136 OUTLINES OF NATURAL PHILOSOPHY. 



three bodies are in a straight line, or not distant 

 from it by more than certain limits, an eclipse 

 always takes place. 



139. Hence it is evident, that an eclipse hap- 

 pens, in consequence of one of the two opaque 

 bodies, the Earth and the Moon, being so placed 

 as to prevent the Sun's light from falling on the 

 other. 



The interposition of the moon between the sun and 

 the earth, produces an Eclipse of the Sun ; and 

 the interposition of the earth between the moon 

 and the sun, so that its shadow falls on the moon, 

 or on any part of the moon, produces an Eclipse 

 of the Moon. The whole of the phenomena of 

 eclipses admit of explanation, on these principles. 



140. As the return of eclipses must depend on 

 the return of the line of the syzygies to the line 

 of the nodes ; and as the mean angular motion 

 of these lines is known, the periods at which 

 eclipses would return, were there no irregulari- 

 ty in the motions of the earth and of the moon, 



may be easily calculated. 





 a. The time of a lunation, or of one revolution of the 



line of the syzygies, is 29 d .530588, as already 

 stated ; and a revolution ol the line of the nodes, 

 relatively to the sun, is 346^.61963. If, by the 

 method of continued fractions, we seek for smaller 



numbers, 



