Part 2

We may look a little further and find whence the clouds have come. It is certain that clouds are merely a form of steam or vapor of water, and as they are so continually sending down rain on the earth, there must be some means by which their supply will be replenished. Here again our excellent friend the sun is to be found ever helping us secretly, if not helping us openly. He pours down his rich and warm beams on the great oceans, and the heat turns some of the water into vapor, which, being lighter than the air, ascends upwards for miles. There the vapor often passes into the form of clouds, and the winds waft these clouds to refresh the thirsty lands of the earth. Thus, you see, it is the sun which procures for us water from the great oceans which cover so much of our globe, and sends it on by the winds to supply our water-works, and fill our teapots. Notice another little kindliness of our great benefactor. The water of the oceans is quite salt. But we could not make tea with salt water, so the sun, when lifting the vapor from the sea, most thoughtfully leaves all the salt behind, and thus provides us with the purest of sweet water.

That nice muffin was baked by the sun, toasted by the sun, and made from wheat grown by the sun. If the wheat was ground in a wind-mill, then the sun raised the wind which turned the mill. Perhaps the flour-mill was driven by steam, in which case the sun, long ago, provided the coal for the boiler. The miller might have lived on a river and used a water-mill, but if he did, then here again the sun actually did the work. The sun raised the water to the clouds, and after it had fallen in rain, and was on its way back to the sea, its descent was utilized to turn the water-wheel. The water derives its power to turn the mill from the fact that it is running downhill, but it could not run down unless it had first been raised up; and thus it is indeed the sun which drives the water-wheel. Nor can the baker dispense with the sun’s aid even if he rejected wind-mills, or steam-mills, or water-mills, and determined to grind the corn himself with a pestle and mortar. Here, at least, it might be thought that it is a man’s sinews and muscles that are doing the work, and so no doubt they are. But you are mistaken if you think the sun has not rendered indispensable aid. The sun has just as surely provided the power which moves the baker’s arms as it has raised the wind which turned the wind-mill. The force exerted in grinding with the pestle has been derived from the food that the man has eaten; that food was grown by the sun, and the man received from the food the energy it had derived from the sun’s heat. So that, look at it any way you please, even for the grinding of the wheat to make the muffin for your tea party, you are wholly indebted to the sun.

It is the sun which has bleached the tablecloth to that snowy whiteness. The sun has given those bright colors which look so pretty in the girls’ dresses. With how much significance can we say and feel that light is pleasant to the eye, and what prettier name than Little Sunbeam can we have for the darling child who makes our home so bright?

THE DISTANCE OF THE SUN.

The sun is a very long way off. It is not easy for you to imagine a distance so great, but if you want to learn astronomy you must make the attempt. This is the first measurement that we shall have to make on our way to that far-off country called Star-Land; but long as we shall find it to be, we shall afterwards have to consider distances very much longer. When you are out in the street, or taking a walk in the country, you can see at once that this man is near, or that house is far, or that mountain is many miles away. This is because you have other objects between to help you to judge of the distances of these different objects. You will see, for example, that there are many houses or farmyards, and you will notice hedges dividing different fields between you and the mountain. You also see that there are woods and parks, and perhaps stretches of moorland extending up the slopes. You have an impression that the farmyards and fields are of considerable size, and that the woods or moors are wide and extensive; and putting these things together, you realize that the mountain must be miles away.

But when we look at the sun we have no aids conveniently placed to help us in judging his distance. There are no intervening objects, and merely gazing at the sun helps us but little in obtaining any accurate knowledge. We must go to the astronomer and ask him to tell us how far he has found the sun to be, and then we must also beg from him some explanation of the method he has used in making his measurements.

It has been found that the sun is, on the average, about ninety-three millions of miles from the earth; but sometimes it is a little further and sometimes it is a little nearer. Let us first try to count 93,000,000. The easiest way will be to get the clock to do this for us; and here is a sum that I would suggest for you to work out. How long will the clock have to tick before it has made as many ticks as there are miles between the earth and the sun? Every minute the clock, of course, makes 60 ticks, and in 24 hours the total number will reach 86,400. By dividing this into 93,000,000 you will find that more than 1076 days, or nearly three years, will be required for the clock to perform the task.

We may consider the subject in another way, and find how long an express train would take to go all the way from the earth to the sun. We shall suppose the speed of the train to be 40 miles an hour; and if the train ran for a whole day and a whole night without stopping, it would then accomplish 960 miles. In a year the distance travelled would reach 350,400 miles, and by dividing this into 93,000,000 we arrive at the conclusion that a train would have to travel at a pace of 40 miles an hour, not alone for days and for weeks and for years, but even for centuries. Indeed, not until 265 years had elapsed would the mighty journey have been ended. Even though King Charles I. had been present when the train began to move, the destination would not yet have been reached. No one who started in the train could expect to reach the end of the trip. That would not occur till the time of his great-great-grandchildren.

HOW ASTRONOMERS MEASURE THE DISTANCES OF THE HEAVENLY BODIES.

I shall so often have to speak of the distances of the celestial bodies that I may once for all explain how it is that we have been able to discover what these distances are. This would be a very puzzling matter if we were to try and describe it fully, but the principle of the method is not at all difficult. Do you know why you have been provided with two eyes? It is undoubted that one of the reasons is to aid you in estimating distances. You see this boy (Fig. 4) judges of the distance of his finger by the inclination of his two eyes when directed at it. In a similar way we judge of the distance of a heavenly body by making observations on it from two different stations.

I shall illustrate our method of measuring the actual distance of a body in the heavens by showing you how we can find the height of that large india-rubber ball which is hanging from the ceiling. Of course, I do not intend to have a measuring tape from the ball itself, because I want to solve the problem on the same principle as that by which we measure the distance of the sun or of any other celestial body which we cannot reach. I will ask the aid of a boy and a girl, who will please stand one at each end of the lecture table. The apparatus we shall want is very simple; it consists of two cards and a pair of scissors. The boy will kindly shape his card to such an angle that when he holds it to his eye one side of the angle shall point straight at the little girl, and the other side shall point straight at the ball, just as you see in the picture (Fig. 5). The girl will also please do the same with her card, so that along one side she just sees the little boy’s face, while the other side points up to the ball. It will be necessary to cut these angles properly. If the angle be too big, then when one side points to the boy’s face, the other will be directed above the ball. If the angle on the card be too small, then one side will be directed below the ball, while the other is pointed to the boy. The whole accuracy of our little observations depends upon cutting the card angles properly. When they have been truly shaped it will be easy to find the distance of the ball. We first take a foot rule and measure the length of our table from one of our young friends to the other. That length is twelve feet, and to discover the distance of the ball we must make a drawing. We get a sheet of paper, and first rule a line twelve inches long. That will represent the length of the table, it being understood that each inch of the drawing is to correspond to a foot of the actual table. Let the end where the girl stood be marked B, and that of the boy, A, and now bring the cards and place them on the line just as shown in the figure. The card the girl has shaped is to be put so that the corner of it lies at B, and one edge along B A. Then the boy’s card is to be so put that its corner is at A and one edge along A B. Next with a pencil we rule lines on the other edges of the cards, taking care that they are kept all the time in their proper positions. These two lines carried on will meet at C; and this must be the position of the ball on the scale of our little sketch. It only now remains to take the foot rule and measure on the drawing the length from A to C. I find it to be twenty inches, and I have so arranged it that the distance from B to C is the same.

I do not intend to trouble you much with Euclid in these lectures, but as many of my young friends have learned the sixth book, I will just refer to the well-known proposition, which tells us that the lengths of the corresponding sides of two similar triangles are proportional. We have here two similar triangles. There is the big one with the boy at one corner, the girl at the other, and the ball overhead. Here is the small triangle which we have just drawn. These triangles are similar because they have got the same angles, and it was to insure that they should have the same angles that we were so careful in shaping the cards. As these two triangles are similar, their sides must be proportional. We have agreed that the line A B, which is twelve inches long, is to represent the length of the table between the little boy and girl. Hence the distance, A C, must, on the same scale, be the interval between the ball and the boy at the end. This is twenty inches on the drawing, and therefore the actual distance from the end of the table to the ball is twenty feet.

Hence you see that without going up to the ball or having a string from it, or in any other way making direct communication with it, we have been able to ascertain how far up in the air the ball is actually hung. This simple illustration explains the principle of the method by which astronomers are able to learn the distances of the different celestial bodies from the earth. You must think of the sun, the moon, and the stars as globes supported in some manner over our heads, and we seek to discover their distances from measurements of angles made at the ends of a base-line.

Of course, astronomers must choose two stations which are far more widely separated than are those in our little experiment. In fact, the greater the interval between the two stations, the better. Astronomers require a much longer distance than from one side of this room to the other, or from one side of London to the other side. If it were merely a balloon at which we were looking, then, when one observer at one side of London and another at the opposite side shaped their cards carefully, we should be able to tell the height of the balloon very easily. But as the sun is so much further off than any balloon could ever be, we must separate the observers much more widely. Even the breadth of England would not be enough, so we have to make them separate more and more until they are as widely divided as it is possible for any two people on this earth to be. One astronomer takes up his position at A (Fig. 7), and the other at the opposite side at B, so that they can both see the sun. They are obliged to use a much more accurate way of measuring the angles than by cutting out cards with pairs of scissors; and as the astronomer at A is not able to see his friend at B, it becomes no easy matter to measure the angles accurately. However, we shall not now trouble ourselves about such difficulties. It may suffice for the present to know that the angles are measured by delicate and very accurate instruments used by astronomers. They will not, indeed, make a little sketch such as sufficed for our purpose. They make a calculation which is a much more accurate way of effecting true measurement. The astronomers know the size of the earth, and thus they know how many thousands of miles lie between the two stations where the observations are made. This distance means in their calculation just what the length of the table did in our sketch. From each end of the line they set off an angle just as we did, and the astronomer must use the principle of similar triangles which he finds in Euclid, just we had to do. At last, when they have calculated the sides of their triangle, they obtain the distance of the sun.

THE APPARENT SMALLNESS OF DISTANT OBJECTS.

I ought here to explain a principle which those who are learning about the stars must always bear in mind. The principle asserts that the further a body is, the smaller it looks. Perhaps this will be understood from the adjoining little sketch (Fig. 8). It represents a great globe, on which oceans and continents are shown, and you see a little boy and a little girl are looking at the globe. The girl stands quite close to it, and I have drawn two dotted lines from her eye, one to the top of the globe, and the other to the under surface. If she wants to examine the entire side of the globe which is visible to her, she must first look along the upper dotted line, and then she must turn her glance downwards until she comes to the lower line, and having to turn her eyes thus up and down she will think the globe is very big, and she will be quite right. The boy is, as you see, on the other side of the globe, but I have put him much further off than the girl. I have also drawn two dotted lines from his eye to the globe, and it is plain that he will not have to turn his head much up and down to see the whole globe. He can take it all in at a glance, and to him, therefore, the globe will appear to be comparatively small, because he is sufficiently far from it. The more distant he is, the smaller it will appear. You can easily imagine that, if the globe were far enough, the two lines that would include the whole would be like those shown (Fig. 9), in which the globe is so distant that it cannot be seen in the picture. The apparent size of the globe, which is really measured by the angle between these two lines, would always be smaller and smaller according as the distance was greater. Now you can understand why an object seems smaller the further away it is; indeed, when sufficiently far, the object ceases to be visible at all.

I could give many illustrations of the diminution of size by distance, and so, doubtless, could you. Every boy knows that his kite looks smaller and smaller the greater the length of string that he lets out. I have seen in the West of Ireland a bird that seemed like a little speck high up near the clouds, but from its flight and other circumstances I knew that the speck was not a little bird. It was, indeed, a great eagle, which was dwarfed by the elevation to which it had soared.

It is in astronomy that we have the best illustrations of this principle. Enormous objects seem to be small because they are so very far off. You must therefore always remember that although an object may appear to be small, this appearance may be only a delusion. It may be that the object is very big, but very distant. In astronomy, this is almost always the case, there is so much room above us, around us, on all sides in space. Look up at the ceiling. It certainly does not bound space, for there is another side to it; and then there is the roof of the house. But the roof is not a boundary, for, of course, there is the air above it, and then, higher up still, there are the clouds, and so we can carry our imagination on and on through and beyond the air up to where the stars are, and still on and on. And as there is unlimited room, the celestial bodies take advantage of it, and are, generally speaking, at distances so gigantic that, no matter how small they may appear, their smallness is merely deceptive.

Let us try to illustrate in another way the exceeding remoteness of the sun. So please imagine that you were on the sun, and that you took a view of our earth from that distance. To find out what we must expect to see, let us think of a balloon voyage. If you were to go up in a balloon, you would at first see only the houses, or objects immediately about you, but as you rose the view would become wider and wider. You would see that London was surrounded by the country, and then, as you still soared up and up, the sea would become visible, and you would be able to trace out the coasts, east and west and south. If, in some way, you could soar higher than any balloon could carry you, the whole of the British Islands would presently lie spread like a map beneath. Still on and on, and then the continent of Europe would be gradually opened out, until the great oceans, and even other continents, would at last be caught sight of, and then you would perceive that our whole earth was indeed a globe. The higher you went, the less distinctly would you be able to see the details on the surface. At last the outlines of the continents and oceans would fade, and you would begin to lose any perception of the shape of the earth itself. Long ere you had reached the distance of the sun, the earth would look merely as the planet Venus now does to us. It is instructive to consider how small our earth would seem if it were possible to view it from the sun. Think of that very familiar little globe, a lawn-tennis ball, which is two and three-quarter inches in diameter. But suppose a tennis ball were at the opposite side of the street, or still further away; suppose, for example, that it were half a mile away, what could you expect to see of it? And yet the earth, as seen from the sun, would appear to be no larger than a tennis ball would look when viewed from a distance of half a mile.

THE SHAPE AND SIZE OF THE SUN.

We have spoken of the heat of the sun, how hot he is; of the distance of the sun, how far he is; and now we must say a little about the size of the sun; and also about his shape. It is plain that the sun is round, that it has the shape of a ball. We are sure of this because, though a plate is circular, yet, if it were placed so that we only saw it edgeways from a distance, it would not appear to be round. The sun is always rotating, and as it always seems to be a circle, we are therefore certain that the true shape of the sun must be globular, and not merely circular like a flat plate.

In the middle of the day, when the sun is high in the heavens, it is impossible for us to form a notion of the size of the sun. People will form very different estimates as to his apparent bigness. Some will say he looks as large as a dinner plate, but such statements are meaningless, unless we say where the plate is to be held. If it be near the eye, of course the plate may hide the sun, and, for that matter, everything else also. If the plate were about a hundred feet away, then it would often hide the sun. If the plate were more than a hundred feet distant, then it could not hide the sun entirely, and the further the plate, the smaller it would seem.

No means of estimating the sun’s size are available when his orb stands high in the heavens. But when he is rising or setting, we see that he passes behind trees or mountains, so that there are intervening objects with which we can compare him; then we have actual proof that the sun must be a very large body indeed.

I give here a picture, by Marcus Codde, taken from a French journal, _l’Astronomie_, which gives a charming illustration of a sunset at Marseilles (Fig. 10). If you wish to see that the sun is bigger than a mountain, you may go to the top of Notre Dame de la Garde, but you must choose either the 10th of February or the 31st of October for your visit, because it is only on the evenings of those days that the sun sets in the right position.

On both these evenings the sun sinks directly behind Mount Carigou in the Pyrenees; this mountain is a long way from Marseilles--no less, indeed, than one hundred and fifty-eight miles. But the mountain is so lofty, that when the sky is clear, the summit can be distinctly seen upon the sun as a background, in the way shown in the picture. This must be a very pretty sight, and it teaches us an important lesson. The sun is further away than the mountain, and yet you see the sun on both sides of the mountain, and above it. Here, then, we learn without any calculations, that the sun must be bigger than the upper part of a great mountain in the Pyrenees.

When we calculate the size of the sun from the measurements made by astronomers, we discover that it is much bigger than Mount Carigou; we see that even the entire range of the Pyrenees, the whole of Europe, and even our whole globe, are insignificant by comparison.