the observators standing in Jupiter, make the self same changes of Figure, that to us upon the Earth, the Moon doth make. You see now how these three things, which at first seemed dissonant, do admirably accord with the Copernican Systeme. Here also by the way may Simplicius see, with what probability one may conclude, that the Sun and not the Earth, is in the Centre of the Planetary conversions. And since the Earth is now placed amongst mundane Bodies, that undoubtedly move about the Sun, to wit, above Mercury and Venus, and below Saturn, Jupiter, and Mars; shall it not be in like manner probable, and perhaps necessary to grant, that it also moveth round?
Simpl.These accidents are so notable and conspicuous, that it is not possible, but that Ptolomy and others his Sectators, should have had knowledge of them, and having so, it is likewise necessary, that they have found a way to render reasons of such, and so sensible appearances that were sufficient, and also congruous and probable, seeing that they have for so long a time been received by such numbers of learned men.
Salv.You argue very well;The Principal scope of Astronomers, is to give a reason of appearances. but you know that the principal scope of Astronomers, is to render only reason for the appearances in the Cælestial Bodies, and to them, and to the motions of the Stars, to accomodate such structures and compositions of Circles, that the motions following those calculations, answer to the said appearances, little scrupling to admit of some exorbitances, that indeed upon other accounts they would much stick at. And Copernicus himself writes,Copernicus restored Astronomy upon the suppositions of Ptolomy: that he had in his first studies restored the Science of Astronomy upon the very suppositions of Ptolomy, and in such manner corrected the motions of the Planets, that the computations did very exactly agree with the Phænomena, and the Phænomena with the supputations, in case that he took the Planets severally one by one. But he addeth, that in going about to put together all the structures of the particular Fabricks, there resulted thence a Monster and Chimæra, composed of members most disproportionate to one another, and altogether incompatible; So that although it satisfied an Astronomer meerly Arithmetical, yet did it not afford satisfaction or content to the Astronomer Phylosophical.What moved Copernicus to establish his Systeme. And because he very well understood, that if one might salve the Cælestial appearances with false assumptions in nature, it might with much more ease be done by true suppositions, he set himself diligently to search whether any amongst the antient men of fame, had ascribed to the World any other structure, than that commonly received by Ptolomy; and finding that some Pythagoreans had in particular assigned the Diurnal conversion to the Earth, and others the annual motion also, he began to compare the appearances, and particulari-ties
