WEIGHING THE WORLDS 



precision that the diameter of the orbit of Jupiter is 

 so many times the radius of the earth's orbit even 

 though we were quite unable to translate either fig- 

 ure into terms of precise miles. 



Similarly we could estimate the relative bulk of 

 the various planets and the relative sizes of the 

 orbits of their various satellites, even though nothing 

 were known as to the exact distances in question. 

 But in order that our chart of the solar system should 

 take on the satisfying quality of a map drawn to a 

 known scale, and in order that we should translate 

 terms of relative bulk into terms of actual bulk, it 

 is essential to know the actual distance between one 

 pair or another of our charted bodies. One such 

 distance known, other distances may be computed. 

 But how are we to measure, in terms of miles or any 

 other precise unit, any one of the planetary distances? 



The most natural method that suggests itself 

 would be to take the "parallax" of the sun by apply- 

 ing the old familiar principle of triangulation the 

 same principle by which the seaman determines the 

 distance of a ship and the surveyor charts the topog- 

 raphy of a region to the measurement of the sun's 

 distance. Theoretically nothing more would be neces- 

 sary than to take the direction of the sun from two 

 stations at opposite points of the earth's surface, using 

 the earth's diameter therefore as a base line, and 

 noting the angular distance between the two lines of 

 observation. In default of stations precisely as locat- 

 ed, any two points fairly distant would seemingly 

 answer, just as the surveyor may use a variable base 

 line, a computation sufficing to make the necessary 



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