366 ASTRONOMY 



not have been justified in introducing into his work an hypothesis 

 which then seemed contrary to all human experience. His work 

 remained a pure geometric conception designed to serve the practical 

 end of grouping under one idea the representation of observed celes- 

 tial motions. From this procedure continued, the truth would be 

 sure to emerge. 



At its inception the invention of the heliocentric hypothesis by 

 Copernicus was virtually no more than a development and improve- 

 ment upon the fundamental conception of Ptolemy. The principle 

 of uniform circular motion was retained. The removal of the centre 

 of reference from the earth to the sun was the characteristic feature 

 of this hypothesis, and this was a distinct improvement in the geo- 

 metric ideal. From the standpoint of the philosopher of that time 

 it could be regarded as no more than this. In defending his new sys- 

 tem Copernicus was not able to advance a single reason more con- 

 vincing than that due to the simplification of planetary computa- 

 tions thus brought about. The telescopic discoveries of Galileo were 

 yet to be made. Nothing was then known of the laws of motion, or 

 of gravitation. The annual apparent motion of stars upon the sky 

 due to reflex effects of the motion of the earth was then unknown 

 and could not have been ascertained by any means of measurement 

 then available. Yet all these sources of proof, and more, are now- 

 needed for the establishment of the theory of the earth's motion on 

 a really sound philosophic basis. 



We may, then, fairly denominate the invention of Copernicus as 

 a step in astronomical development not greatly different in its inner 

 philosophical quality from the steps previously introduced by Hip- 

 parchus and Ptolemy. It resulted in improved representation of plan- 

 etary motion, and brought the astronomer one long stage nearer his 

 goal, the search for a demonstrable, objective reality as the basis 

 of planetary motion. 



The new system, besides admitting of greater simplicity and per- 

 fection in the computations, lent an appearance of reality to repre- 

 sentation of the relative distances of planets from the earth in suc- 

 cessive intervals of time. This was of capital advantage to Kepler in 

 his research. The excellent observations of Tycho Brahe had con- 

 vinced Tycho and Kepler that the existing hypothesis of planetary 

 motion, as resulting from compounded uniform circular motions, was 

 no longer tenable even as a device of computation. Yet it should be 

 observed that the existing planetary tables available for the criti- 

 cism of Kepler served him a most useful purpose in his approxima- 

 tions toward the true elliptical forms of planetary orbits and the equa- 

 ble description of areas. The desire to picture celestial motions as they 

 actually take place in space, and to account in that way for the appar- 

 ent trace of those motions on the sky, as actually observed, appears 



