MIRACLES OF SCIENCE 



in all but the most delicate calculations. The centre 

 of revolution connecting the sun and the earth, for 

 example, lies almost immeasurably near the centre 

 of the sun. The balance is like that of a large man 

 swinging a tiny marble round and round at the end 

 of a string. The same thing is true of the relations 

 between most of the planets and their satellites. As 

 a rule the largest satellites have less than one-thou- 

 sandth the mass of their primaries. The only ex- 

 ception in the solar system, is found in the case of 

 the earth itself, our moon being vastly larger, pro- 

 portionately, than the satellite of any other planet. 



As it happens, the relatively large size of our 

 moon greatly facilitates the task of the astronomer 

 in weighing that body. The moon is, in point of 

 fact, of so significant a size that the centre of its 

 orbit of revolution is considerably removed from the 

 centre of the earth, although still within the circum- 

 ference of the earth itself. This centre of mutual 

 revolution lies, in point of fact, about 2880 miles from 

 the centre of the earth, or, stated otherwise, not 

 much over 1100 miles beneath the earth's surface. 

 About this centre the earth revolves in a monthly 

 orbit just as does the moon, and it is this fact that 

 supplies the astronomer with the easiest method of 

 computing the weight of our satellite. 



If we consider for a moment the mutual relations 

 of the earth and the moon, it will be obvious that the 

 earth, in describing its monthly revolution, in effect 

 wabbles back and forth, like a wheel rotating about 

 a point intermediate between its hub and its periph- 

 ery. If it could be viewed from a stationary point 



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