68 IN STARRY REALMS. 



that in dealing with the history of our earth, as such 

 history is contemplated by the astronomer, we are to con- 

 sider not merely the thousands of years that include the 

 human period, not merely the millions of years that are 

 required by the necessities of geology, but also those un- 

 known periods anterior to geological phenomena to which 

 we have already referred. 



In the course of such vast ages the reaction of the 

 earth on the moon's orbit has not only become per- 

 ceptible, it has become conspicuous ; it has not on]} 

 become conspicuous, but it has become the chief deter- 

 mining agent in making the moon's orbit as we find it 

 at the present day. We have seen that as we look into 

 the past the length of the day seems ever shorter and 

 shorter ; and concurrent with this decline in the day is 

 the diminution in the moon's distance from the earth. 

 There was a tim<- when the moon, instead of revolving at 

 a distance of two hundred and forty thousand miles, as it 

 does at present, revolved at a distance of only two hundred 

 thousand miles. As we think of epochs still earlier we 

 discern the moon ever closer and closer to the earth, until 

 at last, at that critical time in the history of the earth- 

 moon system, when the earth was quickly revolving in a 

 period of a few hours, our satellite seems to have been 

 quite close to the earth ; in fact, the two bodies were 

 almost in contact. 



The study of the tides has therefore conducted us 

 to the knowledge of a remarkable configuration ex- 

 hibited in the primitive earth-moon system. The earth 

 was then spinning round rapidly in a day which was 

 only a few hours long, whilst flose to the earth, or almost 



