Читать книгу Star-land: Being Talks With Young People About the Wonders of the Heavens - Robert S. Ball - Страница 12

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For a great deal of our knowledge about the sun we are indebted to the moon. It will sometimes happen that the moon comes in between us and the sun, and produces an eclipse. At first you might think that an eclipse would only have the effect of preventing us from seeing anything of the sun, but it really reveals most beautiful and interesting objects, of whose existence we should otherwise be ignorant. The great luminary has curious appendages which are quite hidden under ordinary circumstances. In the full glare of day the dazzling splendor of the sun obliterates and renders invisible these appendages, which only shine with comparatively feeble light. It fortunately happens that the moon is just large enough to intercept the whole of the direct light from the sun, or rather, I should say, from the central parts of the sun. Surrounding that central and more familiar part from which the brilliancy is chiefly derived is a remarkable fringe of delicate and beautiful objects which are self-luminous no doubt, but with a light so feeble that when presented to us amid the full blaze of sunlight they are invisible. When, however, the moon so kindly stops all the stronger beams, then these faint objects spring into visibility, and we have the exquisite spectacle of a total eclipse. The objects that I desire to mention particularly are the corona and the prominences.

Fig.18.—Total Eclipse of the Sun, May 6, 1883 (drawn by Trouvelot).

A pretty picture of the total eclipse of the sun which occurred on May 6, 1883, is here shown (Fig.18). It is taken from a drawing made by M. Trouvelot, who was sent out with a French observing party. They went a very long way to see an eclipse, but what they saw recompensed them for all their trouble. The track along which the phenomenon could be best seen lay in the Pacific Ocean, and a place had to be selected which was so situated that the sun should be high in the heavens at the important moment, and also that the duration while the total eclipse lasted should be as long as possible. They accordingly went to Caroline Island, and all this journey to the other side of the earth was taken to witness a phenomenon that only lasted five minutes and twenty-three seconds. Short though these precious minutes were, they were long enough to enable good work to be done. Careful preparations had been made so that not a moment should be thrown away. Each member of the party had his special duty allotted to him, and this had been rehearsed so carefully beforehand that when the long-expected moment of “totality” arrived there was neither haste nor confusion; every one carefully went through his part of the programme. M. Trouvelot, for instance, occupied himself for two minutes and a few seconds in making the sketch that we now show. No doubt an accomplished astronomical artist like M. Trouvelot would gladly have taken longer time for his sketch of so unique a sight, but brevity was imperative. He had already had experience of similar eclipses, so that he was prepared at once to note what ought to be noted, and the picture we have shown is the result. This was completed within less than half of the duration of totality, and the artist had still three minutes left to devote to another and quite different part of the work, which does not concern us at present.

Fig.19.—The Corona of the Sun, 1882 (by Schuster).

I want you particularly to look at these long branches or projections which we see surrounding the sun when totally eclipsed. They shine with a pearly light, and, in fact, it is stated that even during the gloomiest portion of the time there was still as much illumination as on a bright moonlight night. All that light came from this glorious halo round the sun which astronomers call the “corona.” We do not under ordinary circumstances obtain even the slightest glimpse of this object. Even during a partial eclipse of the sun it is not visible, but directly the moon quite covers the sun, so as to cut off all the direct light, then the corona springs into visibility. It is always there, no doubt, though we cannot see it.

One of the most interesting photographs of the eclipsed sun which has ever been taken was that by Professor Schuster in 1882 (Fig.19). The corona is well shown, and also a comet.

The other appendages to the sun which can be seen during an eclipse are the objects which we call “prominences.” They are of a ruddy color, and seem to be great flames, which leap upwards from the glowing surface of the sun below. Though the existence of the prominences was first discovered by their presence during eclipses, it fortunately happens that we are no longer wholly dependent on eclipses for the purpose of making our observations of these remarkable objects. It is true that we may look at the sun with even the biggest and most powerful telescope in the world, and still not be able to perceive anything of the prominences. We require the aid of a special appliance called the spectroscope to render them visible. But I am not now going to describe this ingenious contrivance. I am only going to speak of the results which have been obtained by its means. We shall here again avail ourselves of the experience of M. Trouvelot for a picture of two of these wonderful appendages.

Fig.20.—Solar Prominences (drawn by Trouvelot).

The view (Fig.20) shows the ordinary aspect of the sun diversified with groups of dark spots. The fringe around the margin of the globe is of some ruddy material, forming the base of the flames which rise from the glowing surface. No doubt these flames are also often present on the face of the sun, but we cannot see them against the brilliant background. They are only perceptible when shown against the sky behind. At two points of this ruddy fringe, which happen curiously enough to be nearly opposite to each other, two colossal flames have burst forth. They extend to a vast distance, which is quite one-third of the width of the sun. The vigor of these outbreaks may be estimated by the remarkable changes which are incessantly going on. These great flames may indeed be said to flicker; only, considering their size, we must allow them a little more time than is demanded for the movements of flames of ordinary dimensions. The great flame on the left was obviously declining in brilliancy when first seen. In a quarter of an hour it had broken up into fragments, some of which were still to be seen floating in the sun’s atmosphere. In ten minutes more the light of this flame had almost entirely vanished. Surely these are changes of extraordinary rapidity when we remember the size of this prominence. It was nearly 300,000 miles in height—that is to say, about thirty-seven times the width of our earth.

Great as are these prominences, others have been recorded which are even larger. One of them has been seen to rush up with a speed of 200,000 miles an hour—that is, with more than two hundred times the pace of the swiftest of rifle-bullets.