Its immense disk, hanging overhead, would cover a circle of the firmament twenty degrees in diameter, or, in round numbers, forty times the diameter of the full moon as seen from the earth! It would shed a great amount of light and heat, and thus would more or less effectively supply the deficit of solar radiation, for we must remember that Jupiter and his satellites receive from the sun less than one twenty-fifth as much light and heat as the earth receives.
The maze of contending motions, the rapid flow and eddying of cloud belts, the outburst of strange fiery spots, the display of rich, varied, and constantly changing colors, which astonish and delight the telescopic observer on the earth, would be exhibited to the naked eye of an inhabitant of Jupiter's nearest moon far more clearly than the greatest telescope is able to reveal them to us.
Here, again, the mind is carried back to long past ages in the history of the planet on which we dwell. It is believed by some that our moon may have contained inhabitants when the earth was still hot and glowing, as Jupiter appears to be now, and that, as the earth cooled and became habitable, the moon gradually parted with its atmosphere and water so that its living races perished almost coincidently with the beginning of life on the earth. If we accept this view and apply it to the case of Jupiter we may conclude that when that enormous globe has cooled and settled down to a possibly habitable condition, its four attendant moons will suffer the fate that overtook the earth's satellite, and in their turn become barren and death-stricken, while the great orb that once nurtured them with its light and heat receives the Promethean fire and begins to bloom with life.
One of the first things that persons unaccustomed to astronomical observations ask to see when they have an opportunity to look through a telescope is the planet Saturn. Many telescopic views in the heavens disappoint the beginner, but that of Saturn does not. Even though the planet may not look as large as he expects to see it from what he has been told of the magnifying power employed, the untrained observer is sure to be greatly impressed by the wonderful rings, suspended around it as if by a miracle. No previous inspection of pictures of these rings can rob them of their effect upon the eye and the mind. They are overwhelming in their inimitable singularity, and they leave every spectator truly amazed. Sir John Herschel has remarked that they have the appearance of an "elaborately artificial mechanism." They have even been regarded as habitable bodies! What we are to think of that proposition we shall see when we come to consider their composition and probable origin. In the meantime let us recall the main facts of Saturn's dimensions and situation in the solar system.