298 HISTORY OF FORMAL ASTRONOMY. 



falsehood, and the alternations of hope and sorrow, of vexation and 

 triumph, through which he had gone. It will not be necessary for us 

 to cite many passages of these kinds, curious and amusing as they are. 



One of the most important truths contained in the motions of Mars 

 is the discovery that the plane of the orbit of the planet should be 

 considered with reference to the sun itself, instead of referring it to 

 any of the other centres of motion which the eccentric hypothesis in- 

 troduced : and that, when so considered, it had none of the librations 

 which Ptolemy aud Copernicus had attributed to it. The four- 

 teenth chapter of the second part asserts, " Plana eccentricorum esse 

 drdXavTa ;" that the planes are unlibrating ; retaining always the 

 same inclination to the ecliptic, and the same line of nodes. With 

 this step Kepler appears to have been justly delighted. " Copernicus," 

 he says, " not knowing the value of what he possessed (his system), 

 undertook to represent Ptolemy, rather than nature, to which, how- 

 ever, he had approached more nearly than any other person. For 

 being rejoiced that the quantity of the latitude of each planet was 

 increased by the approach of the earth to the planet, according 

 to his theory, he did not venture to reject the rest of Ptolemy's in- 

 crease 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 together of 

 two orbits, even before I saw the observations of Tycho ; and I there- 

 fore rejoice much that in this, as in others of my preconceived opin- 

 ions, the observations were found to be on my side." Kepler estab- 

 lished his point by a fair and laborious calculation of the results of 

 observations of Mars made by himself and Tycho Brahe ; and had a 

 riafht to exult when the result of these calculations confirmed his views 

 of the symmetry and simplicity of nature. 



We may judge of the difficulty of casting off the theory of eccen- 

 trics and epicycles, by recollecting 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. 7 My first error was, that the path of a planet is a perfect 

 circle ; — an opinion which was a more mischievous thief of my time, 



7 De Stella Martis, iii. 40. 



