WEIGHING THE WORLDS 



having a satellite of measured distance and revolu- 

 tion period may be computed without difficulty. It 

 must not be supposed, however, that it is an alto- 

 gether simple matter to make accurate determina- 

 tion of the orbital distance and period of revolution 

 of a satellite in the case of the more distant planets. 

 It will be recalled that both Uranus and Neptune are 

 invisible to the naked eye. The satellite of Neptune 

 is barely visible under the resolving powers of the 

 largest telescopes; and the same thing is true of the 

 smaller satellites of even such relatively near planets 

 as Jupiter and Saturn. Yet the smallest satellites, 

 particularly if they are relatively far removed from 

 their primaries, are precisely the ones best adapted 

 for accurate observation of their orbital distances. 

 Hence the accurate weighing of the planets has been 

 greatly facilitated by the use of the large modern 

 telescopes; and, indeed, was in the case of the outer 

 planets quite impossible of performance until the 

 great instruments of the Yerkes and Lick and Mt. 

 Wilson Observatories were installed. 



Now, however, it is certain that the masses of 

 Mars, of Jupiter, of Saturn, of Uranus, and of Nep- 

 tune have been determined with a fair degree of 

 accuracy. As the 'diameters of these planets can of 

 course be measured by direct observation (their ex- 

 act distance at the particular time of observation 

 being determined) their actual size and hence the 

 average density of their structures may be computed; 

 average density being, of course, expressed in the 

 ratio between the bulk and the mass of any given 

 structure. 



83 



