30 CLASSICS OF MODERN SCIENCE 

 his discovery of the orbit of Mars, and to the establishment of the 

 first two of his three famous laws: ''i. the planet describes an 

 ellipse, the sun being in one focus; 2. the straight line joining the 

 planet to the sun sweeps out equal areas in equal intervals of time." 

 {Sedgwick and Tyler, pp. 211-21^). He published these laws in 

 idop in his ''Commentaries on the Motions of Mars.'' 



In 1611, when his patron, Emperor Rudolph, was compelled to ab- 

 dicate, Kepler was left penniless, but he moved to Linz where he was 

 appointed to a professorship. In i6ip he published his ''Harmony of 

 the World," which contained his third lazv: "The squares of the times 

 of revolution of any two planets (including the earth) about the sun 

 are proportional to the cubes of their mean distances from the sun." 

 {Sedgwick and Tyler, p. 2ij). This was the triumph about which 

 he wrote in the year of its discovery, 16 18: "What I prophesied 

 twenty-two years ago, as soon as I found the heavenly orbits were of 

 the same number as the five {regular) solids, what I fully believed 

 long before I had seen Ptolemy's Harmonies, what I promised my 

 friends in the name of this book, which I christened before I was six- 

 teen years old, I urged as an end to be sought, that for which I joined 

 Tycho Brake, for which I settled at Prague, for which I have spent 

 most of my life at astronomical calculations — at last I have brought to 

 light, and seen to be true beyond my fondest hopes. It is not eight- 

 een months since I saw the first ray of light, three months since the 

 unclouded sun-glorious sight! burst upon me. Let nothing confine 

 me: I will indulge my sacred ecstasy. I will triumph over mankind 

 by the honest confession that I have stolen the golden vases of the 

 Egyptians to raise a tabernacle for my God far away from the lands 

 of Egypt. If you forgive me, I rejoice; if you are angry, I cannot 

 help it. The book is written; the die is cast. Let it be read now or 

 by posterity, I care not which. It may well wait a century for a 

 reader, as God had waited six thousand years for an observer!* 

 Kepler died at Ratisbon, November 15, 1630. 



ON THE PRINCIPLES OF ASTRONOMY * 



What is astronomy f It is the science of treating of the causes of 

 those celestial appearances which we who live on the earth observe and 

 which mark the changes of times and seasons ; by the studying of 



* From The Epitome of Astronomy 



