Part 3

There is a football on the table, shown in Fig. 11. We shall suppose it to represent the sun; we shall now choose something else to represent the earth. We must, however, exhibit the proportions accurately. A tennis ball will not do; it is far too large. The fact is, the width of the earth is less than the one-hundredth part of the width of the sun. The tennis ball is, however, only a quarter the width of the football, so we must choose something a good deal smaller. I try with a marble, even with the smallest marble I can find, but when I measure it, I find that one hundred such marbles, placed side by side, would be far longer than the width of the football; I must therefore look for something still smaller. A grain of small-sized shot will give the right size for the model of our earth. About one hundred of these grains placed side by side will extend to a length equal to the width of the football. Now you will be able to form some conception of how enormous the sun really is. Think of this earth, how big we find it when we begin to travel. What a tremendous voyage we have to take to get to New Zealand, and even then we have only got halfway round the globe. Then think that the sun is in the same proportion bigger than the earth as that football is bigger than that grain of shot. If a million of such grains of shot were melted and cast into one globe, it would not be so large as that football. If a million globes, as large as our earth, could be united together, no doubt a vast globe would be produced, but it would not be so large as the sun. Think of a single house, with three or four people living in it, and then think of this mighty London, with its millions of inhabitants. The house will represent our earth, while great London represents the sun!

THE SPOTS ON THE SUN.

I have shown you that the sun is intensely hot, and a very long way off, and enormously big. And now we have to describe the appearance of the surface of the sun when we examine it closely.

If you get a piece of very dark glass, or if you smoke a piece of glass over a candle, then you can look directly at the sun with comfort. A nicer plan is to prick a pinhole in a card, through which you can look at the sun without any inconvenience. Generally speaking, a view of the sun in this way will show you only a uniformly bright surface. To study the face of our great luminary carefully, you must use the aid which the telescope gives to the astronomer. A very good way of doing this is shown in Fig. 12. A small telescope, fixed on a stand, is pointed to the sun, and, the eyepiece being drawn out somewhat further than when direct observations are being made, the sun draws its own picture on a screen. This may be examined without any inconvenience, or without the necessity for any protection to the eye, and a number of young astronomers can all view the sun at the same moment. On such a picture you will generally see the brilliant surface marked with dark spots, which are sometimes as numerous as in the case represented in Fig. 13. These spots present very different appearances according to circumstances. One such spot when seen with a very powerful telescope showed the wonderful structure which is represented in Fig. 14.

The visible surface of the sun is entirely formed of intensely heated vapors. We might almost say that the spots are holes, by which we can look through the brilliant surface to the interior and darker parts. Sometimes the spots close up, and fresh ones will open elsewhere. Now and then the whole surface is mottled over in a remarkable way. I give here a picture which was taken from Mr. Nasmyth’s beautiful drawing, in which he shows how the sun sometimes assumes the appearance which has been likened to willow leaves (Fig. 15). This appearance was very noticeable in the great spot of September, 1898.

The spots often last long enough to demonstrate a remarkable fact. We must remember that the sun is a great globe, and that it is poised freely in space. There is nothing to hold it up, and there is nothing to prevent it from turning round. That it does turn round, we can prove by careful observation of the spots. I can best illustrate what I want by Fig. 17, which shows six imaginary pictures. The first represents the sun on the 1st day of the month; the next shows it five days later, on the 6th; another view is five days later still, on the 11th; and so on until the last picture, which corresponds to the 26th. You see, on the first day there is a spot near the left edge; by the 6th, this spot is near the middle; by the 11th, it is near the right edge; then you do not see it at all on the 16th, or on the 21st; but on the 26th it is back in the same place from which it started. We find other spots to have a similar history. They appear to move across the face, and then to return in a little less than four weeks to the same place where they were originally noticed. These appearances can be illustrated very simply by cutting a small hole through the rind of an orange down to the white interior skin, which may be darkened with ink. Put a knitting needle through the axis of the orange, and then turn it slowly round. The spot will be found to go through the changes that we have seen. We start with the spot near the left, it moves across the face, and then passes to invisibility by moving behind the globe until it reappears again, after having moved round the back. As the same may be observed with every spot which lasts long enough, we learn that the changes in the places must be produced by the turning round of the sun. Here you see is the way in which an astronomical discovery is made. We first observe the fact that the spots do always appear to move. Then we try to account for this, and we find a very simple explanation, by supposing that the whole sun, spots and all, turns steadily round and round. It can also be proved in a very conclusive manner that no other explanation is possible. This rotation of the sun is always going on uniformly, and some curious consequences follow from it. The view of the sun which is turned towards us to-day is quite different from that which was towards us a fortnight ago, or from that which we shall see in a fortnight hence. There is no actual or visible axis about which the sun rotates. In this the sun is like the earth and other celestial bodies.

APPEARANCES SEEN DURING A TOTAL ECLIPSE OF THE SUN.

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.

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.

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.

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.

NIGHT AND DAY.

The sun is bright, and the earth is dark. The sun gives light and heat, and the earth receives light and heat. We should be in utter darkness were it not for the sun; at least, all the light we should have, beyond our trivial artificial light, would come from the feeble twinkle of the stars. The moon would be no use, for the brightness of the moon is merely the reflection of the sunbeams. Were the sun’s light completely extinguished we could never again see the moon, and we should also miss from the sky a few other bodies, which we call planets, such as Jupiter and Venus, Mars and Saturn. But the stars would be the same as before, for they do not depend upon the sun for their light. We shall, indeed, afterwards see that each star is itself a sun.

Picture to yourself the earth as receiving a stream of sunbeams. These beams fall on one half of our globe, and give to it the brilliance of day. The other half of the earth of course receives no sunlight. It is in the shadow, and consequently the darkness of night there prevails. The boundary between light and darkness is not quite sharply defined, for the pleasant twilight softens it a little, so that we pass gradually from day to night. Looking at the progress of the sun in the course of the day, we see that he rises far away in the east, then he gradually moves across the heavens past the south, and in the evening declines to the west, sets, and disappears. All through the night the sun is gradually moving round the opposite side of the earth, illuminating New Zealand and Japan and other remote countries, and then gradually working round to the east, where he starts afresh to give us a new day here.

Our ancestors many ages ago did not know that the earth was round. They thought it was a great flat plain, and that it extended endlessly in every direction. They were, however, much puzzled about the sun. They could see from the coasts of France and Spain or Britain that the sun gradually disappeared in the ocean; they thought that it actually took a plunge into the sea. This would certainly quench the glowing sun; and some of the ancients used to think they heard the dreadful hissing noise when the great red-hot body dropped into the Atlantic. But there was here a difficulty. If the sun were to be chilled down every evening by dropping into the water hundreds of miles away to the west, how did it happen that early the next morning he came up as fresh and as hot as ever, hundreds of miles away to the east? For this, indeed, it seemed hard to account. Some said that we had an entirely new sun every day. The gods started the sun far off in the east, and after having run its course it perished in the west. All the night the gods were busy preparing a new sun to be used on the succeeding day. But this was thought to be such a waste of good suns that a more economical theory was afterwards proposed. The ancients believed that the continents of the earth, so far as they knew them, were surrounded by a limitless ocean. At the north, there were high mountains and ice and snow, which they thought prevented access to this ocean from civilized regions. Vulcan was the presiding deity who navigated those wastes of waters, and to him was intrusted the responsible duty of saving the sun from extinction. He had a great boat ready, so that when the sun was just dropping into the ocean at sunset he caught it, and during all the night he paddled with his glorious cargo round by the north. The glow of the sun during the voyage could even be sometimes traced in summer over the great highlands to the north. This, at all events, was their way of accounting for the long midsummer twilight. After a tedious night’s voyage Vulcan got round to the east in good time for sunrise. Then he shot the sun up with such terrific force that it would go across the whole sky, and then the industrious deity paddled back with all his might by the way he had come, so as to be ready to catch the sun in the evening and thus repeat his never-ending task.

THE DAILY ROTATION OF THE EARTH.

Vulcan and his boat seemed a pretty way of accounting for the sun’s apparent motion. The chief drawback was that it was all work and no play for poor Vulcan. There were also a few other difficulties. Captains of ships told us that they had sailed out on the great sea, and that so far from finding that the ocean extended on and on in one flat plain forever, the water seemed to bend round, so that, in fact, after sailing far enough in the same direction, they found that they would be brought back again to the place from which they started. They also knew a little about the north. They told us that there could be no such ocean as that which Vulcan in this fable was supposed to navigate. It also appeared that ships had been voyaging all over the globe night and day in every direction, and that no captain had ever seen the sun coming down to the sea, and still less had he ever met with Vulcan in the course of his incessant voyages. Thus it was discovered that the earth could not be a never-ending flat, but that it must be a globe, poised freely in space without any attachment to hold it up. It was thought that the change from day to night might be accounted for by supposing that the sun actually went round the earth through the space underneath our feet. This is, indeed, what it seems to do. But there was a great difficulty about this explanation, which began to be perceived when the size and distance of the sun were considered. It required the sun to possess an alarming activity. He would actually have to rush round a circle one hundred and eighty million miles in diameter and complete this astonishing voyage once every day.

A little reflection will show that a very much simpler explanation was available. It was shown that the sun need not revolve round the earth once every day, but that everything would be explained if the earth itself turned round in such a way as to produce the changes from day to night. We may illustrate the case by this figure (Fig. 21). The small globe is the earth, which I can turn by the handle. The lamp will represent the sun, and, as at present shown, the side of the earth, on which England lies, is towards the lamp and in full day. On the opposite side of the globe are other countries such as New Zealand, and there it is dark. You see that by simply turning the handle I can move England gradually round so that it passes into the dark side, and then night falls over the country. At the same time New Zealand is turned round to enjoy the smiles of day. This is a very simple method of accounting for the succession of day and night, and it is also the true method. We have already seen that the sun turns round, and now we find that the earth also turns, but the little body, the earth, goes much the faster, for it makes twenty-five turns while the sun goes round once.

Our earth is at this moment spinning round at a speed so great that London moves many hundreds of miles every hour. A town near the equator would gallop round at a pace of more than a thousand miles an hour--quicker, in fact, than a rifle-bullet. Don’t you think that we ought to perceive that we are being whirled about in this terrific fashion? We know that when we are flying along in a railway train, we feel the jolting and we hear the noise, and we feel the blast of air if we put our heads out of window, and we see the trees as they appear to rush past. All these things tell us that we are in rapid motion. But suppose these sensations were absent. Imagine a line so perfectly laid that no jolts are perceptible, and that no racket is heard; draw down the blinds so that nothing can be seen, how then are we to know that we are moving? Indeed, your grandfathers used to be able to enjoy such a tranquil locomotion. I remember seeing in my childhood the fly-boats, as they were called, on the Royal Canal, wherein passengers were conveyed from Dublin to the West of Ireland, before the railway was made. The fly-boat was a sort of Noah’s ark in appearance, drawn by a horse cantering along the towing-path. In the cabin of such a vessel, where there was not the slightest motion of rolling or pitching--nothing but noiseless gliding along the canal--no one would be conscious of motion, so long as he did not look through the cabin windows. No one was ever seasick in a fly-boat; it was the perfection of travelling for those who loved ease and quiet.

The motion of the earth round its axis is, so far, like that of the fly-boat. It is so absolutely smooth that we do not feel anything, and we only become conscious of it by looking at outside objects. These are the sun, or the moon, or the stars. We see these bodies apparently going through their unvarying rising and setting, just as, in looking out from the fly-boat, the passengers in that quaint old conveyance could see the houses and trees as they passed.

Seeing is believing; and I should like here, in this very theatre, to show you that we are actually turning round; and this I am enabled to do by the kindness of my distinguished friend, Professor Dewar.