292 The Outline of Science 



not true that they are more powerful than those produced by the 

 moon. The sun's tide-producing power is, as a matter of fact, 

 less than half that of the moon. The reason of this is that distance 

 plays an enormous role in the production of tides. The mass of 

 the sun is 26,000,000 times that of the moon; on the other hand 

 it is 386 times as far off as the moon. This greater distance more 

 than counterbalances its greater mass, and the result, as we have 

 said, is that the moon is more than twice as powerful. Some- 

 times the sun and moon act together, and we have what are called 

 spring tides; sometimes they act against one another, and we have 

 neap tides. These effects are further complicated by a number 

 of other factors, and the tides, at various places, vary enormously. 

 Thus at St. Helena the sea rises and falls about three feet, 

 whereas in the Bay of Fundy it rises and falls more than fifty 

 feet. But here, again, the reasons are complicated. 



17 

 Origin of the Moon 



But there is another aspect of the tides which is of vastly 

 greater interest and importance than the theory we have just 

 been discussing. In the hands of Sir George H. Darwin, the son 

 of Charles Darwin, the tides had been made to throw light on 

 the evolution of our solar system. In particular, they have il- 

 lustrated the origin and development of the system formed by our 

 earth and moon. It is quite certain that, long ages ago, the earth 

 was rotating immensely faster than it is now, and that the moon 

 was so near as to be actually in contact with the earth. In that 

 remote age the moon was just on the point of separating from the 

 earth, of being thrown off by the earth. Earth and moon were 

 once one body, but the high rate of rotation caused this body to 

 split up into two pieces; one piece became the earth we now 

 know, and the other became the moon. Such is the conclusion 

 to which we are led by an examination of the tides. In the first 

 place let us consider the energy produced by the tides. We see 



