The same thing holds in regard to comets. If one of these infrequent fires of unusual shape have made its appearance, everybody is eager to know what it is. Blind to all the other celestial bodies, each asks about the newcomer; one is not quite sure whether to admire or to 5 fear it. Persons there are who seek to inspire terror by forecasting its grave import. And so people keep asking and wishing to know whether it is a portent or a star. But, by my honour, no one could embark on a more exalted study, or master a more useful branch of knowledge than that which treats of the nature of the stars and planets. Are they a concentration of flame as our vision avers, and as the very light that streams from them,[91] and the heat that descends from them suggest? 6 Or are their orbs not of flame, but, as it were, solid bodies of earth that glide through tracts of fire, and having no light of their own draw thence their brightness and heat? That is an opinion that has been held by great men who have believed the stars to be compact of hard material, and to be nourished by fire that is not their own. Flame by itself, they argue, would be dissipated and would have nothing to hold or to be held by. If it were merely massed and not attached to a solid body, the universe would assuredly long since have scattered it in its impetuous whirl. [91] The common reading, _aliis_ = others, seems an error for _illis_ = them. II In view of this inquiry it will be well to ask whether comets are 1 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 2 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 3 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.