WEIGHING THE WORLDS 



mass of matter if at rest will remain forever at rest 

 unless operated upon by some disturbing force, and 

 that a mass in motion tends if undisturbed to move on 

 in a straight line at an unvarying rate of speed for- 

 ever. Applied to any body that is revolving in an 

 orbit, the result is this: at any given moment the 

 body tends to take a tangential course which would 

 carry it off in a right line farther and farther from 

 its primary; but the attraction of gravitation pulls 

 it away from the tangential course, so that in effect 

 it falls toward the primary. If the two tendencies 

 just balance, the body neither flies off into space nor 

 falls actually nearer its primary, but maintains an 

 orbital course at a uniform distance. Such is, in 

 point of fact, the condition of the earth itself in re- 

 volving about the sun, and of the moon in its course 

 about the earth. 



It is clear that the rate of revolution of a small 

 body at a given distance from its primary is largely 

 dependent upon the mass of the primary itself. If, 

 for example, the mass of the sun were to be mate- 

 rially changed, the entire solar system would at once 

 be thrown out of equilibrium, and all the planets 

 would of necessity seek new orbits before that equi- 

 librium could be restored. 



But it must be borne in mind that a planetary 

 body does not in a strict interpretation revolve about 

 its primary; the fact being that primary and satellite 

 revolve about a common center. Where the primary 

 is enormously larger than the satellite, however, as 

 in the case of our sun and his planetary family, the 

 motion of the larger body may practically be ignored 



