NAVIGATION. 99 



able for finding longitude at sea was that great 

 natural chronometer presented by the moon in 

 her orbital motion round the earth. 



Imagine a line joining the centres of inertia 

 of the earth and moon to be, as it were, the 

 hand of a great clock, revolving round the 

 common centre of inertia of the two bodies, 

 and showing time on the background of stars 

 for dial. If the centres of inertia of the moon 

 and earth moved uniformly in circles round the 

 common centre of inertia of the two, the moon, 

 as seen from the earth, would travel through equal 

 angles of a great circle among the stars in equal 

 times ; and thus our great lunar astronomical 

 clock would be a perfectly uniform timekeeper. 

 This supposition is only a rough approximation 

 to the truth ; and the moon is, in fact, a very 

 irregular chronometer. But thanks to the mathe- 

 maticians, who, from the time of Newton, have 

 given to what is called the Lunar Theory in 

 Physical Astronomy the perfection which it now 

 possesses, we can tell, for years in advance, 

 where the moon will be relatively to the stars, at 



H 2 



