We inhabitants of the earth enjoy a piece of good fortune to which we give very little thought, which, indeed, we take almost as much for granted as the air we breathe—I mean the fact that we have a transparent atmosphere. Some of the other planets, for instance Venus and Jupiter, have atmospheres which are so thick with clouds as to be totally opaque. If we had been born on Venus or Jupiter, we should have lived our lives without ever seeing through the clouds, and so should have known nothing of the beauty and poetry of the night sky, and nothing of the intellectual excitement and joy of trying to decipher the meaning of the vast panorama of lights which are scattered round us in all directions in space.

It will not form a bad approach to our subject, if we imagine that until to-night our earth had also been covered in by an opaque blanket of clouds. Suddenly this is rolled back, and we see the glory, and the tantalising puzzle, of the night-sky for the first time.

Our first impression would probably be that the stars were some sort of illumination of lamps or lanterns suspended above our heads, perhaps at only a few miles, or even yards, distance—rather like the lights in the roof of a vast tent or hall.