CHAPTER XII.

MORE ABOUT THE SUN.

Not among the least of the wondrous things of creation are the tremendous disturbances taking place upon the surface of the sun--that raging, roaring sea of flame.

A good many explanations have been from time to time offered as to the dark spots seen to move across the face of the sun. Some one or more of these explanations may be true; but we know little about the matter. A sun-spot does not commonly consist of merely one black patch. There is the dark center, called the _umbra_--plural, _umbræ_. There is the grayish part surrounding the umbra, called the _penumbra_. Also, in the center of the umbra there is sometimes observable an intensely black spot, called the _nucleus_. Sometimes a spot is made up of nucleus and umbra alone, without any penumbra. Sometimes it is made of penumbra alone, without any umbra. Sometimes in one spot there are several umbræ, with the gray penumbra round the whole, and gray bridges dividing the umbræ.

The enormous size of these spots has been already described in an earlier chapter. Fifty thousand to one hundred thousand miles across is nothing unusual. In the year 1839 a spot was seen which measured no less than one hundred and eighty-six thousand miles in diameter.

One explanation proposed was, that the sun might be a cool body, covered over with different envelopes or dense layers of cloudy form, one above another. The inside envelope--or, as some say, the inside atmosphere--would then be thick and dull-colored, protecting the solid globe within and reflecting light, but having none of its own. The next envelope would be one mass of raging, burning gases--the _photosphere_, in fact. The outer envelope would be a transparent, surrounding atmosphere, lighted up by the sea of fire within. A sun-spot would then consist of the tearing open of one or more of these envelopes, so as to give glimpses of the gray, inner atmosphere, or even of the dark, cool globe at the center.

There may be some truth in this explanation; but the notion of a cool and dark body within is now pretty well given up. The apparent blackness of a spot-nucleus does not prove actual blackness or absence of heat. A piece of white-hot iron, held up against the sun, looks black; and it may be merely the contrast of the glowing photosphere which makes the nucleus seem so dark. It is even believed that the blackest parts may be the most intensely hot of all.

Another proposed explanation was of dark clouds floating in the sun’s atmosphere. There, again, are found difficulties, particularly in the fact that, as the spots move across the sun, the changes which regularly take place in their appearance make it pretty clear that their shape is not flat, but hollow and cave-like. The changes here spoken of are seeming changes of shape, caused by change of position. There are also real changes constantly taking place. Although the spots often keep their general outlines long enough to be watched across the face of the sun, and even to be known again after spending nearly a fortnight hidden on the other side, still they are far from being fixed in form.

[Illustration: A TYPICAL SUN-SPOT.]

The alterations are at times not only very great, but very rapid. Sometimes in a single hour of watching, an astronomer can see marked movement going on--as you or I might in an hour observe movements slowly taking place in a high layer of clouds. For movement to show at all in one hour, at so immense a distance, proves that the actual rate of motion must be very great.

The first great fact which was got from the study of these spots was this, that this great sun is very much like our own earth, in so far as it rotates on an axis in exactly the same way that our earth does. Not only do the spots change their position on the face of the sun, in consequence of the sun’s rotation on its axis, but they change very much from day to day, and even from hour to hour; so that we have evidence not only that the sun is rotating like our earth, but that the atmosphere of the sun is subjected to most tremendous storms--storms so tremendous, in fact, that the fiercest cyclones on our own earth are not for one moment to be compared with them. The fact that the spots do really move with the sun, and are really indentations, saucer-like hollows, in the photosphere, is shown by the appearance, which is always presented by a spot when it is near the edge of the sun. You know if you take a dinner-plate, and look it full in the face, it is round; but if you look at it edgeways, it is not round. Take two different views of the same spot: In one, you look the sun-spot straight in the face, and you see into it and can learn all about it; but in the other, when it has nearly gone round the corner, and is disappearing on the sun’s edge, you see it in the same way that you would see a plate looked at edgeways.

These sun-cyclones must indeed be of terrific force and extent, compared with anything we see on earth. It was calculated that the speed of movement perceived in one spot was about three hundred and sixty-three miles each second.

And still all this is nothing, or almost nothing, in comparison with the real power of the sun! The liquid state of the ocean; the gaseous state of the atmosphere; the currents of the sea; the raising of the clouds, the rains, storms, streams, rivers; the calorific value of all the forests of the globe and all the coal-mines of the earth; the motion of all living beings; the heat of all humanity; the stored-up power in all human muscles, in all the manufactories, in all the guns,--all that is almost nothing compared with that of which the sun is capable. Do we think that we have measured the solar power by enumerating the effects which it produces on the earth? Error! profound, tremendous, foolish error! This would be to believe still that this star has been created on purpose to illuminate terrestrial humanity. In reality, what an infinitesimal fraction of the sun’s total radiation the earth receives and utilizes! In order to appreciate it, let us consider the distance of ninety-three millions of miles which separates us from the central star, and at this distance let us see what effect our little globe produces, what heat it intercepts. Let us imagine an immense sphere _traced at this distance from the sun_, and entirely surrounding it. Well, on this gigantic sphere, the spot intercepted by our little earth is only equivalent to the fraction 1/2,138,000,000; that is to say, that the dazzling solar hearth radiates all round it through immensity a quantity of light and heat two thousand one hundred and thirty-eight million times more than that which we receive, and of which we have just now estimated the stupendous effects. The earth only stops in its passage the _two thousand millionth part of the total radiation_.

Sometimes the storms or outbursts come in the shape of a bright spot instead of a dark one.

[Illustration: A SOLAR ERUPTION.]

Two astronomers were one day watching the sun from two different observatories, when they saw such an event take place. An intense and dazzling spot of light burst out upon the surface of the sun--so intense, so dazzling, as to stand quite apart from the radiant photosphere. To one astronomer it looked like a single spot, while the other saw two spots close together. In about a minute the light grew more dim, and in five minutes all was over. But in those five minutes the spot or spots had traveled a distance of thirty-five thousand miles.

It was a very remarkable thing that the _magnets_ on earth--those delicate little needles which point so steadily yet perseveringly towards the North Pole--seemed to be strongly agitated by the distant solar outburst. This brings us to another interesting fact.

The spots on the sun are not always the same in number. Sometimes they are many, sometimes they are few. Long and close watching has made it clear that they pass through a regular _order_ of changes: some years of many spots being followed by other years of less and less spots; then some years of very few spots being followed by other years of more and more spots,--decrease and increase being seemingly regular and alternate.

This turn or _cycle_ of changes--from more to less, and then from less to more again--is found to run its course about once in every eleven years, with some variations.

Now, it has long been known that the magnetic-needle goes through curious variations. Though we speak of it as pointing always north, yet it does not always so point exactly. Every day the needle is found to make certain tiny, delicate motions, as if faintly struggling to follow the daily movements of the sun--just a little towards the east, or just a little towards the west. These tiny motions, having been long watched and measured, were found to go through a regular course of changes--some years more, and some years less, waxing and waning by turns. It was discovered that the course of changes from more to less, and from less to more again, took place in about eleven years.

These two things, you see, were quite independent of one another. Those who watched the sun-spots were not thinking of the magnets, and those who watched the magnets were not thinking of the sun-spots. But somebody did at last happen to think of both together. He was laughed at, yet he took the trouble carefully to compare the two. And, strange to say, he found that these two periods in the main agreed--the eleven years of alternate changes in the number of sun-spots, and the eleven years of alternate changes in the movements of the magnetic-needle. When the spots are most, the needle moves most. When the spots are least, the needle moves least. So much we know. But to explain the why and the wherefore is beyond our power.

This study of the magnetism of our wandering planet is very interesting, and one which is still very little known. Here is a weak needle, a slip of magnetic iron, which, with its restless and agitated finger, incessantly seeks a region near the north. Carry this needle in a balloon up to the higher aerial regions, where human life begins to be extinguished; shut it up in a tomb closely separated from the light of day; take it down into the pit of a mine, to more than a thousand yards in depth,--and incessantly, day and night, without fatigue and without rest, it watches, trembles, throbs, seeks the point which attracts it across the sky, through the earth, and through the night. Now--and here is a coincidence truly filled with notes of interrogation--the years when the oscillation of this innocent little steel wire is strongest are the years when there are more spots, more eruptions, more tempests in the sun; and the years when its daily fluctuations are weakest are those when we see in the day-star neither spots, eruptions, nor storms. Does there exist, then, a magnetic bond between the immense solar globe and our wandering abode? Is the sun magnetic? But the magnetic currents disappear at the temperature of red-hot iron, and the incandescent focus of light is at a temperature incomparably higher still. Is it an electrical influx which is transmitted from the sun to the earth across a space of ninety-three millions of miles? On September 1, 1859, two astronomers--Carrington and Hodgson--were observing the sun, independently of each other; the first on a screen which received the image; the second directly through a telescope,--when, in a moment, a dazzling flash blazed out in the midst of a group of spots. This light sparkled for five minutes above the spots without modifying their form, as if it were completely independent, and yet it must have been the effect of a terrible conflagration occurring in the solar atmosphere. Each observer ascertained the fact separately, and was for an instant dazzled. Now, here is a surprising coincidence: at the very moment when the sun appeared inflamed in this region the magnetic instruments of the Kew Observatory, near London, where they were observing, manifested a strange agitation; the magnetic needle jumped for more than an hour as if infatuated. Moreover, a part of the world was on that day and the following one enveloped in the fires of an aurora borealis, in Europe as well as in America. It was seen almost everywhere,--at Rome, at Calcutta, in Cuba, in Australia, and in South America. Violent magnetic perturbations were manifested, and at several points the telegraph-lines ceased to act. Why should these two curious events not be associated with each other? A similar coincidence was observed on August 3, 1872, by Professor Charles A. Young, of Princeton, N. J.: a paroxysm in the solar chromosphere, magnetic disturbances everywhere.

There is a very singular appearance seen upon the sun which must not be passed over without mention. Some astronomers speak of the whole surface as being _mottled_ all over with a curious rough look when examined through a powerful telescope. This “mottling” is described by various observers in various ways. One speaks of “luminous spots shaped like rice-grains.” Another, of “luminous spots resembling strokes made with a camel’s-hair pencil.” Another, of “luminous objects or granules.” Others, of “multitudes of leaves,” “nodules,” “crystalline shapes,” “leaves or scales, crossing one another in all directions, like what are called spills in the game of spillikens.” They have also been pictured as “certain luminous objects of an exceedingly definite shape and general uniformity of size, whose form is that of the oblong leaves of a willow-tree.” These cover the whole disk of the sun, excepting the space occupied by the spots, in countless millions, and lie crossing each other in every imaginable direction.

In size they are said to be about one thousand miles long, by two or three hundred broad, but they vary a good deal. Where there is a spot, the willow-leaves at its edge are said to point pretty regularly towards the center. Whether they are, as they seem to be, solid in form; whether they are, as some suppose, the chief source of the sun’s light and heat; whether they lie on his surface or float in his atmosphere; what is their real nature, and what is their real use,--about these questions we are at present quite in the dark.

We have next to think a little more about the edge or limb of the sun, and the stormy flames and outburst there seen. Until quite lately the only time for observing such appearances was during a total eclipse of the sun. Lately, by means of the new instrument called the “spectroscope,” it has been found possible to take observations when no eclipse is going on.

[Illustration: SUN-FLAMES, MAY 3, 1892.]

A few words of explanation as to eclipses of the sun seem needful, before going further. An eclipse of the sun is caused simply by the round body of the moon passing exactly between the sun and the earth, so as to hide the sun from us.

Let there be a candle on the table, while you stand near. The rays of light from the candle fall upon your face. Now move slowly, to and fro, a round ball between you and the candle. So long as it is not precisely in the line between--so long as it is a little higher, or a little lower, or a little to one side--then you can still see the flame. Once you let the ball come just between the light and your eyes, and you see it no more. In other words the candle-flame is eclipsed--hidden, veiled, cut off--by the ball.

It may seem curious at first sight that the moon, which is very small compared with the sun, should have power to cover the sun. But remember the difference of the distance. The sun is very far, and the moon is very near. Any small object very near will easily hide from your sight a large object at a considerable distance. You may hold up a shilling-piece at arm’s length, and make it cover from sight a man, or even a house, if the latter be far enough away. The sun at a distance of ninety-three millions of miles, and the moon at a distance of two hundred and forty thousand miles, have to our vision the same seeming size. So, when the moon glides between, her round face just about covers the sun’s round face.

If the moon were traveling exactly in the same plane as the plane of the earth’s orbit, an eclipse would be a very common affair indeed. But the plane of the moon’s orbit being not quite the same as the plane of the earth’s orbit, she passes sometimes a little above, and sometimes a little below, the exact spot where she would hide the sun’s rays from us. Now and then, at certain intervals, she goes just between. And so well is the moon’s path in the heavens understood that astronomers can tell us, long years beforehand, the day and the hour an eclipse will take place.

An eclipse of the sun is sometimes partial, sometimes total, sometimes annular. In a partial eclipse, the moon does indeed pass between, but only so as to hide from us _part_ of the sun. She is a little too low, or a little too high, to cover his face. In a total eclipse, the moon covers the sun completely, so that for a few minutes the bright photosphere seems blotted out from the heavens, a black round body, surrounded by light, taking its place. In an annular eclipse, the moon in like manner crosses the sun, but does not succeed in covering him entirely, a rim of bright photosphere showing round the black moon. For in an annular eclipse, the moon, being a little farther away from the earth than at the time of a total eclipse, has too small a disk quite to hide the sun’s disk.

The blackness of the moon during an eclipse is caused by the fact that her bright side is turned towards the sun, and her dark side toward us. An eclipse of the sun can take place only at new moon, never at full moon. At her full, the moon is outside the earth’s orbit, away from the sun, and can not by any possibility pass between.

Eclipses, like comets, have always been interpreted as the indication of inevitable calamities. Human vanity sees the finger of God making signs to us on the least pretext, as if we were the end and aim of universal creation. Let us mention, for example, what passed even in France, with reference to the announcement of an eclipse of the sun, on August 21, 1560. For one, it presaged a great overthrow of States and the ruin of Rome; for another, it implied another universal deluge; for a third, nothing less would result than a conflagration of the globe; finally, for the less excited, it would infect the air. The belief in these terrible effects was so general that, by the express order of the doctors, a multitude of frightened people shut themselves up in very close cellars, well heated and perfumed, in order to shelter themselves from these evil influences. Petit relates that the decisive moment approached, that the consternation was at its height, and that a parish priest of the country, being no longer able to confess his parishioners, who believed their last hour had come, was obliged to tell them in a sermon “not to be so much hurried, seeing that, on account of the wealth of the penitents, the eclipse had been postponed for a fortnight.” These good parishioners found no more difficulty in believing in the postponement of the eclipse than they had in believing in its unlucky influence.

History relates a crowd of memorable acts on which eclipses have had the greatest influence. Alexander, before the battle of Arbela, expected to see his army routed by the appearance of a phenomenon of this kind. The death of the Athenian general Nicias and the ruin of his army in Sicily, with which the decline of the Athenians commenced, had for their cause an eclipse of the moon. We know how Christopher Columbus, with his little army, threatened with death by famine at Jamaica, found means of procuring provisions from the natives by depriving them in the evening of the light of the moon. The eclipse had scarcely commenced when they supplied him with food. This was the eclipse of March 1, 1504. We need not relate other facts of this nature, in which history abounds, and which are known to every one.

Eclipses no longer cause terror to any one, since we know that they are a natural and inevitable consequence of the combined motions of the three great celestial bodies--the sun, the earth, and the moon; especially since we know that these motions are regular and permanent, and that we can predict, by means of calculation, the eclipses which will be produced in the future as well as recognize those which have occurred in the past.

The following description of the total eclipse of 1860 will be found interesting. Mr. Lowe, who observed it at Santander, Spain, thus writes:

“Before totality commenced, the colors in the sky and on the hills were magnificent beyond all description. The clear sky in the north assumed a deep indigo color, while in the west the horizon was first black like night. In the east the clear sky was very pale blue, with orange and red, like sunrise. On the shadow creeping across, the deep blue in the north changed, like magic, to pale, sunrise tints of orange and red, while the sunrise appearance in the east had changed to indigo. The darkness was great; the countenances of men were of a livid pink. The Spaniards lay down, and their children screamed with fear; fowls hastened to roost, ducks clustered together, pigeons dashed against the houses; flowers closed; many butterflies flew as if drunk, and at last disappeared. The air became very humid, so much so that the grass felt to one of the observers as if recently rained upon.”