Part 4

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.

ANNUAL MOTION OF THE EARTH ROUND THE SUN.

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.

THE CHANGES OF THE SEASONS.

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.

SUNSHINE AT THE NORTH POLE.

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.