234 A CENTURY'S PROGRESS IN ASTRONOMY. 



of our present days. The Moon will revolve 

 round the Earth in the same period that the 

 Earth rotates on its axis, and the two bodies 

 will perform their circuit round the Sun as if 

 united by a bar. 



Not only can we foresee the future of the 

 Earth-Moon System, but we can also read the 

 past. According to Darwin's theory, the Earth, 

 in the remote past, was probably rotating on its 

 axis in a very short period, between three and 

 five hours. The Moon must then have been 

 much nearer us than it is now, and was prob- 

 ably revolving round its primary in the same 

 period that the Earth took to rotate on its axis. 

 The two globes, then gaseous, must have been 

 revolving almost in actual contact. Had the 

 month been even a second shorter than the day, 

 the Moon must inevitably have fallen back on 

 the Earth. As it was, the condition of affairs 

 could not endure. The condition of the Moon 

 resembled that of an egg balanced on its point. 

 The Moon must either recede from the Earth or 

 fall back upon it. The solar tide here interfered, 

 and caused the Moon to recede from its primary 

 until it reached its present distance of 239,000 

 miles. 



The fact that the Earth and Moon were almost 

 in contact suggests that they were probably in 



