17. It has been alleged, as a proof of the likeness of the Fixed Stars to our Sun, that like him, they revolve upon their axes.[15] This has been supposed to be proved with regard to many of them, by their having periodical recurrences of fainter and brighter lustre; as if they were revolving orbs, with one side darkened by spots. Such facts are not very numerous or definite in the heavens. _Omicron_[16] in the constellation _Cetus_, is the longest known of them; and is held to revolve in 831 days. From the curious phenomena now spoken of, it has been called _Mira Ceti_.[17] _Algol_, the second star (_Beta_) of _Perseus_, called also _Caput Medusæ_, is another, with a period of 2 days 21 hours; and in this case, the obscuration of the light, and the restoration of it, are so sudden, that from the time when it was first remarked, (by Goodricke, in 1782,) it suggested the hypothesis of an opaque body revolving round the star. The star _Delta_, in the constellation _Cephus_, is another, with a period of 5 days 9 hours. The star _Beta_ in the _Lyre_, has a period of 6 days 10 hours, or perhaps 12 days 21 hours, one revolution having been taken for two. Another such star is _Eta Aquilæ_, with a period of 7 days 4 hours. These five are all the periodical stars of which astronomers can speak with precision.[18] But about thirty more are supposed to be subject to such change, though their periods, epochs, and phases of brightness, cannot at present be given exactly. 18. That these periodical changes in certain of the fixed stars are a curious and interesting astronomical fact, is indisputable. Nothing can be more probable also, than that it indicates, in the stellar masses, a revolution on their axes; which cannot surprise us, seeing that revolution upon an axis is, so far as we know, a universal law of all the large compact masses of matter which exist in the universe; and may be conceived to be a result derived from their origin, and a condition of any permanent or nearly permanent figure. But this can prove little or nothing as to their being like the sun, in any way which implies their having inhabitants, in themselves or in accompanying planets. The rotation of our Sun is not, in any intelligible way, connected with its having near it the inhabited Earth. 19. If we were to suppose some of the stars to be centres of planetary systems, we can hardly suppose it likely that these alone rotate, and that the others stand still. Probably all the stars rotate, more or less regularly, according as they are permanent or variable in form; but the most regular may still have no planets; and if they have, those planets may be as blank of inhabitants as our moon will be proved to be.