316 Astronomy. [Lecture 20. 



continue at A when the moon had completed its 

 orbit from D, through M, and to D again, it 

 would be exactly in conjunction, and the pe- 

 riodical and synodical month would be the same. 

 But as the earth does not continue at A, but 

 moves to B, and as the moon's orbit moves with 

 it, the diameter of that orbit will then be in the 

 position m d, and the moon will be at d. If 

 then the moon is at d, while the sun is at S, it 

 will be seen by the figure, that it cannot be in 

 conjunction, but must move to e 9 in the diameter 

 fe, and consequently describe the arc d e to bring 

 it in conjunction with the sun. To do this oc- 

 cupies at a mean about 2 days 5 hours and 51 

 seconds; and the synodical is just so much 

 longer than the periodical month. 



It is almost unnecessary to mention to you 

 that the diurnal potation of the earth about its 

 axis occasions an apparent daily revolution of 

 the moon from east to west, or, in common 

 language, the rising and setting of that lumi- 

 nary. But, during this apparent revolution of 

 the moon from east to west, she in reality ad- 

 vances in her orbit about 13 degrees from west 

 to east. There is therefore an apparent daily 

 retardation in the course of the moon, as she 

 rises and sets each day about 49 minutes later 

 than the preceding. This, however, is strictly 

 true only as to the equatorial regions, and under 

 circumstances to be afterwards explained. The 

 moon turns round on her own axis in the same 



