[20] Hersch. _Outl. Astr._ 821. Another explanation of the variable period of Algol, is that the star is moving towards us, and therefore the light occupies less and less time to reach us.
[21] Humboldt, very justly, regards the force of analogy as tending in the opposite direction. "After all," he asks, (_Cosmos_, III. 373,) "is the assumption of satellites to the Fixed Stars so absolutely necessary? If we were to begin from the outer planets, Jupiter, &c., analogy might seem to require that all planets have satellites. But yet this is not true for Mars, Venus, Mercury." To which we may further add the _twenty-three_ Planetoids. In this case there is a much greater number of bodies which have not satellites, than which have them.
[23] What is said in Art. 15, that in consequence of the time employed in the transmission of visual impressions, our seeing a star is evidence, not that it exists now, but that it existed, it may be, many thousands of years ago; may seem, to some readers, to throw doubts upon reasonings which we have employed. It may be said that a star which was a mere chaos, when the light, by which we see it, set out from it, may, in the thousands of years which have since elapsed, have grown into an orderly world. To which bare possibility, we may oppose another supposition at least equally possible:--that the distant stars were sparks or fragments struck off in the formation of the Solar System, which are really long since extinct; and survive in appearance, only by the light which they at first emitted.
1. When it was discovered, by Copernicus and Galileo, that Mercury, Venus, Mars, Jupiter, Saturn, which had hitherto been regarded only as "wandering fires, that move in mystic dance," were really, in many circumstances, bodies resembling the Earth;--that they and the Earth alike, were opaque globes, revolving about the Sun in orbits nearly circular, revolving also about their own axes, and some of them accompanied by their Satellites, as the Earth is by the Moon;--it was inevitable that the conjecture should arise, that they too had inhabitants, as the Earth has. Each of these bodies were seemingly coherent and solid; furnished with an arrangement for producing day and night, summer and winter; and might therefore, it was naturally conceived, have inhabitants moving upon its solid surface, and reckoning their lives and their employment by days, and months, and years. This was an unavoidable guess. It was far less bold and sweeping than the guess that there are inhabitants in the region of the Fixed Stars, but still, like that, it was, for the time at least, only a guess; and like that, it must depend upon future explorations of these bodies and their conditions, whether the guess was confirmed or discredited. The conjecture could not, by any moderately cautious man, be regarded as so overwhelmingly probable, that it had no need of further proof. Its final acceptance or rejection must depend on the subsequent progress of astronomy, and of science in general.