The Evolution of the Sciences 



them by establishing a certain number of precise 

 facts. This will be the more easy since we have 

 reached our knowledge of the sun by a few paths 

 only: by descriptive astronomy, measurement 

 of solar radiation, direct telescopic observation, 

 spectroscopy, and finally by observation of the 

 sun's eclipses. 



The chief purpose of descriptive astronomy 

 is to determine the chart of the heavens and the 

 movements of the stars. It shows us that the 

 sun is a sphere of 1,300,000 times greater volume 

 than the earth. Around the sun revolve our 

 globe and the other planets, according to the 

 laws discovered by Kepler and summed up by 

 Newton in the wonderful synthesis of universal 

 gravitation. These conclusions, which recapitu- 

 late an enormous number of observations and 

 measurements, have enabled us to determine 

 other facts of the greatest importance for our 

 investigations. Thus the elliptical trajectory 

 of the planets may be considered as due to a fall 

 of these bodies caused by solar attraction, in the 

 same manner that the orbit of a projectile fired 

 from a cannon is the result of terrestrial attrac- 



152 



