446 HISTORY OF FORMAL ASTRONOMV. 



reject the rest of Ptolemy's increase of latitude, 

 but in order to express it, devised librations of the 

 planes of the eccentric, depending not upon its own 

 eccentric, but (most improbably) upon the orbit 

 of the earth, which has nothing to do with it. I 

 always fought against this impertinent tying to- 

 gether of two orbits, even before I saw the obser- 

 vations of Tycho ; and I therefore rejoice much 

 that in this, as in others of my preconceived 

 opinions, the observations were found to be on my 

 side." Kepler established his point by a fair and 

 laborious calculation of the results of observations 

 3f Mars made by himself and Tycho Brahe; and 

 had a right to exult when the result of these cal- 

 culations confirmed his views of the symmetry and 

 simplicity of nature. 



We may judge of the difficulty of casting off 

 the theory of eccentrics and epicycles, by recollect- 

 ing that Copernicus did not do it at all, and that 

 Kepler only did it after repeated struggles; the 

 history of which occupies thirty-nine chapters of 

 his book. At the end of them he says, " This 

 prolix disputation was necessary, in order to pre- 

 pare the way to the natural form of the equations, 

 of which I am now to treat 6 . My first errour was, 

 that the path of a planet is a perfect circle ; an 

 opinion which was a more mischievous thief of my 

 time, in proportion as it was supported by the 

 authority of all philosophers, and apparently agree- 



6 De Stella Martis, iii. 40. 



