"Astronomers have the same kind of troubles," said the Professor. "They incur great labor and expense to take some particular observation that is possible only once in a number of years, and then for only a few minutes. And after their instruments are all carefully set up, and their calculations made, the clouds spread over the sky, and hide everything they wish to see. People, too, are very apt to laugh at their disappointment.
"There would, however, be no science of astronomy if those who pursued it were discouraged by common difficulties. To explain the heavenly bodies they sometimes try to make little systems or images of the sun and the planets; but they are never able to show the sizes and distances correctly. If they were to begin by making the sun one inch in diameter, then the earth would have to be three yards off, and as small as a grain of dust; some of the planets would have to be across the street, and others away beyond the opposite houses. So when you look at these little solar systems, as they are called, you must remember that the sizes and distances are all wrong.
"Still, you can get from them some idea how the sun stands in the middle, and the earth and other planets go round, and how the earth, while going round the sun, keeps also turning itself around. You have seen how a top, while spinning, sometimes runs round in a circle. That is just the way our earth does. And if you imagine a candle in the centre of the circle that the top makes, you will see why it is sometimes day and sometimes night. When the side of the earth we are on is turned toward the sun, we have day; and when we have spun past the sun, night comes.
"The sun seems to go past us, and people used to think it really did. But we know now that it is as if we were in a rail-car, and the trees and houses seemed to be rushing along, when we ourselves are the ones that are moving. The sun and all the stars seem to move through the sky from east to west; but it is only our earth that is turning itself the other way, and carrying us with it."
"What makes summer and winter?" asked Joe.
"I think that the top will help you to understand that too. You have noticed that when it spins it does not always stand straight up, but often leans over to one side. So sometimes the upper part of it would be over toward the candle, and sometimes over away from it. The earth leans over too in this same manner; and that is the reason why we have summer and winter. When by this leaning our part of the earth is toward the sun, we get more heat, and have a warm season; when we are leaning away from the sun, and are more in the shadow, the cold weather comes, and continues until we get into a good position to be warmed up again.
"A kind Providence brings this all around very regularly, and there is no danger of our being kept so long in the cold that we would freeze to death. Everything works like a clock that is never allowed to run down or get out of order. In spinning, the earth carries us round twelve or fifteen times as fast as the fastest railway train has ever yet been made to run; and in making its circle round the sun, it moves as fast as a shot from a gun."
"Oh! oh!" exclaimed the children; and Joe asked, "Why are we not all dashed to pieces?"
"Because," said the Professor, "we do not run against anything large enough to do any harm; and we do not realize how fast we are moving, or that we are moving at all, because we do not pass near anything that is standing still. You know that in riding we look at the trees and fences by the road-side to see how rapidly we are going. The hills in the distance do not show our speed, but seem to be following us. Unless we look outside we can not know anything about it, excepting, perhaps, we may guess from the noise and jostling of the vehicle. But as the earth moves smoothly and without the least noise, we would think it stood entirely still did not astronomers assure us of its wonderfully rapid motion. It took them a great while to find it out. When they began to suspect it there was a great dispute over it. Some said it moved; others said it did not. The two parties were for a time very bitter against each other; but now all agree in the belief of its rapid motion."
"A queer thing to quarrel about, I must say," remarked Gus. "I wouldn't have cared a straw whether it moved or not, if I could only have been allowed to move about on it as I pleased."