Copernicus, whose lead Moestlin also followed with regard to the motion of the earth.
That the great Danish astronomer did not become convinced of the truth of the Copernican system, but, on the contrary, set up a system of his own founded on the immovability of the earth, may appear strange to many who are unacquainted with the state of astronomy in the sixteenth century, and it may to them appear to show that he cannot have been such a great reformer of astronomical science, as is generally supposed. But it is not necessary to concoct an apology for Tycho; we shall only endeavour to give an intelligible and correct picture of the state of science at that time with regard to the construction of the universe.
That Copernicus had precursors among the ancients who taught that the earth was in motion, is well known, and he was well aware of this fact himself. But none of those precursors had done more than throw out their ideas for the consideration of philosophers; they had not drawn the scientific conclusions from those ideas, and had not worked them into a complete system by which the complicated motions of the planets could be accounted for and made subject to calculation. Neither had this been done by the philosophers who made the earth the centre of the universe, and let it be surrounded by numerous solid crystal spheres to which the heavenly bodies were attached. All this was only philosophical speculation, and was not founded on accurate observations; but the only two great astronomers of antiquity, Hipparchus and Ptolemy, have handed down to posterity a complete astronomical system, by which the intricate celestial motions could be explained and the positions of the planets calculated. But this "Ptolemean system," in which a planet moved on an epicycle, whose centre moved on another circle (the deferent), with a velo-
