NOTES, REFERENCES, AND STATISTICS. Titles of books referred to. - Figure and dimensions of the Earth. Latitudes and longitudes of some observatories. - Refraction and dip. Sun's rotation, parallax, dimensions, mass, spots, and heat. Number of stars; data respecting variable and other stars. — Ele- ments of planets and of satellites. Notes relating to subjects men- LIST OF ILLUSTRATIONS. IN the following List, the letters H. C. O. show that the Plate opposite ERRATA. - Owing to an error in the printing of this edition, Plates IX., X., XI., and XII. have been placed between pages 193 and 194, instead of between pages 207 and 208. The number of pages between Plates I. and IX. should be about in order to make the explanation of Plate IX. correct. OUTLINES OF ASTRONOMY. CHAPTER I. OBJECTS OF ASTRONOMY. 1. ALL the countries and oceans about which we learn by the study of geography make up the outside of that great ball, or globe, eight thousand miles thick, which is called the Earth. Eight thousand miles is so great a distance that we cannot well understand how great it is without the help of some comparison, like this. Suppose that, on some winter day, when the ground is covered with damp snow, we roll up a snowball a yard thick or more, so as to reach up to the waist of a man standing beside it. Let us make this snowball as round and smooth as possible, and then stick some rather small pins into it up to their heads. The heads of those pins will stand out as far from the snowball, with respect to its size, as the highest mountains in South America or India stand out from the Earth. So, too, if we make a little hollow in the snowball, just large enough to hold the head of a pin, that hollow will go as deep into the snowball as the hollows which contain the oceans go into the Earth, and deeper than the deepest mines go into it. Of course it would be very hard to make a snowball so perfectly round and smooth as not to have little lumps and hollows upon it many times larger than pin-heads; and yet, in spite of these little roughnesses, the ball would look smooth enough to any one standing a few steps away from it. So, too, the Earth's roughness, caused by its mountains and its valleys, both on land and under the sea, is really so slight in comparison with its size, that we may regard it as a smooth round ball when we are considering the whole of it at once. We can now understand a little better what is meant when we are told that the Earth's diameter is eight thousand miles; for if a pin's head on a ball a yard thick is large enough to represent a great mountain on the Earth, we may imagine how small a house or even an ordinary hill would be when it was represented on the same scale. We see, besides, that the Earth's mountains cannot be represented in the right proportion by raised figures on a globe or map made on any ordinary scale. 2. The Earth seems to us to stand still; but, in fact, it is always moving along in much the same way that a ball is moving after it has rolled off the roof of a house and before it has come to the ground. Such a ball has at least three different kinds of movement at the same time. First, it is falling to the ground: but this is not all; for it does not drop directly downwards, as it would if it had been stopped just when it came to the edge of the roof and let go again. Instead of this, it has gained some headway by rolling down the roof; and while it is falling, this headway carries it farther and farther from the house. This is its second kind of movement. But besides falling and moving away from the house, it goes on rolling in the air just as it did on the roof. This third kind of movement is not so easy to see as the others, because it is generally too quick for the eye to follow. But if we take a rather large and light ball, like a football, for instance, — place it near the edge of a smooth board a few feet from the ground, and then tilt the board just enough to make the ball roll off, we shall see that it turns over a little while it is falling. This can be seen more plainly if the ball is marked in some way; by pasting pieces of paper upon it, for example. 3. It is seldom that any ball moves in the air without having all the kinds of movement just described; that is, a dropping movement, a forward movement, and a rolling movement. The Earth is like any other ball in these respects; it is always falling, moving on, and rolling. How it |