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In view of this inquiry it will be well to ask whether comets are wholly analogous to stars and planets. They seem to have certain elements in common with them—for example, rising and setting—as well as their general form, although comets are more scattered, and end in a longer tail. They are alike, too, in their fiery bright appearance. So, if all the stars are earthy bodies, comets must share the same lot. But if the stars are pure fire and nothing else, remaining for six months at a time unbroken by the rapid whirl of the universe, then comets, too, may consist of some rarefied material, which is not broken up by the constant revolution of the sky. It will also tend to clear up this point if we endeavour to ascertain whether the earth stands still while the universe revolves round it, or whether the converse is the truth, the universe standing still while the earth revolves. There have been persons who made bold to say that it is we that all unwitting are borne round by the frame of things, that risings and settings are not produced by a movement of the heavens, but that we ourselves rise and set. The subject well deserves our study, if we are to know where we really stand, whether the abode we have obtained as ours is the most sluggish or the swiftest of motion, whether God causes all things to revolve round us or causes us to revolve. Now, for this it is essential that we have a record of all the appearances of comets in former times. For, on account of their infrequency, their orbit cannot as yet be discovered or examined in detail, to see whether they observe periodic laws, and whether some fixed order causes their reappearance at the appointed day. Such a development of astronomy is recent, having been lately introduced into Greece.