Origin of Eight-Year Generating Cycle 99 



upon which he based his conclusions as to the long pe- 

 riod of rotation were verified at the Lowell Observatory, 

 and in addition new markings were discovered and pho- 

 tographed. ''By watching them assiduously it was pos- 

 sible to note that no change in position occurred in 

 them, first through an interval of five hours, then 

 through one of days, then of weeks. Care was taken to 

 guard against illusion. It thus became evident that 

 they bore always the same relation to the illuminated 

 portion of the disk. This illuminated part, then, never 

 changed. In other words, the planet turned always the 

 same face to the Sun. The fact lay beyond a doubt, 

 though of course not beyond a doubter." ^ Thirteen 

 years after these first discoveries Professor Lowell re- 

 peated, in 1909, the account of his early work and then 

 said ''the years that have passed since these observa- 

 tions were made have brought corroboration of them. 

 Several observers at Flagstaff have seen and drawn 

 them and added discoveries of their own." ^ 



The conclusion that the period of rotation of Venus 

 upon its axis is the same as its period of revolution 

 around the Sun was reached by Schiaparelli and Lowell 

 from telescopic observations. There was, of course, 

 the possibility, though not the probability, that both 

 observers had been misled by visual errors. But no 

 such deception could occur with the spectroscope, and to 

 make sure of his ground Lowell installed at his observa- 

 tory a spectroscopic equiqm.ent. The investigator was 

 Dr. Slipher, the present Director of the Lowell Observa- 

 tory. The accuracy with which Dr. Slipher could de- 



1 Percival Lowell, The Evolution of World, pp. 77-79. 

 ^ Ibid., p. 79. 



