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.
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.
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.
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.
I am tempted to wish that I had Aladdin’s lamp for the moment, for I would rub it, and when the great genie appeared, I would bid him take the Royal Institution, and all of us here, to a place which everybody has heard of, and nobody has seen—I mean the North Pole. It would be so easy to describe the experiment I am about to show you, there. It is not so easy here. But it will be sufficiently accurate for our purpose to suppose that we actually have made the voyage, and that this is the Pole at the centre of the lecture-table. The direction of the axis round which the earth is turning is a line pointing up straight to the ceiling. This lecture-table and all the rest of the theatre is going round. In about six hours it will have moved a quarter of the way, and in twenty-four hours it will have gone completely round. That is, at least, what would happen if we were actually at the Pole. As we are not there, for the Pole is many miles away from the Royal Institution, I must slightly modify this statement, and say that the table here takes more than twenty-four hours to go round. And now I want some way of proving that such is actually the case. There is no use in our merely looking at it, because we ourselves, and this whole building, and the whole of London, are all turning together. What we want is something which does not partake of the motion. Here is a heavy leaden ball (Fig. 22). It is fastened to the roof by a fine steel wire, and you see it swings to and fro with a deliberate and graceful motion. I want it to oscillate very steadily, so I draw it to one side and tie it by a piece of thread to a support, and then I burn the thread, and the great ball begins to swing to and fro. It would continue to do so for an hour, or indeed for several hours, and it is a peculiarity of this motion that the vibration always remains in the same direction in space. Even the rotation of the earth will not affect the plane of this great pendulum, so far at least as our experiment is concerned. Here, then, we have a method of testing my assertion about the turning round of this theatre. I mark a line on the table, directly underneath the motion of the ball to and fro. If we could wait for an hour or so, we should see that the motion of the ball seemed to have altered to a direction inclined to its original position, but it is really the table that has moved, for the direction of the motion of the ball is unaltered. We cannot, however, wait so long, therefore I show you the ingenious method which Professor Dewar has devised. By a beam from the electric light, he has succeeded in so magnifying the effect that even in a single minute it is quite obvious that the whole of this room is distinctly turning round, with respect to the oscillations of the pendulum. This celebrated experiment proves by actual inspection that the earth must be rotating. By measuring the motion we might even calculate the length of the day, though I do not say it would be an accurate method of doing so.
The proper way of finding how long the earth takes to turn round is by observing the stars. Fix on any star you please, and note it in a certain position to-night; if you then observe the moment when the star is in the same place to-morrow, the interval of time that has elapsed is the true duration of one complete rotation. When accurately measured its length is found to be 23 hours 56 minutes 4 seconds, or about four minutes shorter than the ordinary day, measured from one noon to the next.
I have as yet only been speaking of the daily movements by which the sun appears to go across the heavens between morning and evening. We next consider the annual movements which give rise to the changes of the seasons. It is now Christmastide, when the days are short and dark, while six months ago the days were long and glorious in the warmth and brightness of summer. A similar recurrence of the seasons takes place every year, and thus we learn that some great changes alter the relation between the earth and the sun year after year. We must try and explain this. Why is it that we enjoy warmth at one season, and suffer from frost and snow at another?
Note first a great difference between the sun in summer and the sun in winter. I will ask you to look out at noon any day when the clouds are absent, and you will then find the sun at the highest point it reaches during the day. All the morning the sun has been gradually climbing from the east; all the afternoon it will be gradually sinking down to the west. Let us make the same observation at different parts of the year. Suppose we take the shortest day in December. You will look out about twelve o’clock from some situation which affords a view towards the south, and there, as shown in the adjoining sketch (Fig. 23), is the midwinter sun.
But now the spring approaches, and the days begin to lengthen. If you watch the sun you will see it pass higher and higher every noon until Midsummer Day is reached, and then the sun at noon is found quite high up in the sky. As autumn draws near, the sun at noon creeps downwards again until, when the next shortest day has come round, we find that it passes just where it did at the previous midwinter. With unceasing regularity year after year the sun goes through these changes. When he is high at noon we have days both long and warm; when he is low at noon we have days both short and cold.
Vulcan with his golden boat was naturally expected to give an explanation of this. As the summer drew on, each day Vulcan shot out the sun with a stronger impulse, so that it should ascend higher and higher. His greatest effort was made on Midsummer Day, when, after rowing but a little way round from the north towards the east, he drove off the sun with a terrific effort. The sun soared aloft to the utmost height it could reach, and in the meantime Vulcan returned to the west to be ready to catch the sun as it descended. On the other hand, in midwinter, he came round much further through the east to the south, and then shot up the sun with his feeblest effort, and had to paddle as hard as ever he could so as to complete his long return voyage during the brief day.
It is evident that there are two quite distinct kinds of motion of the sun. There is first the daily rising and setting, for which we have accounted by showing that it is merely an appearance produced by the fact that the earth is turning round. But now we have been considering quite a different motion by which the sun seems to creep up and down in the heavens, and this takes a whole year to go through its changes.
There is still another point which we must consider before we can understand all these puzzling movements of the sun. We shall ask the stars to help us by their familiar constellations. You know, perhaps, the Great Bear, or the Plough as it is often called, and Orion. There are also Aries the Ram, Taurus the Bull, and other fancifully named systems. These constellations have been known for countless ages, and for our present purposes we may think of them as permanent groups in the heavens, which do not alter either their own shapes or their positions relatively to each other. These groups of stars extend all around the sky. They are not only over our heads and on all sides down to the horizon, but if we could dig a deep hole through the earth, coming out somewhere near New Zealand, and if we then looked through, we should see that there was another vault of stars beneath us. We stand on our comparatively little earth in what seems the centre of this great universe of stars all around. It is true we do not often see the stars in broad daylight, but they are there nevertheless. The blaze of sunlight makes them invisible. A good telescope will always show the stars, and even without a telescope they can sometimes be seen in daylight in rather an odd way. If you can obtain a glimpse of the blue sky on a fine day from the bottom of a coal pit, stars are often visible. The top of the shaft is, however, generally obstructed by the machinery for hoisting up the coal, but the stars may be seen occasionally through the tall chimney attached to a manufactory when an opportune disuse of the chimney permits of the observation being made (Fig. 24). The fact is that the long tube has the effect of completely screening from the eye the direct light of the sun. The eye thus becomes more sensitive, and the feeble light from the stars can make its impression, and produce vision. From all these various lines of reasoning we see that there can be no doubt of the continuous presence of stars above and around us, and below us, on every side, and at all times.
If you look out at Christmas time, towards the south, you will see the Belt of Orion and the Dog Star in a splendid portion of the heavens. These stars you will see every winter in the same place. But you may look in vain for them in summer. No doubt you can see stars in the summer evenings, but they will be totally different from those that adorned the skies in winter. Each season has its own constellations. This simple fact was known to the ancients, and we shall find its explanation full of meaning. Let us select four well-known constellations which will best answer our purpose. They lie in a circle round the heavens. They are Orion, Virgo, Scorpio, and Pisces. I am supposing that you are looking out at midnight towards the south. In December you will see Orion; in March, Virgo; in June, Scorpio; and in September, Pisces; and then next December you will be looking at Orion again. See what this proves. At midnight, of course, the sun is at the other side of the earth, so that if I am looking at Orion in midwinter the sun must be behind my back. Look at our little picture (Fig. 25). The earth is in the middle, and the sun must be on the opposite side to Orion. That is, the sun must be somewhere about the position I have marked at A. In March we see Virgo in the south at midnight, when, of course, the sun is at the other side of the earth; so that the sun must be somewhere at B. In June Scorpio is seen, so that the sun must be at the other side, at C. That is to say, in midsummer the sun is in that part of the sky where Orion is situated. If, therefore, on a bright June day we could see the stars, we should find Orion in the south. But, of course, the light of the sun makes Orion invisible. We can, however, see the stars by our telescopes, and on rare occasions an eclipse of the sun will occur, by which he is temporarily extinguished, and then we can see the stars without the help of a telescope, even though it is daytime.
Thus it would seem as if the sun were first at A and then at B, C, and D, and then began to go round again. I say it would seem as if the sun had these movements, and the ancients thought there was no doubt about the matter. Even after it was plain that the earth turned round on its axis so as to give the changes of day and night, it was still thought necessary to suppose that the sun went round the earth once in the year, in order to explain how the changes in the stars during the different seasons were produced.
Here is another case in which we must be careful to distinguish between what appears to be true and what is actually the case. Everything that we undoubtedly see would be just as well explained by supposing that the sun remained at rest, and that the earth revolved around it, as in Fig. 26. If, for instance, the earth were at A in midwinter, then the sun is on the opposite side to Orion, and of course at midnight we shall be able to see Orion. So in spring the earth is at B, and we see Virgo, and similarly in summer we have Scorpio, and in autumn Pisces. Thus all that is actually visible could be fully accounted for by regarding the sun as fixed in the centre, and the earth as travelling round it from A to B, to C and to D respectively, and completing the journey in a twelvemonth. Which idea are we to adopt? Shall we say that the earth goes round the sun, or the sun goes round the earth?
I remember an old college story, which I cannot help giving you at this place. It may serve to lighten what I fear you must otherwise have thought rather a tedious part of our subject. There were three students brought up for examination in astronomy, and they showed a lamentable ignorance of the subject, but the examiner, being a kind-hearted man, wished, if possible, to pass them; and so he proposed to the three youths the very simplest question that he could think of. Accordingly, addressing the first student, he said: “Now tell me, does the earth go round the sun, or the sun go round the earth?” “It is—the earth—goes round the sun.” “What do you say?” he inquired, turning rather suddenly on the next, who gasped out: “Oh, sir—of course—it is the sun goes round the earth.” “What do you say?” he shouted at the third unhappy victim. “Oh, sir, it is—sometimes one way, sir, and sometimes the other!”
But which is it? Well, we must remember that the earth is comparatively a very little body and the sun a very big one, so it is not at all surprising to learn that the earth goes round the sun, which remains, practically speaking, at rest in the centre. Thus our great earth and all it contains are continually bound in what is very nearly a circular course round the great luminary. You will find it instructive to work out this little sum. How fast is the earth moving, or how far do we go in a second? We are about 93,000,000 miles from the sun, and the great circle that we go round has a diameter twice as great as this—that is, about 186,000,000 miles. The circumference of a circle is nearly three and one-seventh times its diameter, and accordingly the whole length of the voyage in the year is about 585,000,000 miles. This has to be accomplished in 365 days, so that the daily run must be about 1,600,000 miles. We divide this by 24, to find the distance journeyed each hour, which we find to be about 67,000 miles; and we must divide this again by 60 to find the length covered in a minute, and by 60 again for the progress made each second. It is truly startling to find that, night and day, this great earth has to travel more than eighteen miles every second in order to get round its mighty path in the allotted time.
I began this lecture about forty minutes ago, and I think from what I have said you will be able to calculate a result that will, I dare say, astonish you. In these forty minutes we have moved about 45,000 miles. No doubt my lecture commenced in this hall, and in your presence; but can I truly say I began it here? Well, no; I began it not here, but at a place 45,000 miles away; but we have all been travelling together, and the journey has been so very smooth and free from all jolts, that we never thought anything about the motion.
I am sure many of those to whom I am now speaking have read accounts of voyages in the Arctic regions. You have been told of the sufferings of the crews during the long winters, amid the ice and snow; and you have heard how, during that dismal period, there is total darkness, for the sun never rises for weeks and months together. On the other hand, these northern regions often present a more cheerful picture. During midsummer, the long darkness of winter is atoned for by perpetual sunshine. At midnight there is still the full brilliance of day, and the sun, though low, no doubt, has not passed below the horizon. Even in the northerly parts of Europe we can see the midnight sun. Lord Dufferin, in his delightful narrative of a cruise, entitled “Letters from High Latitudes,” gives an interesting illustration of the perplexities arising from endless daylight. It appears that everything went on happily until the fatal moment when the yacht crossed the Arctic circle. Then it was that dire tribulation arose among the poultry. A fine cock was the cause of the trouble. Knowing his duty, he always liked to be particular about performing the important task of crowing at sunrise. This he could do regularly, so long as the yacht remained in reasonable latitudes, where the sun behaved properly. But when they crossed the Arctic circle, the cock was confronted with a wholly new experience. The sun never set in the evening, and consequently never had to rise in the morning. What was the distracted bird to do? He did everything. He burst into occasional fits of terrific crowing at all sorts of hours, then he gave up crowing altogether, but finding that did not mend matters, he took to crowing incessantly. Exhaustion was succeeded by delirium, and rather than live any longer in a universe where the sun was capable of pranks so heartless, the indignant fowl flung himself from the vessel and perished in the Arctic Ocean.
In the adjoining figure, I show a little sketch (Fig. 27), by which I try to explain the changes of the seasons. It exhibits four positions of the earth, one on each side of the sun. The left. A, represents the earth when summer gladdens the northern hemisphere; while the right, C, shows winter in the same region. You will see the two central lines which represent the axis about which the earth rotates. Of course, the earth has no visible axis. The line which runs through the globe from the North to the South Pole is imaginary. It remains fixed in the earth, for we can prove in our observatories that the Pole does not shift its position to any considerable extent in the earth itself. In fact, if we could reach the North Pole and drive a peg into the ground year after year to mark the exact spot, we should find that the position of the Pole was sensibly the same. Does it not seem strange that we should be able to know so much about the Pole, though we have never been able to get there; have never, in fact, been able to get within less than 400 miles of it? I think you will be able to understand the point quite easily. The latitude of a place, as you know from your geography, is the number of degrees, and parts of a degree, between that place and the equator. In our observatories, we can determine this so accurately that the difference between the latitude of one side of a room and of the other side of the same room is quite perceptible. As we find that the latitudes of our observatories remain sensibly unchanged from year to year, we are certain that the Pole must remain in the same place. Indeed, if the Pole were to alter its position by the distance of a stone’s throw, the careful watchers in many observatories would speedily detect the occurrence.
And now I must direct your attention to something apparently quite different. When the battle of Waterloo was fought, the great victory was won with the aid of the old-fashioned musket, a smooth-bore gun which was loaded at the muzzle with a good charge of powder, and then a round bullet was rammed down. “Brown Bess,” as the musket was called, was a most efficient weapon at close quarters, and indeed at any distance when the bullet hit; but there was the difficulty. The round bullets, rushing up the tube and out into the air in a somewhat vague manner, had a habit of roaming about, which was quite incompatible with the accurate shooting of our modern rifles.
One great improvement in small arms consisted in giving to the bullet a rapid rotation about an axis which is in the line of fire. This is what the rifle accomplishes. The grooves in the barrel of the rifle twist round, and though they only give half a complete turn in the length of the barrel, yet the speed of the bullet is so great that when it flies off it is actually spinning with the tremendous velocity of about one hundred and fifty revolutions a second. Even with the old-fashioned round bullet, the rifling of the barrel effected great improvement in the accuracy of the shooting. The introduction of the elongated bullets was another great improvement, while the adaptation of breech-loading enabled a bullet to be used rather larger than that which could have been forced down the barrel, and thus it was insured that the grooves should bite into the bullet as it hurries past and impart the necessary spin.
A body rapidly rotating about an axis has a tendency to preserve the direction of that axis, and powerfully resists any attempt to change it. Our earth is spinning in this fashion. It is true that the rotation is, in one sense, a slow one, for it requires almost an entire day for each rotation. But when we remember the dimensions of our earth, we shall modify this notion. We have already stated that any place on the equator has to travel more than one thousand miles each hour in order to accomplish the journey within the required time. So far, therefore, the earth moves like a rifle-bullet, and the direction of its axis remains constant.
In the course of the great voyage between summer and winter, the earth travels from one side of the sun to the opposite side, and in doing so it still continues to spin about an axis parallel to the original direction. See the consequences which follow. The sun illuminates half the earth, and in the left position in Fig. 27, representing summer, the North Pole is turned over towards the sun, and lies in the bright half of the earth. There is continual day at the North Pole, and night is unknown there at this time of year, because the turning of the earth about its axis will not bring the Pole nor the regions near the Pole into the dark hemisphere. Thus it is that the Arctic regions enjoy perpetual day at this season. Look now at the position of England when the northern hemisphere is tilted towards the sun, and is consequently enjoying the full splendor of midsummer. As the earth turns round, England will gradually cross the boundary between light and shade, and will enter upon the darkened hemisphere. Then there will be night in England, but you will see from the figure that the day is much longer than the night, and hence it is that we enjoy the fine long days in summer.
We next look at a different scene six months later. The earth has reached the other side of the sun, but the axis has remained parallel to itself, consequently the North Pole is now inclined entirely away from the sun. The earth continues to turn round as before, but its movements do not bring the North Pole or the surrounding Arctic regions out of the dark hemisphere, and consequently the night must be unbroken in these dismal circumstances. The long continuous day which forms the Polar midsummer is dearly purchased by the gloom and cold of a winter in which there is no sun for many weeks in succession. Observe also the changed circumstances of England. In the course of each twenty-four hours it lies much longer in the dark half of the earth than in the bright, and consequently there is only a short day succeeded by a long night.
It is a privilege of astronomers to be able to predict events that will happen in thousands of years to come, and to describe things accurately though they never saw them, and though nobody else has ever seen them either. No one has ever yet got to the North Pole, but whenever they do, we are able to tell them much of what they will see there. We may leave it to Jules Verne to describe how the journey is to be made, and how the party are to be kept alive at the North Pole. I shall give a picture of the changes of the seasons, and of the appearance in the stars, as seen from thence.
We shall, therefore, prepare to make observations from that very particular spot on this earth—the North Pole. I suppose that eternal ice and snow abide there. I don’t think it would be a pleasant residence. However, we shall arrange to arrive on Midsummer Day, prepared to make a year’s sojourn. The first question to be settled is the erection of the hut. In a cold country it is important to give the right aspect, and we are in the habit of saying that a southerly aspect is the best and warmest, while the north and the east are suggestive only of chills and discomfort. But what is a southerly aspect at the North Pole, or, rather, what is not a southerly aspect? Whatever way we look from the North Pole we are facing due south. There is no such thing as east or west; every way is the southward way. This is truly an odd part of the earth. The only other locality at all resembling it would be the South Pole, from which all directions would be north.
The sun would be moving all through the day in a fashion utterly unlike its behavior in our latitudes. There would, of course, be no such thing as rising and setting. The sun would, indeed, at first seem neither to go any nearer to the horizon nor to rise any higher above it, but would simply go round and round the sky. Then it would gradually get lower and lower, moving round day after day in a sort of spiral, until at last it would get down so low that it would just graze the horizon, right round which it would circulate till half the sun was below, and then until the whole disk had disappeared. Even though the sun had now vanished, a twilight glow would for some time be continuous. It would seem to come from a source moving round and round below the horizon, then gradually the light would become fainter and fainter until at last the winter of utter and continuous blackness had set in. The first indications of the return of spring would be detected by a feeble glow near the horizon, which would seem to move round and round day after day. Then this glow would pass into a continuous dawn, gradually increasing until the sun’s edge crept into visibility, and the great globe would at last begin to climb the heavens by its continual spiral until midsummer was reached, when the change would go on again as before.
Our first excursion to the country of Star-land has now been taken, and we have naturally commenced by studying that sun to which we owe so much. But we shall have to learn that though our sun is of such vital importance to us, yet, in magnificence and size, he has many rivals among the host of stars.