We know that in our own atmosphere there is a continual downfall of meteoric dust, and astronomers have long believed that in the sun's neighbourhood meteoric streams are much more densely aggregated than near the earth ; so that there is no want of material basis to the theory we have here ventured to propound.
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