ASTRONOMY. 



numbers, that may nearly express this ratio, we 

 shall find 19 and 223 ; so that after 223 lunations, 

 the node has nearly performed 19 revolutions. 

 Jn 223 lunations, therefore, or 18 Julian years 10 

 days and 7 hours, the sun, the moon, and the node 

 are nearly in the same position with respect to one 

 another ; and the series of eclipses returns nearly 

 in the same order. 



This period is thought to be the SAROS of the Chal- 

 dean astronomers ; and their predictions of eclip- 

 ses were probably founded on it. It is particular- 

 ly mentioned by PLINY. A period of 521 Julian 

 years, is considerably more exact. LA LANDE, 

 Astron. 1503. 



I. The mean motions of the lines of the syzygies, and 

 of the nodes, are also used in another way in the 

 calculation of eclipses. Though the mean places 

 of those lines are different from the true, and 

 though it be on the latter that the phenomena of 

 eclipses depend, it is useful to have the mean places, 

 in order to know whether the circumstances are 

 such, that an eclipse can possibly happen or not at 

 a given new or full moon. The mean motions in 

 astronomical tables, afford the means of perform- 

 ing this calculation, which is also much abridged 

 by a table of what are called the Epacts, that is, 

 of the ages of the moon, (reckoning from the last 

 mean conjunction, and supposing her motion uni- 

 form), at the beginning of every yt ar. LA LANDE, 

 Astron. 1732. ; FftftQUSSOX, Astron. 



Eclipses 



