68 FROM NEBULA TO NEBULA 



coveries of Copernicus b<pen delayed a half century or so 

 longer; for by that time Galileo (1564-1642) had im- 

 proved the telescope, and with its help had observed the 

 phases of Venus and followed the satellite system of Ju- 

 piter through several cycles of rotation. Had Copernicus 

 been in possession of these significant facts, much of the 

 glory of his achievement would have been lost. 



Fortunately the cycle-and-epicycle fancy was likewise 

 destined not to survive long; for John Kepler (1571- 

 1630), in the year 1627, gave it its quietus when he pub- 

 lished his three laws of planetary motion, as follows : 



1. The planets revolve in ellipses. 



2. The radius vector passes over equal areas in 

 equal times. 



3. The cubes of their mean distances are propor- 

 tional to the squares of their periodic times. 



Although later researches have shown these laws not 

 to be exactly true, they are so close an approximation to 

 the truth that modern astronomers seem to have tacitly 

 agreed among themselves to shut their eyes to the ob- 

 served discrepancies as immaterial alas, to the very 

 great detriment of the science ! In estimating the true 

 value of Kepler 's contribution to astronomy, it is impor- 

 tant to remember that his discoveries were purely empiri- 

 cal, and that they were arrived at without reference either 

 to the principle of gravitation or to the fact of the sun's 

 flight. The story of Kepler 's labors and privations in the 

 pursuit of his quest is a tragic one, all the more so because 

 of the perverse genius he displayed in so long eluding the 

 obvious. If only he had had the sagacity when he began 

 his labors to ask himself the seemingly self -propounding 

 question, May not the planets move in curves other than 

 circles? he would unquestionably have shortened to a 

 period of weeks, or even days, the score of years that his 

 task actually consumed. What a travesty on human in- 

 telligence ! Fourteen centuries to evolve the simple idea 

 that the earth, instead of the universe, rotates on its axis 

 once in twenty-four hours ! Fifteen centuries to prompt 



