The Sun 



tion; our knowledge of Kepler's laws enables us 

 to calculate the velocity and the acceleration of 

 this fall and to deduce from them the force 

 which causes it. This force, solar attraction, 

 depends on the magnitudes of the two masses 

 attracting each other, and if one of these is 

 known it enables us to calculate the other. Now 

 we know the actual mass of the earth, thanks to 

 an experiment of the English physicist, Caven- 

 dish, repeated and improved by many subsequent 

 experimenters, and we are thus able to calculate 

 the mass of the sun. 



We are able then to weigh the stars, or, to be 

 more accurate, to compare their masses with the 

 mass of our kilogram. I do not reproduce the 

 formidable numbers which result from the neces- 

 sary series of calculations, because what interests 

 us is less the magnitude of the masses than 

 their ratio to the volumes of the stars, that is 

 to say, the densities of these bodies. We find 

 thus that, whereas the density of the earth is 

 approximately 5.5, that is to say five times 

 greater than the density of water, the density 

 of the sun is about 1.45, or four times less; these 

 are, of course, averages. On the earth, for 



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