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 quantities 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.