THE SUN'S SURROUNDINGS

We have now reached the point beyond which mere telescopic power will not carry us, a point as definite for the largest instrument as for the smallest. We have traced what can be seen on the visible sun, but beyond the familiar disc, and invisible at ordinary seasons or with purely telescopic means, there lie several solar features of the utmost interest and beauty, the study of which very considerably modifies our conception of the structure of our system's ruler. These features are only revealed in all their glory and wonder during the fleeting moments in which a total eclipse is central to any particular portion of the earth's surface.

A solar eclipse is caused by the fact that the moon, in her revolution round the earth, comes at certain periods between us and the sun, and obscures the light of the latter body either partially or totally. Owing to the fact that the plane of the orbit in which the moon revolves round the earth does not coincide with that in which the earth revolves round the sun, the eclipse is generally only partial, the moon not occupying the exact line between the centres of the sun and the earth. The dark body of the moon then appears to cut off a certain portion, larger or smaller, of the sun's light; but none of the extraordinary phenomena to be presently described are witnessed. Even during a partial eclipse, however, the observer may find considerable interest in watching the outline of the dark moon, as projected upon the bright background of the sun. It is frequently jagged or serrated, the projections indicating the existence, on the margin of the lunar globe, of lofty mountain ranges.

Occasionally the conditions are such that the moon comes centrally between the earth and the sun (Fig. 19), and then an eclipse occurs which may be either total or annular. The proportion between the respective distances from us of the sun and the moon is such that these two bodies, so vastly different in real bulk, are sensibly the same in apparent diameter, so that a very slight modification of the moon's distance is sufficient to reduce her diameter below that of the sun. The lunar orbit is not quite circular, but has a small eccentricity. It may therefore happen that an eclipse occurs when the moon is nearest the earth, at which point she will cover the sun's disc with a little to spare; or the eclipse may occur when she is furthest away from the earth, in which case the lunar diameter will appear less than that of the sun, and the eclipse will be only an annular one, and a bright ring or 'annulus' of sunlight will be seen surrounding the dark body of the moon at the time when the eclipse is central.

All conditions being favourable, however—that is to say, the eclipse being central, and the moon at such a position in her orbit as to present a diameter equal to, or slightly greater than, that of the sun—a picture of extraordinary beauty and wonder reveals itself the moment that totality has been established. The centre of the view is the black disc of the moon. From behind it on every side there streams out a wonderful halo of silvery light which in some of its furthest streamers may sometimes extend to a distance of several million miles. In the Indian Eclipse of 1898, for example, one streamer was photographed by Mrs. Maunder, which extended to nearly six diameters from the limb of the eclipsed sun (Plate VIII.). The structure of this silvery halo is of the most remarkable complexity, and appears to be subject to continual variations, which have already been ascertained to be to some extent periodical and in sympathy with the sun-spot period. At its inner margin this halo rests upon a ring of crimson fire which extends completely round the sun, and throws up here and there great jets or waves, which frequently assume the most fantastic forms and rise to heights varying from 20,000 to 100,000 miles, or in extreme instances to a still greater height. To these appearances astronomers have given the names of the Corona, the Chromosphere, and the Prominences. The halo of silvery light is the Corona, the ring of crimson fire the Chromosphere, and the jets or waves are the Prominences.

The Corona is perhaps the most mysterious of all the sun's surroundings. As yet its nature remains undetermined, though the observations which have been made at every eclipse since attention was first directed to it have been gradually suggesting and strengthening the idea that there exists a very close analogy between the coronal streamers and the Aurora or the tails of comets. The extreme rarity of its substance is conclusively proved by the fact that such insubstantial things as comets pass through it apparently unresisted and undelayed. Its structure presents variations in different latitudes. Near the poles it exhibits the appearance of brushes of light, the rays shooting out from the sun towards each summit of his axis, while the equatorial rays curve over, presenting a sort of fish-tail appearance. These variations are modified, as already mentioned, by some cause which is at all events coincident with the sun-spot period. At minimum the corona presents itself with polar brushes of light and fish-tail equatorial rays, the latter being sometimes of the most extraordinary length, as in the case of the eclipse of July 29, 1878, when a pair of these wonderful streamers extended east and west of the eclipsed sun to a distance of at least 10,000,000 miles.

When an eclipse occurs at a spot-maximum, the distribution of the coronal features is found to have entirely changed. Instead of being sharply divided into polar brushes and equatorial wings, the streamers are distributed fairly evenly around the whole solar margin, in a manner suggesting the rays from a star, or a compass-card ornament. The existence of this periodic change has been repeatedly confirmed, and there can be no doubt that the corona reflects in its structure the system of variation which prevails upon the sun. 'The form of the corona,' says M. Deslandres, 'undergoes periodical variations, which follow the simultaneous periodical variations already ascertained for spots, faculÆ, prominences, and terrestrial magnetism.' Certainty as to its composition has not yet been attained; nor is this to be wondered at, for the corona is only to be seen in the all too brief moments during which a total eclipse is central, and then only over narrow tracts of country, and all attempts to secure photographs of it at other times have hitherto failed. When examined with the spectroscope, it yields evidence that its light is derived from three sources—from the incandescence of solid or liquid particles, from reflected sunshine, and from gaseous emissions. The characteristic feature of the coronal spectrum is a bright green line belonging to an unknown element which has been named 'coronium.'

The Chromosphere and the Prominences, unlike the elusive corona, may now be studied continuously by means of the spectroscope, and instruments are now made at a comparatively moderate price, which, in conjunction with a small telescope—3 inches will suffice—will enable the observer to secure most interesting and instructive views of both. The chromosphere is, to use Miss Clerke's expression, 'a solar envelope, but not a solar atmosphere.' It surrounds the whole globe of the sun to a depth of probably from 3,000 to 4,000 miles, and has been compared to an ocean of fire, but seems rather to present the appearance of a close bristling covering of flames which rise above the surface of the visible sun like the blades of grass upon a lawn. Any one of these innumerable flames may be elevated into unusual proportions in obedience to the vast and mysterious forces which are at work beneath, and then becomes a prominence. On the whole the constitution of the chromosphere is the same as that of the prominences. Professor Young has found that its normal constituents are hydrogen, helium, coronium, and calcium. But whenever there is any disturbance of its surface, the lines which indicate the presence of these substances are at once reinforced by numbers of metallic lines, indicating the presence of iron, sodium, magnesium, and other substances.

The scale to which these upheavals attain in the prominences is very remarkable. For example, Young records the observation of a prominence on October 7, 1880. When first seen, at about 10.30 a.m., it was about 40,000 miles in height and attracted no special attention. Half an hour later it had doubled its height. During the next hour it continued to soar upwards until it reached the enormous altitude of 350,000 miles, and then broke into filaments which gradually faded away, until by 12.30 there was nothing left of it. On another occasion he recorded one which darted upwards in half an hour from a moderate elevation to a height of 200,000 miles, and in which clouds of hydrogen must have been hurled aloft with a speed of at least 200 miles per second. (Plate IX. gives a representation of the chromosphere and prominences from a photograph by M. Deslandres.) Between the chromosphere and the actual glowing surface of the sun which we see lies what is known as the 'reversing layer,' from the fact that owing to its presence the dark lines of the solar spectrum are reversed in the most beautiful way during the second at the beginning and end of totality in an eclipse. Young, who was the first to observe this phenomenon (December 22, 1870), remarks of it that as soon as the sun has been hidden by the advancing moon, 'through the whole length of the spectrum, in the red, the green, the violet, the bright lines flash out by hundreds and thousands, almost startlingly; as suddenly as stars from a bursting rocket-head, and as evanescent, for the whole thing is over within two or three seconds.'

The Chromosphere and Prominences, April 11, 1894. Photographed by M. H. Deslandres.

The spectrum of the reversing layer has since been photographed on several occasions—first by Shackleton, at Novaya Zemlya, on August 9, 1896—and its bright lines have been found to be true reversals of the dark lines of the normal solar spectrum. This layer may be described as a thin mantle, perhaps 500 miles deep, of glowing metallic vapours, surrounding the whole body of the sun, and normally, strange to say, in a state of profound quiescence. Its presence was of course an integral part of Kirchhoff's theory of the mode in which the dark lines of the solar spectrum were produced. Such a covering was necessary to stop the rays whose absence makes the dark lines; and it was assumed that the rays so stopped would be seen bright, if only the splendour of the solar light could be cut off. These assumptions have therefore been verified in the most satisfactory manner.

Thus, then, the structure of the sun as now known is very different from the conception of it which would be given by mere naked-eye, or even telescopic, observation. We have first the visible bright surface, or photosphere, with its spots, faculÆ, and mottling, and surrounded by a kind of atmosphere which absorbs much of its light, as is evidenced by the fact that the solar limb is much darker than the centre of the disc (Plate V.); next the reversing layer, consisting of an envelope of incandescent vapours, which by their absorption of the solar rays corresponding to themselves give rise to the dark lines in the spectrum. Beyond these again lies the chromosphere, rising into gigantic eruptive or cloud-like forms in the prominences; and yet further out the strange enigmatic corona.

It must be confessed that the reversing layer, the chromosphere, and the corona lie somewhat beyond the bounds and purpose of this volume; but without mention of them any account of the sun is hopelessly incomplete, and it is not at all improbable that a few years may see the spectroscope so brought within the reach of ordinary observers as to enable them in great measure to realize for themselves the facts connected with the complex structure of the sun. In any case, the mere recital of these facts is fitted to convey to the mind a sense of the utter inadequacy of our ordinary conceptions of that great body which governs the motions of our earth, and supplies to it and to the other planets of our system life and heat, light and guidance. With the unaided eye we view the sun as a small tranquil white disc; the telescope reveals to us that it is a vast globe convulsed by storms which involve the upheaval or submersion, within a few hours, of areas far greater than our own world; the spectroscope or the total eclipse adds to this revelation the further conception of a sweltering ocean of flame surrounding the whole solar surface, and rising in great jets of fire which would dissolve our whole earth as a drop of wax is melted in the flame of a candle; while beyond that again the mysterious corona stretches through unknown millions of miles its streamers of silvery light—the great enigma of solar physics. Other bodies in the universe present us with pictures of beautiful symmetry and vast size: some even within our own system suggest by their appearance the presence within their frame of tremendous forces which are still actively moulding them; but the sun gives us the most stupendous demonstration of living force that the mind of man can apprehend. Of course there are many stars which are known to be suns on which processes similar to those we have been considering are being carried on on a yet vaster scale; but the nearness of our sun brings the tremendous energy of these processes home to us in a way that impresses the mind with a sense almost of fear.

'Is it possible,' says Professor Newcomb, 'to convey to the mind any adequate conception of the scale on which natural operations are here carried on? If we call the chromosphere an ocean of fire, we must remember that it is an ocean hotter than the fiercest furnace, and as deep as the Atlantic is broad. If we call its movements hurricanes, we must remember that our hurricanes blow only about 100 miles an hour, while those of the chromosphere blow as far in a single second. They are such hurricanes as, coming down upon us from the north, would, in thirty seconds after they had crossed the St. Lawrence, be in the Gulf of Mexico, carrying with them the whole surface of the continent in a mass not simply of ruin, but of glowing vapour.... When we speak of eruptions, we call to mind Vesuvius burying the surrounding cities in lava; but the solar eruptions, thrown 50,000 miles high, would engulf the whole earth, and dissolve every organized being on its surface in a moment. When the mediÆval poets sang, "Dies irÆ, dies illa, solvet sÆclum in favilla," they gave rein to their wildest imagination without reaching any conception of the magnitude or fierceness of the flames around the sun.'

The subject of the maintenance of the sun's light and heat is one that scarcely falls within our scope, and only a few words can be devoted to it. It is practically impossible for us to attain to any adequate conception of the enormous amount of both which is continually being radiated into space. Our own earth intercepts less than the two thousand millionth part of the solar energy. It has been estimated that if a column of ice 2¼ miles in diameter could be erected to span the huge interval of 92,700,000 miles between the earth and the sun, and if the sun could concentrate the whole of his heat upon it, this gigantic pillar of ice would be dissolved in a single second; in seven more it would be vaporized. The amount of heat developed on each square foot of solar surface is 'equivalent to the continuous evolution of about 10,000 horse-power'; or, as otherwise stated, is equal to that which would be produced by the hourly burning of nine-tenths of a ton of anthracite coal on the same area of 1 square foot.

It is evident, therefore, that mere burning cannot be the source of supply. Lord Kelvin has shown that the sun, if composed of solid coal, would burn itself out in about 6,000 years.

Another source of heat may be sought in the downfall of meteoric bodies upon the solar surface; and it has been calculated that the inrush of all the planets of our system would suffice to maintain the present energy for 45,604 years. But to suppose the existence near the sun of anything like the amount of meteoric matter necessary to account, on this theory, for the annual emission of heat involves consequences which are quite at variance with observed facts, though it is possible, or even practically certain, that a small proportion of the solar energy is derived from this source.

We are therefore driven back upon the source afforded by the slow contraction of the sun. If this contraction happens, as it must, an enormous amount of heat must be developed by the process, so much so that Helmholtz has shown that an annual contraction of 250 feet would account for the total present emission. This contraction is so slow that about 9,500 years would need to elapse before it became measurable with anything like certainty. In the meantime, then, we may assume as a working hypothesis that the light and heat of the central body of our system are maintained, speaking generally, by his steady contraction. Of course this process cannot have gone on, and cannot go on, indefinitely; but as the best authorities have hitherto regarded the date when the sun shall have shrunk so far as to be no longer able to support life on the earth as distant from us by some ten million years, and as the latest investigations on the subject, those of Dr. See, point in the direction of a very large extension of this limit, we may have reasonable comfort in the conviction that the sun will last our time.