WEIGHING THE WORLDS 



any other known member of the sun's planetary 

 family. Moreover it fortunately chanced that Eros 

 was at the time of its discovery approaching the 

 portion of its orbit which would bring it nearer the 

 earth. Sir David Gill, at the Cape Town Observa- 

 tory, had made very accurate determinations of the 

 sun's distance by testing the planetoids Victoria, 

 Isis, and Sappho, notwithstanding their distance. It 

 seemed obvious that Eros would afford still better 

 opportunity for making the all-important measure- 

 ment. " i< 



The expectations were fully realized. In the 

 winter of 1900-1901, Eros was at the nearest point, 

 and elaborate series of observations were made, 

 chiefly by the photographic method, at many 

 different observatories, with an eye to the detection 

 of the solar parallax. The computations from these 

 observations are matters of complex mathematics, 

 but the preliminary discussions gave results closely 

 in accord with the previous estimates of the sun's 

 parallax. 



THE SPECTROSCOPIC METHOD 



The newest method of measuring the sun's dis- 

 tance is what seems to the layman the rather curious 

 one of aiming a telescope at a distant star and record- 

 ing the positions of the spectroscopic lines by pho- 

 tography, and then aiming again at the same star six 

 months later and again recording the positions of 

 the lines. 



The explanation of this seeming puzzle is that the 

 lines in the spectrum (as we have elsewhere noted) 



77 



