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.

CHAPTER VII

SATURN, A PRODIGY AMONG PLANETS

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.

Saturn is the second of the major, or Jovian, group of planets, and is situated at a mean distance from the sun of 886,000,000 miles.  We need not consider the eccentricity of its orbit, which, although relatively not very great, produces a variation of 50,000,000 miles in its distance from the sun, because, at its immense mean distance, this change would not be of much importance with regard to the planet’s habitability or non-habitability.  Under the most favorable conditions Saturn can never be nearer than 744,000,000 miles to the earth, or eight times the sun’s distance from us.  It receives from the sun about one ninetieth of the light and heat that we get.

[Illustration:  SATURN IN ITS THREE PRINCIPAL PHASES AS SEEN FROM THE EARTH.  From a drawing by Bond.]

Saturn takes twenty-nine and a half years to complete a journey about the sun.  Like Jupiter, it rotates very rapidly on its axis, the period being ten hours and fourteen minutes.  Its axis of rotation is inclined not far from the same angle as that of the earth’s axis (26 deg. 49 min.), so that its seasons should resemble ours, although their alternations are extremely slow in consequence of the enormous length of Saturn’s year.