CHAPTER V THE MOON The Sun and Moon offer to our sight almost exactly the same apparent diameters; to the eye, they look the same size. But as we know the Sun to be 400 times as distant as the Moon, it is necessarily 400 times as large; its surface must exceed that of the Moon by the square of 400, or 160,000; its volume by the cube of 400, or 64,000,000. As the Sun is of low mean density, its mass does not exceed that of the Moon in quite the same high ratio; but it is equal in mass to 27,000,000 moons. Compared with the Sun, the Moon is therefore an insignificant little ball--a mere particle; but as a world for habitation it possesses some advantages over the Sun. The first glance at it in a telescope is sufficient to assure the observer that he is looking at a solid, substantial globe. It is not only substantial, it is rugged; its surface is broken up into mountains, hills, valleys, and plains; the mountains stand out in sensible relief; it looks like a ball of solid silver boldly embossed and chased. So far all is to the good for the purpose of habitation. Wherever men are, they must have a solid platform on which to stand; they must have a stable terrene whereon their food may grow, and this the Moon could supply. "The Earth's gloom of iron substance" is necessary for man here, and the Moon appears to offer a like stability. Another favourable condition is that we know that the Moon receives from the Sun a sufficient supply of light and heat. Each square yard of its surface receives, on the average, the same amount of light and heat that would fall upon a square yard on the Earth that was presented towards the Sun at the same inclination; and we know from our own experience that this is sufficient for the maintenance of life. And the Moon is near enough for us to subject her to a searching scrutiny. Every part of the hemisphere turned toward us has been repeatedly examined, measured, and photographed; to that extent our knowledge of its topography is more complete than of the world on which we live. There are no unexplored regions on our side of the Moon. The great photographs taken in recent years at the observatories of Paris and of the University of Chicago have shown thousands of "crater-pits," not more than a mile across; and narrow lines on the Moon's surface have been detected with a breadth less than one-tenth of this. An elevation on the Moon, if it rose up abruptly from an open plain, would make its presence apparent by the shadow which it would cast soon after sunrise or near sunset; in this way an isolated building, if it were as large as the great pyramid of Ghizeh, would also show itself, and all our great towns and cities would be apparent as areas of indistinct mottling, though the details of the cities would not be made out.