THE SUN'S CORONA.
ONE problem after another presented by the study of the Sun has been mastered by astronomers and physicists. They
have ascertained what are the substances of which his globe is con- stituted, they have learnt something of the condition in which those substances exist ; they have detected the secret of the rose-coloured prominences which spring suddenly into view when his orb is eclipsed. Later came the simultaneous discovery by Jannsen and our ingenious countryman Lockyer, that the spectrum of the prominences can be studied when the sun is not eclipsed, a discovery fruitful in promise, if it has not yet brought with it all the results which had been looked for. More important still was the invention by Mr. Huggins, facile princeps amongst astronomical spectroscopists, of a method by which the prominences can be seen (" not merely rendered sensible," as Sir John Herschel justly says,) by means of the spectroscope. Thus it has become possible to inspect the figure, and what is yet more striking, the changes of figure, of these amazing objects. Astronomers have availed them- selves at once of this ingenious method, and so far as the pro- mineuces are concerned little seems left to be discovered.
But one imposing phenomenon presented during total solar eclipses remains yet to be interpreted. The crown of glory, which suddenly bursts into view when the sun's orb is totally concealed, has proved a source of perplexity to astronomers and physicists until now, and may, perchance, yet foil their attempts at explana- tion for many years to come. It has been regarded as a lunar appendage by some, as a solar appendage by others, while others again have supposed it to be merely an optical phenomenon. In August, 1868, during the eclipse which afforded such important information tweeting the coloured prominences, the Corona was left uninterpreted. In August, 1869, though several observers scrutinized it attentively with the powerful instruments now available to the astronomer, it still foiled their efforts. The question which is perhaps of all others most in the thoughts of the astronomer, is whether the observations to be made during the eclipseof December next will suffice to master this stubborn problem.
If we consider the matter aright, we shall see that the solution of the difficulty can hardly fail to afford most interesting informa- tion respecting the physical habitudes of the sun, nay even respecting relations affecting the whole economy of the Solar system. Regarded as a solar appendage, the corona is, in some respects, the most amazing object within the limits of the planetary scheme. It has been seen (under favourable con- ditions) to extend several degrees from the eclipsed sun ; and a degree, in the case of an object situated at the sun's distance, corresponds to a length of seventeen hundred thousands of miles. When we remember that the object, whatever it is, is not a mere plane surface (as one is so apt to conceive in regarding a pheno- menon of the sort), but surrounds the sun on every side, we begin to recognize the enormous volume which it occupies. Those radiations whose structure is so perplexing are in reality enormous streams of matter, whether continuous or discrete remains yet to be determined. These streams, too, are not necessarily directed towards or from the sun, as one is apt to suppose from their ordinary aspect. It is not even probable that they are so situated, since observers of repute have noticed that the radiations of the corona are not symmetrical.
Is it safe to hazard a guess as to the real nature of the corona, when a few months may afford positive information respecting its structure? Let us consider what we know already. It has some- times happened that astronomers have remained in doubt respect- ing a problem whose solution was in reality in their hands. Galileo was content to remain perplexed by the strange changes of Saturn's appearance, when it was possible for him to have anticipated by simple reasoning the discovery of the real nature of the planet's appendage. The predecessors of Sir W. Herschel suffered discovery after discovery to escape them, because they would not be at the pains to discuss in full their own obser- vations. The wonderful success of that great astronomer was due to his appreciation of the fact that observations can only be fruitful when submitted to scrutiny and analysis. It appears to us that the observations already made upon the corona only require careful consideration to reveal with tolerable distinct- ness the real nature of this object.
Setting aside the theories which associate the corona with the moon or with our own atmosphere, as altogether untenable in the face of recent discoveries, let us inquire whether we have any reason to believe that the portion of space apparently occupied by the corona is really tenanted by material substance. We cannot suppose that a solar atmosphere of any sort occupies this region. For it is incredible that objects of such a nature as the coloured prominences—flames thousands of miles in height—should exist at the bottom of an atmosphere whose depth must be estimated by hundreds of thousands of miles, and which would be subject also to the enormous attractive energies of the sun's mass. If our own atmosphere, with a probable depth of about a hundred miles, and attracted only by the relatively insignificant forces of terrestrial gravity, is yet capable of exerting a pressure of nearly a ton on every square foot of surface, how inconceivably vast would be the pressure of an atmosphere thousands of times as deep (even supposing the visible bounds of the corona to indicate its true limits), and attracted by the sun, at whose surface four of our pounds would weigh a hundredweight !
The material particles, then, which form the corona, must be of such a nature as not to press towards the sun. In other words, they must travel around him. As to the nature of these motions, we are led by the irregular appearance of the corona to believe that the bodies forming the corona travel in paths having every variety of form.
Thus we are led to the conclusion that they must be somewhat of the nature of those meteoric bodies which reach our own atmosphere. We know that the meteors reach us along paths so eccentric in figure that many of these bodies must have come from beyond distauces exceeding those at which Uranus and Neptune circle round the Sun. And what is more to the purpose, we know that large numbers of those which are intercepted by the earth would otherwise have passed to the immediate neighbourhood of the sun.
Now for one meteor which the earth intercepts there must be millions on millions which pass on their course, past her path, without being interfered with by her. And for each meteor- system which passes close past the earth's track there must (according to all reasonable probability) be millions of systems which make no such approach. Is it conceivable that these count- less millions of meteors and meteor-systems, undoubtedly existing in the sun's neighbourhood, should remain wholly invisible when the sun is eclipsed? Supposing them,—brilliantly illuminated as they must be, owing to their proximity to the sun,—to be visible during total eclipses, we should expect them to present precisely the appearance actually exhibited by the corona. Thus we are led at once by a priori and by a posteriori considerations to judge that the corona consists of multitudes of meteoric bodies, travelling in orbits having every variety of figure and position around the central luminary.
But if this be the case, we should expect that some traces of the outlying portions of this vast congeries of systems would be visible after sunset and before sunrise. We find accordingly, that there can be traced in the evening towards the west, and in the morning towards the east, that faint luminous gleam known as the zodiacal light. This light grows brighter towards the horizon beneath which the sun lies at the time. Are we to suppose this increase of light stops short suddenly at that point? It is reasonable to con- clude, on the contrary, that the light increases in brilliancy up to the very place occupied by the sun. So that here, again, we have evidence accounting satisfactorily for the existence of a glory of light round the sun during total eclipses.
We venture to predict that if the corps of observers now being formed under the auspices of the Royal Astronomical Society should be successful in finding new evidence respecting the corona, that evidence will not be found opposed to the views we have expressed above.
It should be widely known, let us remark, in conclusion, that the means of transit to and from the Mediterranean stations suitable for observing the eclipse will, in all probability, be placed by Govern- ment at the disposal of an observing party of about sixty persons. The Royal Astronomical Society invite all who take interest in the eclipse, and are willing and able to take part in observing it, to send in their names to the secretaries before the next meeting of the Society (May 13th). The invitation is not by any means
restricted to Fellows of the Society ; indeed, it is almost certain that the required number of observers cannot be made up from the list of Fellows.
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