WEIGHING THE WORLDS 



a few more years" the spectroscope determination of 

 the sun's distance with a degree of accuracy equal 

 to that of any other method. 



Meantime the various types of measurement are 

 so closely in harmony that we may look with much 

 confidence on the figure 92,897,000 miles as represent- 

 ing the earth's average distance from the sun. This, 

 then, is the unit distance which serves as a yard stick 

 in measuring the planetary distances. With its aid 

 we compute the mass of the sun, and also, as will 

 appear in a moment, the masses of all but two of the 

 fraternity of major planets. 



WEIGHING THE PLANETS 



In determining the mass of a planet, no new 

 principle is involved. All that is necessary is to note 

 carefully the distance from the planet of one of its 

 satellites, and the'precise period of revolution of that 

 satellite. As we have seen, the rate of revolution at 

 any given distance is determined by the combined 

 masses of the planet and satellite. So what will really 

 be determined when the other data are known is the 

 joint mass of planet and satellite. But with the single 

 exception of the moon, the satellites are so small in 

 comparison with the bulk of their primaries that their 

 weight may virtually be disregarded, and for the pur- 

 poses of rough calculation it may be assumed that 

 the entire mass of the system is located in the planet 

 itself. 



Speaking in astronomical terms, the planets are 

 relatively near the earth, and it is possible, with the 

 modern instruments, to measure the orbits of their 



81 j 



