18 FROM NEBULA TO NEBULA 



These laws were to Kepler himself purely empir- 

 ical; he knew their truth only from observation, BUT 



NOT THEIR REASON WHY. 



In the latter part of the same century (seven- 

 teenth) Sir Isaac Newton published his celebrated 

 work, the Principia, in which, by a series of exquisite 

 theorems, he proved that Kepler's laws are precisely 

 consistent with the supposition that the body occupy- 

 ing the focus attracts the circulating one "directly as 

 the product of their masses and inversely as the square 

 of the intervening distance." He went even farther, 

 and showed that no otherwise conditioned force would 

 answer. This law is known as the law of gravitation, 

 and lies at the very foundation of astronomical science. 



But why doesn't the planet fall into the sun, if it is 

 so attracted? What is the counteracting or sustaining 

 force? These questions presented themselves to New- 

 ton, and he made an effort to answer them, taking the 

 specific case of the earth and moon as the most con- 

 venient for analysis and illustration. 



First, he distinguished the two counteracting forces 

 by name : that drawing the moon to the earth he called 

 centripetal, and the other propelling the moon forward 

 in its course he named centrifugal. 



Now, in Newton's time belief in the literal day of 

 the Mosaic cosmogony was all but universal. Hence 

 (although he did not say so in precise words) he saw 

 nothing unreasonable in assuming as a matter of course 

 that the Creator set the planets in motion at the same 

 time that he gave them their siibstance and form. Mod- 

 ern science is not satisfied with this naive supposition 

 of Newton's, but having not even a guess to offer in 

 its place as to how, when, or where the motion orig- 

 inated, it tacitly ignores altogether the question of 

 origin, while yet implicitly believing in the reality and 



