Читать книгу Star-land: Being Talks With Young People About the Wonders of the Heavens - Robert S. Ball - Страница 8

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Fig.8.—The nearer you are, the bigger the Globe looks.

Fig.9.—The Globe is so far off that it lies beyond the Picture. The dotted lines show how small it seems.

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.