26 Astronomy. [Lecture 24. 



junction with the sun and moon at the beginning 

 of the first of these lunations, will be within less 

 than half a degree of the line of conjunction with 

 the sun and moon again, when the last of these 

 lunations is completed. In that time, therefore, 

 there will be a regular period of eclipses, or re- 

 turns of the same eclipses, for many ages. But 

 the falling back of the line of conjunction of the 

 sun and moon, with respect to the line of the 

 nodes in every period, will at length exhaust it, 

 and after that it will not return again in less than 

 12,492 years*. 



If these principles are properly considered, it 

 will not be difficult to conceive how astronomers 

 are able to foretel the exact time when any phae- 

 nomenon of this kind will happen. For, as an 

 eclipse can only take place at the time of a new 

 or full moon, the chief requisites are to determine 

 the number of mean conjunctions and oppositions 

 that will occur in every year, and the true places 

 of the sun and moon in their orbits at each of 

 those times. And, if from this it appears that 

 the two luminaries are within the proper limits of 

 the node, there will be an eclipse, or otherwise 

 not, agreeably to what has been already ob- 

 served upon this subject-)-. 



But in order to facilitate these operations, we 

 have astronomical tables ready computed, by 

 which the places of the heavenly bodies and 



* Bonnycastlc's Astronomy. f Ibid. 



