Even at this unthinkable speed, it would take us about four years to reach the nearest star. Does it really take light several years to travel from a star to us? It does; it takes light hundreds of years to reach us from some of the distant stars. If one of these far-distant stars should disappear suddenly, you and I would know nothing about the change. When we looked up into the sky we should still see the star shining away year after year. Our children and our grandchildren would still believe the star to be where it had been always, for the last rays of light, sent out by the star before it disappeared, would take hundreds of years to reach this world. There is no hope of our ever reaching these very distant stars in our imaginary flying machine; it would be too big a tax on our imagination to live in the cabin of our machine for hundreds of years.

For the present we shall be content with a trip to the moon. But one boy says that he cannot imagine our flying machine forcing its way through the air at the enormous speed at which light travels. Even when traveling by motor car, at the comparative snail-pace of, say, forty miles per hour, we have to see that hats and veils are well fastened on. This boy says that he has been told that a good deal of the additional energy required to drive a train at sixty miles an hour is used up in forcing the train through the ocean of air.

We quite agree with this boy that it would be very difficult to imagine our machine traveling through the air at the speed of light. We shall get over this difficulty by making our machine not exceed one hundred miles per hour so long as we are in the ocean of air. When we get beyond this great blanket of air, we can increase our speed to that of light.

I am very glad that we have come to this arrangement, for I had just been thinking that our journey to the moon would be a very disappointing one, as it would have taken only about one second to get there. Now our journey will be much slower and we shall have time to look about us.

When we emerge from the misty clouds we cannot see the instruments for the moisture on the window. By the time it clears we are about two miles upwards off the earth. We find that the temperature has fallen down below freezing-point. We are rather surprised, as it looks lovely and warm in the sunshine. The moment we shoot above the clouds we are in the sunshine, with the sea of clouds below us. Our thermometer is placed in the shade, so that the direct rays of the sun cannot reach it, as we wish to know the temperature of the surrounding air.

The reason why it is colder here than down on the surface of the earth is that we have climbed out of the thickest part of the great blanket of air which serves to keep you warm. The air is what you might describe as much thicker near the earth, but, to speak more correctly, we should say that the air is much denser at the surface of the earth. Up here it is much flimsier. One girl says that she climbed a high mountain with her father, on a beautiful summer day, and it was so cold on the top of the mountain they could not stay there many minutes.

We keep rushing upwards and in half an hour we know that we must be about fifty miles from the earth, as we have been traveling at the rate of one hundred miles per hour. The air at this height is so rare that we could not possibly live in it. It is a good thing that we are in an enclosed cabin, and have plenty of fresh air in liquid form, which gives off ordinary air just at the rate we require it.

We know that in about an hour and a half we should be clear of the atmosphere altogether, and that we may travel thereafter at the speed of light, which means about eleven million miles per minute.

One little girl says she is feeling very sleepy, and as she was too excited to sleep much last night, I advise her to lie down and have a sleep now; we shall waken her when we arrive at the moon.

Imagine then that we have arrived at the moon. It looks like our own world, "the earth," only there is nothing growing on it; it seems to be all bare rocks. But the mountains look very high; some of them are really higher than the highest mountains on the earth.

Most of the children in our cabin say that they thought the moon was quite smooth, and had a sort of polished surface. When they were told that the moon did not send out light, but merely reflected the light of the sun, they pictured some sort of mirror-like surface.

We are startled by a sudden cry of surprise behind us; the little girl who was sleeping has wakened up, and looking out of the window behind us, she calls: "Oh! look how huge the moon has grown! It is far, far bigger than the sun!"

Looking out at her window, we see what does look like a giant moon.

But our little friend is very much surprised when I tell her that that is old Mother Earth, and that the rocky mountains close to us are part of the moon. But how can the earth shine like the moon? In exactly the same way as the moon shines; both merely reflect the light sent out by the sun.

We can scarcely believe that this "huge half-moon", standing out so clearly against a jet-black sky, is really the earth which we left a few hours ago in broad daylight. But the people on the part of it which we speak of as shining like the moon are still in broad daylight. Those people living on the half of the great globe which appears dark to us are in darkness except for the light which is reaching them from this moon up here where we are at present.

One boy says that some of the astronomers on the earth may be looking at us through their telescopes, and he starts making Boy Scout signals with his arms. He is disappointed when I tell him that the very best telescope on earth would not enable the astronomers to see our flying machine even as a speck on the face of the moon. If our machine had been as big as one of our largest buildings, it could then have been seen as a tiny dot.

But we cannot help staring at that great ball on which we live, and which we left so recently. This distant view gives us a good idea of how much bigger the earth is than the moon. Of course if you look down at the moon at present you can get no idea of its size; you can see only a small part of it, just as you could see only a small part of the earth until you made this imaginary voyage skywards. But you can remember what the moon looked like when you saw it from the earth.

Looking back now at the far-distant earth, you would say that it was much more than four times the size of the moon. When we are comparing the sizes of the earth and the moon, and the other heavenly bodies, it is usual to speak of their diameters. The word _diameter_ is made out of two Greek words which mean _a measure through_. The diameter of the moon or of the earth is just the distance right through the center from one side to the other. We say that the diameter of the earth is four times as great as the diameter of the moon, but the earth could contain sixty-four moons.

Suppose we take a quiet cruise about the moon in our imaginary flying machine. One boy remarks that he is glad it is a fine day, for, although we cannot leave the cabin of our flying machine, we can see much better on a fine day. Another boy says he guesses that if we were to stay here for a hundred years it would be fine every day. He is quite right. There are no clouds around the moon, and so there cannot be any rain.

When we looked at the moon from the earth, the moon's surface seemed to be beautifully smooth, but now that we have traveled to it in our flying machine we find it to be anything but smooth. It seems to be covered entirely with high mountains, and great rings of mountains. One boy says that those large basins formed by the rings of mountains remind him of some mountains that he saw on the earth, and that he was told the mountains had formed the crater of a great volcano in the long ago.

But what is a volcano? Although a volcano is described often as a burning mountain, it is not really burning, but it has a large chimney in the center of it reaching far down into the earth. Great quant.i.ties of molten rock-material are hurled up this chimney while the volcano is active. There are a few active volcanoes on the earth today, but there are traces of very many more that have long ceased to be active.

One boy hopes that we may find an active volcano as we cruise around the moon, but he will be disappointed. The moon is no longer a hot body like the earth, and the earth used to be very much hotter than at present, when there were a great many volcanoes at work.

Looking at the moon, we can see that it must have been very hot at one time also; it seems so covered with volcanic mountains that one ring sometimes overlaps another.

But what is this huge dark-colored part of the moon? One boy guesses that it is a sea or a lake, and when I tell him that there is no water on the moon he is very much surprised. He says that there must have been water at one time, for he has seen a map of the moon, and it contained a lot of Latin names which his tutor told him meant the Sea of Rains, the Sea of Tranquillity, the Lake of Dreams, and so on.

No doubt the map which the boy saw was quite a correct map of the moon, for we speak of those dark patches as seas and lakes, although we know now that no water exists, and probably never existed, on the moon.

It is those great dark patches on the moon which seem to form the man's face; this dark patch which we are coming to is the one which forms his right eye. It is called the Sea of Rains.

How bright the mountains look in the sunshine, and how black their shadows are. That is because there is no air. Notice how very clearly you can see the far-distant mountain ranges. You can scarcely believe me when I tell you that the mountains on either side of that great crater are more than one hundred miles apart. But do you not remember how the mountains on the earth have appeared to us sometimes to be very much nearer than usual? If you asked the reason for this, you were told that it was because the atmosphere happened to be particularly clear. Well! here on the moon we have no air at all and that is why we can see everything so very clearly.

One little girl says that she had been wondering why there is nothing growing on the moon, but now she sees that not even gra.s.s, nor moss, nor heather could grow, since there is no air and no water. Not only are the mountains bare rock, but the whole surface of the land is the same.

--_From "The Stars and Their Mysteries", by Charles R. Gibson, F.R.S.E._

QUESTIONS

1. Why can no aeroplane ever fly to the moon?

2. How long would it take an aeroplane flying 100 miles an hour to reach the moon?

3. How much farther away is the sun than the moon?

4. Give some idea how far away the fixed stars are.

5. Why is it colder as we rise up in the air?

6. What does the surface of the moon look like?

7. How would the earth look to a person on the moon?

8. How much larger is the earth than the moon?

9. What is the weather on the moon?

10. What is a volcano?

11. Why could you see a long distance on the moon?

12. Why does nothing grow on the moon?