CHAPTER V.. The Moon by Night.

THE MOON BY NIGHT.

How far off would you guess the Moon to be from our Earth?

A mile or two, perhaps you will say. Or twenty miles! Or forty miles! Or one hundred miles!

Even on Earth it is often puzzling to tell distances. If one is looking across a smooth surface, with nothing to break it, one cannot easily judge. I remember going in a sailing-boat, as a child, and after a good while saying, “Why, what a little way we have come! The shore looks only a mile or two off!” And I was told that it was at least ten miles off.

You see, there was nothing between to break the smooth water-surface, and so to show how far we had sailed.

If it is perplexing down here on Earth it is much more so up in the Sky. There, nothing lies between to break the great distance; and the stars seem so much alike, except that some are a little brighter and some a little dimmer. One might very easily suppose that the Moon and the Sun and all the Stars were at much the same distances from the Earth.

Yet nothing could be a greater mistake. Some are very near, and some are enormously far away.

That is to say, some are very near compared with others. But even the very nearest is a great deal farther off than fifty or a hundred miles.

Of all bright bodies in the sky, seen day after day and night after night from our Earth, not one ever comes so close as the round silvery Moon.

The Moon is our own especial companion. She always journeys with us, and never goes away.

Before we learn the distance of the Moon we have to think a little about her size. You have not yet learned about the size of our Earth, so we will take the two friends together.

There are two ways of measuring a ball or globe. We may say how big it is through the middle, from one side to the other. Or we may say how big it is round the outside. The outside measure is always about three times as much as the through-measure.

A large grape may be one inch through, and three inches round outside. A small orange may be two inches through, and six inches round outside. A large apple might be three inches through, and nine inches round outside. A small cocoa-nut might be four inches through and twelve inches round outside. A school-globe might be one foot through and three feet round outside; or two feet through, and six feet round outside. A balloon might be twenty feet through and sixty feet round outside. Or it might be thirty feet through, and ninety feet round outside.

The through measure is called the Diameter of a ball, and the outside measure is called its Circumference. A globe may be of any size; and it can be measured according to its size in inches, or feet, or yards, or miles.

Our Earth is a little less than eight thousand miles through, from side to side, or from north pole to south pole. Its outside measure, right round the equator, is nearly twenty-five thousand miles.

This will not give you any clear idea. It only sounds very large.

Think first of a mile. One mile is a good way for a little child to walk; but not much for a big boy. Some people count five or six miles a very long walk, while others think nothing of ten or twelve miles. Not many men can do as much as thirty or forty miles in a day.

But even fifty miles are only half of one hundred. And it takes ten hundreds to make one thousand. And the through-measure of our Earth is eight thousands of miles.

If that man, whose story you heard in the first chapter, ever had really done as the story says, and walked round the whole world, he would have gone about twenty-five thousand miles!

How long would that have taken him? Certainly very much longer than twenty-four hours. He could not possibly have got along at the rate of one thousand miles and more each hour. The fastest express train does not manage over sixty or seventy miles in an hour.

If he had journeyed all the while, Sundays and week-days alike, twenty miles each day, then he would have got round the world in three years and a half. And if he had only done ten miles a day he would have been nearly seven years getting round.

Of course no man could really cross the oceans on foot; but this will help you to a little notion of what the size of our Earth is.

A very big globe, is she not? yet not truly large, compared with other larger worlds in the sky.

Our Moon is not one of those larger worlds, however.

While the through-measure of the Earth is eight thousand miles that of the Moon is only a little more than two thousand miles. And while the Earth is nearly twenty-five thousand miles round outside, the Moon is only about six thousand miles.

So if we could put a knitting-needle straight through the Moon, with the ends just showing, one on each side, it would need to be only a quarter as long as a needle to go through the Earth. And a ribbon to fold round the Moon should be scarcely a quarter as long as a ribbon which could be folded just round the Earth.

This makes a good deal of difference in the sizes of the two globes; perhaps more than you would suppose.

I want you now to bring down the Moon, in your mind, to the size of a very small ball; only one inch through or three inches round outside. Picture her to yourself as getting smaller--and smaller--and smaller, till she is only the size of a very big grape.

Then think of the Earth also, as getting smaller and smaller, just in the same way. Only the Earth must not get so small as the little Moon, in your mind. It must still be four times as long in its through-measure--four inches instead of one inch. And while a little piece of tape, only three inches long, would go just round the tiny Moon, a tape to go round the little Earth would have to be twelve inches long.

Then, if the Moon is about the size of a big grape, or a small walnut, the Earth will be the size of a very large apple, or of a small cocoa-nut.

It would be a good plan to get a walnut and cocoa-nut of the right sizes, or, if you like, to find two balls, and to place them side by side. The little one must be one inch through, the bigger one must be four inches through. Looking upon them, you will see in a moment how great is the difference between the Earth and the Moon.

I shall often speak of these sizes, so it would be as well to fix them now in your mind, and have them there, ready for use.

Now as to the distance of the Moon from the Earth:

It is about two hundred and forty thousand miles!

A rope twenty-five thousand miles long would reach once round the whole Earth, if laid down on the equator.

But a rope to reach all the way from our Earth to the Moon would have to be more than nine times as long as the equator-rope.

You have tried to picture the Earth in your mind as brought down to the size of a small cocoa-nut, and the Moon as brought down to a walnut. In doing this we make one little half-inch do duty for a thousand miles; so that one inch stands for two thousand miles, and four inches means eight thousand miles.

The Moon is two thousand miles through; therefore a ball or walnut, to picture the Moon, must be one inch through. The Earth is eight thousand miles through; therefore a very big apple or small cocoa-nut, to picture the Earth, must be four inches through.

In the same way we will bring down the distance of the Moon from the Earth. We will let each thousand miles of all that space in the sky shrink into a tiny half-inch. Then, instead of two hundred and forty thousand miles, we shall only have to think of one hundred and twenty inches, which make ten feet.

So the smaller ball, or walnut, must be put ten feet off from the larger ball, or cocoa-nut. That will give you a picture, not only of the size of the earth, compared with the size of the Moon, but also of the distance between the two.

Besides putting the two balls ten feet apart you have to think of them as two little shining worlds.

That is not quite so easy, is it? Why should they shine?

We know that bodies in the sky do shine; but bodies on the Earth more commonly do not. By “bodies” I mean “things.” A marble does not shine, nor a grape, nor a walnut, nor an apple, nor an orange, nor a school-globe, nor a balloon.

At least they do not shine of themselves. Any of them can be made to shine a little, if not much, by being placed in bright sunshine.

Suppose that the two balls--the little imitation-Earth and the little imitation-Moon--were made of glass, or of some smooth metal, such as tin or silver. And suppose you were to hang them up, by wires, out-of-doors, in pitch darkness. Would they shine?

Certainly not. How could they?

But suppose there was another ball, also out in the darkness; a much larger ball, shining with great brilliance, like an electric light.

Would the glass or metal balls show any brightness then at all?

Yes; for the shining of the large brilliant ball would light them up, at least on one side, and would make them bright.

Light is always thrown back from a smooth surface. If you have a looking-glass in a dark room it does not shine; but if you hold it in full sunlight it flashes radiantly. Yet the looking-glass has no brightness of its own. It only takes and gives out again of the Sun’s light.

That is just how our Earth shines, and how the Moon shines. In themselves both are dull and dark worlds; but, like the looking-glass, they receive radiance from the Sun and give it out again.

Before we go on I want you to be quite clear in your mind as to what is really meant by this bringing down of large sizes to small sizes. In coming pages you will often hear of it again.

Suppose you have two very large toy-carts, big and heavy. One of them is four feet long and two feet wide, the other of them is three feet long and one foot and a half wide. And suppose that you are trying to explain, to somebody who has not seen them, how much bigger one cart is than the other cart.

You may do it by talking, and by showing with your hands about how high they each stand.

Or you may do it in quite a different manner, and much more exactly, by making a kind of little model of each cart--in paper, or cardboard, or wax.

The models would not of course be of the same size as the big carts, but they would have to keep what is called the same proportion of sizes. The bigger must still be the bigger; and the smaller must still be the smaller.

You could let one inch stand for one foot. Then the tiny model of the bigger cart--the cart which is four feet long and two feet broad--would only be four inches long and two inches broad. And the tiny model of the lesser cart--the cart which is three feet long and one foot and a half broad--would be only three inches long and one inch and a half broad.

Anybody looking on those two tiny model carts could not possibly tell how big the real carts are. But he could tell one thing. He could know how much bigger one cart is than the other.

This is what I hope to show you, by bringing down the sizes of worlds and moons--not how large they really are, but how much larger or how much smaller one is than another.

Also, by this means we learn to understand distances better.

If you are looking at a map of the world, or of a part of the world, the miles in that map have to be brought down into a very tiny space. A map of a country, made as large as the country itself, would take up a great deal too much room. So half-an-inch is made to do duty for perhaps fifty real miles, or a hundred real miles, or even a thousand real miles. In quite a small map, a continent or an ocean which is really two thousand miles across might be only one inch across.

And yet, looking at that map, small though it is, you are able to see how near one country is to your own, and how very much farther off another country is.

This is the way in which we are going to think about different worlds--those which are nearer and those which are farther. We have to make a sort of little map or model of them in our minds; letting one inch always picture two thousand real miles.

QUESTIONS.

1. What is meant by the diameter of a ball?

Its “through measure” from one side to the other, straight through the centre.

2. What is the circumference of a ball?

Its measure round the outside.

3. Which is larger, the through measure or the measure round outside?

The outside measure is about three times as large as the through measure.

4. Give an example or two.

A ball one inch through is about three inches round outside. A ball four inches through is about twelve inches round outside.

5. What is the Earth’s diameter?

The Earth is nearly 8,000 miles through.

6. And the Earth’s circumference?

The Earth is nearly 25,000 miles round at the equator.

7. What is the Moon’s diameter?

The Moon is about 2,000 miles through.

8. And the Moon’s circumference?

The Moon is about 6,000 miles round outside.

9. How far is the Moon from the Earth?

About 240,000 miles.

10. How long should a rope be to lie round the Earth on the equator?

About 25,000 miles.

11. How many such ropes would reach all the way from here to the Moon?

About nine such ropes joined together.

12. If we should let one inch stand for 2,000 miles, how large would the Moon be?

A ball one inch through.

13. How large would the Earth be?

A ball four inches through.

14. In that case, how far would the Moon be from the Earth?

About ten feet off.

15. How do the Earth and the Moon shine?

By giving out again, or throwing back, the sunlight which falls upon them.