The mention of eclipses reminds us, too, of the use which the moon has been in increasing, through them, our knowledge of the physical condition of the sun. If the moon had never intervened to cut off the blinding glare of the solar disc, we should have been to this day left to assume that the sun is all-contained by the dazzling globe that we ordinarily see. But, thanks to the moon’s intervention, we now know that the sun is by no means the mere naked sphere we should have suspected. Eclipses have taught us that it is surrounded by an envelope of glowing gases, and that it has a vast vaporous surrounding, beyond its glowing atmosphere, which appears to be composed of matter streaming away from the sun into surrounding space. With these discoveries still in their infancy, it is impossible to foresee the knowledge to which they will eventually lead, but they can hardly be barren of fruit, and whatever they ultimately teach will be so much insight gained into the sublimest problem that human science has before it—the determination of the source and maintaining power of the light and heat and vivifying agency of the sun. In according our thankful reflections to the moon for these revelations, we must not forget that, should there be inhabitants upon our neighbouring worlds, Mercury, Venus, and Mars, which have no satellites, they, the supposed inhabitants, can gain no such knowledge upon the surroundings of the ruler of the solar system. On the other hand, any rational being who may be supposed to dwell upon Saturn or Jupiter, would, through the intervention of their numerous moons, have, in the latter case especially, far more abundant opportunities of acquiring the knowledge in question than we have.
Finally, there is a use of the moon which touches us, author and reader, very closely. It has taught us of a world in a condition totally different from our own; of a planet without water, without air, without the essentials to life development, but rather with the conditions for life destruction; a planet left by the Creator—for wise purposes that we cannot fully know—as it were but half-formed, with all the igneous foundations fresh from the cosmical fire, and with its rough-cast surface in its original state, its fire and mould-marks exposed to our view. From these we have essayed to resolve some of the processes of formation, and thus to learn something of the cosmical agencies that are called forth in the purely igneous era of a planet’s history. We trust that we, on our part, have shown that the study of the moon may be a benefit not merely to the astronomer, but to the geologist; for we behold in it a mighty “medal of creation” doubtless formed of the same material and struck with the same die that moulded our earth; but while the dust of countless ages and the action of powerful disintegrating and denuding elements have eroded and obliterated the earthly impression, the superscriptions on the lunar surface have remained with their pristine clearness unsullied, every vestige sharp and bright as when it left the Almighty Maker’s hands. The moon serves no second-rate or insignificant service when it teaches us of the variety of creative design in the worlds of our system, and exalts our estimation of this peopled globe of ours by showing us that all the planetary worlds have _not_ been deemed worthy to become the habitations of intelligent beings.