THE SUN.* "Tn a copiousness," says Sir Robert Ball, "of
the light radiated from the sun always seems to me to be one of the most astonishing facts in nature." This sentence, which is
the first of that chapter devoted to the consideration of the light of the sun, strikes the keynote of Sir Robert's discourse,
and surely it contains a truth that can never weary us in the telling. It is impossible to compare it with any artificial light that we can produce ; such comparisons are made for us by industrious mathematicians, but the numbers used are too great for us to grasp their significance. A general statement, however indefinite, is more impressive, and therefore in one sense truer. An American astronomer has said that though the State of Pennsylvania could supply the United States with coal for centuries to come, such a mass of fuel would not furnish the heat of the sun for the one-thousandth part of a second. Some one has calculated what portion of that vast energy eventually reaches us, but we need not concern our- selves about the figures ; they quite justified the humourist in the use of the expression applied to sunshine, "regardless of expense." It is indeed a magnificently infinite waste, and even an astronomer who carefully explains a theory to account for the continuation of it, never ceases to feel astonishment at its manifestation.
The remarkable fact, however, in connection with the sun, is that we really know so little that is definite about it, though a great number of astronomers study nothing else. Human ingenuity has discovered the laws by which the great universe of stars is ordered, has penetrated almost in- conceivable distances, can even tell us the constituent elements of a star from the nature of its light, though the star itself is so distant that it may have been ex- tinguished thousands of years ago; yet we do not know the sun's distance from us to a thousandth part. Sir Robert Ball thinks the distance is 93,700,000 miles; it is quite possible, he says, that the seven may be in error by one or even two digits. Our first computation of the sun's dis- tance was obtained from the Transit of Venus, but this cele- brated and, according to astronomers, overrated method, possesses only a historical interest now; there are many errors belonging to it, and, moreover, it will not be available for more than a century,—not more than a score of human beings alive now will see the transit of 2004. Sir Robert hopes with some confidence that we shall have ascertained the figures correctly long before that time. Two very wonderful physical facts, the Velocity of Light and the Aberration of Light, give us more assistance than the time-honoured Transit. The mass of the sun we do not require to know with such accuracy, when obtained from observations of Mars; we should be content, says Sir Robert, if we could ascertain the mass of the sun within one-three-hundredth part of the whole. Curiously enough, an error in the mass as large as this will have little effect on the distance ; that is to say, if the sun's mass is altered by the fraction we have mentioned, in calcu- lating the sun's distance it would only alter it by a thousandth part. Sun-spots seem to be the most fascinating as well as the most elusive pursuit of the observing astronomer. The immense energy which they indicate, their apparent connec- tion with terrestrial magnetic storms, and their periodicity, fill the waking and sleeping hours of baffled observers. What violent forces they represent, Lockyer's and Jannssen's researches with the spectroscope showel us. Lockyer, in his 016omistry of the Sun, told us how the bending of a line in the spectrum to right or left meant an inconceivably rapid nprush or downrnsh of vast masses of flaming vapour or gas. Perhaps we should say that the prominences seen on the edge of the great sphere divide with the sun-spots the atten- tion of astronomers, for the bright disc that we see by no means includes the whole sun, gases in a state of extreme tenuity extend the actual dimensions of the sun, and every now and then some great disturbance takes place, on the brilliant margin of the sun as seen in the telescope, and a jet of shining vapour is thrown out. There is no sharp line between the globe and its atmosphere, as on our earth ; the sun is in a molten condition, and moreover, bodies exist there in intermediate states between water and steam, liquid in the centre of .the sun by reason of the immense pressure of gravity, and getting thinner and thinner the further they are from the The Story of the Sun. By Sir Robert S. Ball, LL.D., P.R.S., F.R.L.S. With 8 Coloured Plates Ana other Illustrations. London : Cassell and Co. centre. We can only make fantastic guesses at the tempera- ture of the sun. The small portion of that furnace heat that reaches us is sufficient to maintain the cycle of life ; and we cannot imagine -what the fiery furnace itself must be like.
Lord Kelvin believes that there is no real connection between the breaking out of sun-spots and magnetic storms, regarding the concurrence of sun-spot energy and magnetic storms as simply coincidence. He pointed out that it is almost incon- ceivable that the sun should be able as a magnet to produce such an alteration in the earth's magnetic force as the instru- ments for measuring the amount of magnetism present in our earth, sometimes record. The magnetism of the earth has been known to vary by as much as one-twentieth of the whole amount ; and Lord Kelvin, taking "a not very severe storm," showed that to produce such an effect the sun must have exerted a horse-power three hundred and sixty-four times as great as the entire solar radiation ! This reasoning of a great physicist is conclusive ; yet circumstantial evidence cannot be denied, and Sir Robert Ball is fain to suggest some force exterior to both the sun and the earth as a possible agent. The real use of sun-spots has been to determine the rotation of the sun on its axis ; and here we are landed in another perplexity, as the spots revolve in different periods, those nearer the sun's equator having a shorter revolution. This simply means that the sun does not revolve like a solid body, its equatorial regions travelling much faster than regions nearer the poles. Jupiter in this respect somewhat resembles the sun. Now, observations of " faculw," which are bright spots as opposed to the dark or ordinary "sun-spot," seem to point to a regular rotation, that of a solid shell, and of the same length as that indicated by spots 10° north or south of the equator. This unexpected result, pointing to a rigid shell, is not, however, conclusive. We would suggest that these observations are taken of markings on the sun's so-called surface, whereas the sun has no well-defined surface, and they may be at most various and constantly varying distances from his centre. After all, we are not all Laplaces, and if we were, it is quite possible for some physical law to have escaped us. The rigidity of mathematic laws has made man blasphemously conceited as to what is possible and what is not.
There is a beautiful effect called the Zodiacal Light, which many people must have noticed after sunset and before sun- rise in the spring and autumn, a broad beam of faint, pearly light, extending into the sky, sometimes as much as 30°. This is supposed to be part of the Solar Corona, that luminous envelope seen to surround the sun on the occasion of an eclipse, and which is quite distinct from the fiery flames and brilliant eruptions that are always upheaving the sun's surface. The Corona takes the form of diverging streamers at either pole of the sun, and immense equatorial streamers of light. Yet, remarkable as these streamers of the Corona about the region of the Equator are, they have not the extent of the Zodiacal Light, so that one is fain to disconnect them. One remarkable thing about the Corona is that the unaided eye has noted peculiarities in it which the resources of photography have failed to record.
Perhaps the most interesting and impressive study to a reflective mind is that of the sun as a star. The time is long past when we regarded him as unique. Yes indeed, for we must now think of him only as a second-rate star. All that brilliant nightly splendour that the eye is never tired of gazing at, comes from suns compared to which ours is insignificant. What of Sirius, the Dog-star, the most brilliant star of the heavens, a star of immemorial fame, as bright now as it was generations ago ? yet it recedes from us at the rate of twenty miles a second! Our sun is placed in the second class on the series of star-types arranged by Lockyer ; there are stars hotter than he, such as Sirius and Alpha Lyra:. To us there seems no apparent diminution of the sun's heat, yet some effect must be ascribed to the continuous expenditure of heat. A con- traction of its bulk to the extent of 220 ft. yearly, we are told, would be sufficient to maintain its heat, and such an altera- tion would not be detected by us. Yet the stage of brilliant incandescence must form but a small portion of the history of a star ; this reflection at least should help us to appre- ciate the contrast of human life to eternity. We have spoken of time, of what must have been an almost infinite period, which, indeed, is difficult to grasp. Distance is more easily understood, and some things which Sir Robert has to say about the distance of the stars from us, will assist us in comparing them to the sun. Of these the most strikixtg is Arcturus, and Dr. Elkin has put this star at such a distance from the solar system, that the orbit of the earth round the sun must seem from Arcturus as large as a penny-piece would, seen at a hundred miles. Arcturus, in other words, is perhaps a dozen times as far off from us as Procyon is, and Procyon, one of the nearest of bright stars, is a million times the distance of the sun from us. But the marvellous thing about Arcturus is its movement; it has, comparatively speaking, a very distinct "proper motion" across the sky, though not as large as some stars. Lately, however, the spectroscope has ascertained for us the pace of stars along the line of sight, and Arcturus travels, it is now believed, at the rate of three hundred and eighty miles a second. Saab speed as this is truly terrific, and we may well ask where this furious star is hurrying to. As Arcturus, ten generations hence, will not have moved to the eye by as much as the diameter of the moon, we shall have plenty of opportunity of discussing the question.
The motion of these stars leads us to the consideration of the chapter in which Sir Robert Ball tells us of the efforts made to determine the motion of the sun with regard to the stars and his connection with them. Is the sun a member of some brilliant constellation ? No; but he is one of the stars in the Milky Way. Is he also racing along towards some point in space P The spectroscope has shown to us that he and his system of planets are travelling towards the constellation Lyra, to a point near Vega, the most brilliant gem of the Northern Heavens, and not far from a star in Cygnus, the nearest as yet measured in our hemisphere to our earth. With this striking conclusion we take leave of a sound and philoso- phical treatise on the sun, written in a most interesting and popular manner, and illustrated no less by the clearness of the style than by the numerous cuts and diagrams.
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