Curiosities of the Sky - Garrett P. Serviss (best book clubs txt) 📗

two very different parts — first, the red “prominences,” which resemble tongues of flame ascending thousands of miles above the sun’s surface; and, second, the “corona,” which extends to distances of millions of miles from the sun, and shines with a soft, glowing light. The two combined, when well seen, make a spectacle without parallel among the marvels of the sky. Although many attempts have been made to render the corona visible when there is no eclipse, all have failed, and it is to the moon alone that we owe its revelation. To cover the sun’s disk with a circular screen will not answer the purpose because of the illumination of the air all about the observer. When the moon hides the sun, on the other hand, the sunlight is withdrawn from a great cylinder of air extending to the top of the atmosphere and spreading many miles around the observer.

There is then no glare to interfere with the spectacle, and the corona appears in all its surprising beauty. The prominences, however, although they were discovered during an eclipse, can now, with the aid of the spectroscope, be seen at any time. But the prominences are rarely large enough to be noticed by the naked eye, while the streamers of the corona, stretching far away in space, like ghostly banners blown out from the black circle of the obscuring moon, attract every eye, and to this weird apparition much of the fear inspired by eclipses has been due. But if the corona has been a cause of terror in the past it has become a source of growing knowledge in our time.

 

The story of the first scientific observation of the corona and the prominences is thrillingly interesting, and in fact dramatic. The observation was made during the eclipse of 1842, which fortunately was visible all over Central and Southern Europe so that scores of astronomers saw it. The interest centers in what happened at Pavia in Northern Italy, where the English astronomer Francis Baily had set up his telescope. The eclipse had begun and Bailey was busy at his telescope when, to quote his own words in the account which he wrote for the Memoirs of the Royal Astronomical Society: I was astounded by a tremendous burst of applause from the streets below, and at the same moment was electrified by the sight of one of the most brilliant and splendid phenomena that can well be imagined; for at that instant the dark body of the moon was suddenly surrounded with a corona, or kind of bright glory, similar in shape and magnitude to that which painters draw round the heads of saints…

 

Pavia contains many thousand inhabitants, the major part of whom were at this early hour walking about the streets and squares or looking out of windows in order to witness this long-talked-of phenomenon; and when the total obscuration took place, which was instantaneous, there was a universal shout from every observer which “made the welkin ring,” and for the moment withdrew my attention from the object with which I was immediately occupied. I had, indeed, expected the appearance of a luminous circle round the moon during the time of total obscurity; but I did not expect, from any of the accounts of preceding eclipses that I had read, to witness so magnificent an exhibition as that which took place…

 

Splendid and astonishing, however, as this remarkable phenomenon really was, and although it could not fail to call forth the admiration and applause of every beholder, yet I must confess that there was at the same time something in its singular and wonderful appearance that was appalling…

 

But the most remarkable circumstance attending the phenomenon was the appearance of three large protuberances apparently emanating from the circumference of the moon, but evidently forming a portion of the corona. They had the appearance of mountains of a prodigious elevation; their color was red tinged with lilac or purple; perhaps the color of the peach-blossom would more nearly represent it. They somewhat resembled the tops of the snowy Alpine mountains when colored by the rising or the setting sun. They resembled the Alpine mountains in another respect, inasmuch as their light was perfectly steady, and had none of that flickering or sparkling motion so visible in other parts of the corona…

 

The whole of these protuberances were visible even to the last moment of total obscuration, and when the first ray of light was admitted from the sun they vanished, with the corona, altogether, and daylight was instantly restored.

 

I have quoted nearly all of this remarkable description not alone for its intrinsic interest, but because it is the best depiction that can be found of the general phenomena of a total solar eclipse. Still, not every such eclipse offers an equally magnificent spectacle. The eclipses of 1900 and 1905, for instance, which were seen by the writer, the first in South Carolina and the second in Spain, fell far short of that described by Bailey in splendor and impressiveness. Of course, something must be allowed for the effect of surprise; Bailey had not expected to see what was so suddenly disclosed to him. But both in 1900 and 1905 the amount of scattered light in the sky was sufficient in itself to make the corona appear faint, and there were no very conspicuous prominences visible. Yet on both occasions there was manifest among the spectators that mingling of admiration and awe of which Bailey speaks. The South Carolinians gave a cheer and the ladies waved their handkerchiefs when the corona, ineffably delicate of form and texture, melted into sight and then in two minutes melted away again. The Spaniards, crowded on the citadel hill of Burgos, with their king and his royal retinue in their midst, broke out with a great clapping of hands as the awaited spectacle unfolded itself in the sky; and on both occasions, before the applause began, after an awed silence a low murmur ran through the crowds. At Burgos it is said many made the sign of the cross.

 

It was not long before Bailey’s idea that the prominences were a part of the corona was abandoned, and it was perceived that the two phenomena were to a great extent independent. At the eclipse of 1868, which the astronomers, aroused by the wonderful scene of 1842, and eager to test the powers of the newly invented spectroscope, flocked to India to witness, Janssen conceived the idea of employing the spectroscope to render the prominences visible when there was no eclipse. He succeeded the very next day, and these phenomena have been studied in that way ever since.

 

There are recognized two kinds of prominences — the “erruptive” and the “quiescent.” The latter, which are cloud-like in form, may be seen almost anywhere along the edge of the sun; but the former, which often shoot up as if hurled from mighty volcanoes, appear to be associated with sunspots, and appear only above the zones where spots abound. Either of them, when seen in projection against the brilliant solar disk, appears white, not red, as against a background of sky.

The quiescent prominences, whose elevation is often from forty thousand to sixty thousand miles, consist, as the spectroscope shows, mainly of hydrogen and helium. The latter, it will be remembered, is an element which was known to be in the sun many years before the discovery that it also exists in small quantities on the earth. A fact which may have a significance which we cannot at present see is that the emanation from radium gradually and spontaneously changes into helium, an alchemistical feat of nature that has opened many curious vistas to speculative thinkers. The eruptive prominences, which do not spread horizontally like the others, but ascend with marvelous velocity to elevations of half a million miles or more, are apparently composed largely of metallic vapors — i.e. metals which are usually solid on the earth, but which at solar temperatures are kept in a volatilized state. The velocity of their ascent occasionally amounts to three hundred or four hundred miles per second. It is known from mathematical considerations that the gravitation of the sun would not be able to bring back any body that started from its surface with a velocity exceeding three hundred and eighty-three miles per second; so it is evident that some of the matter hurled forth in eruptive prominences may escape from solar control and go speeding out into space, cooling and condensing into solid masses. There seems to be no reason why some of the projectiles from the sun might not reach the planets. Here, then, we have on a relatively small scale, explosions recalling those which it has been imagined may be the originating cause of some of the sudden phenomena of the stellar heavens.

 

Of the sunspots it is not our intention here specifically to speak, but they evidently have an intimate connection with eruptive prominences, as well as some relation, not yet fully understood, with the corona. Of the real cause of sunspots we know virtually nothing, but recent studies by Professor Hale and others have revealed a strange state of things in the clouds of metallic vapors floating above them and their surroundings. Evidences of a cyclonic tendency have been found, and Professor Hale has proved that sunspots are strong magnetic fields, and consist of columns of ionized vapors rotating in opposite directions in the two hemispheres. A fact which may have the greatest significance is that titanium and vanadium have been found both in sunspots and in the remarkable variable Mira Ceti, a star which every eleven months, or thereabout, flames up with great brilliancy and then sinks back to invisibility with the naked eye. It has been suggested that sunspots are indications of the beginning of a process in the sun which will be intensified until it falls into the state of such a star as Mira. Stars very far advanced in evolution, without showing variability, also exhibit similar spectra; so that there is much reason for regarding sunspots as emblems of advancing age.

 

The association of the corona with sunspots is less evident than that of the eruptive prominences; still such an association exists, for the form and extent of the corona vary with the sunspot period of which we shall presently speak. The constitution of the corona remains to be discovered. It is evidently in part gaseous, but it also probably contains matter in the form of dust and small meteors. It includes one substance altogether mysterious — “coronium.” There are reasons for thinking that this may be the lightest of all the elements, and Professor Young, its discoverer, said that it was “absolutely unique in nature; utterly distinct from any other known form of matter, terrestial, solar, or cosmical.” The enormous extent of the corona is one of its riddles. Since the development of the curious subject of the “pressure of light” it has been proposed to account for the sustentation of the corona by supposing that it is borne upon the billows of light continually poured out from the sun. Experiment has proved, what mathematical considerations had previously pointed out as probable, that the waves of light exert a pressure or driving force, which becomes evident in its effects if the body acted upon is sufficiently small. In that case the light pressure will prevail over the attraction of gravitation, and propel the attenuated matter away from the sun in the teeth of its attraction. The earth itself would be driven away if, instead of consisting of a solid globe of immense aggregate mass, it were a cloud of microscopic particles. The reason is that the pressure varies in proportion to the surface of the body acted upon, while the gravitational attraction is proportional to the volume, or the total amount of matter in the body.