ASTRONOMY. 133 



133. If the angular velocity with which the 

 moon revolves on her axis is uniform, that with 

 which she revolves in her orbit, being, as we 

 have seen, alternately accelerated and retarded, 

 small segments on the east and west sides ought 

 alternately to come in sight and to disappear. 

 This is conformable to observation. 



Thus there is produced in the orb of the moon, an 

 apparent libration backward and forward, called 

 her Libration in Longitude. 



134. If the axis on which the moon turns, 

 were perpendicular to the plane of her orbit, or 

 if her equator coincided with that plane, we 

 should perceive no other libration than that 

 which has now been described. But in fact the 

 spots on the north and south of the plane of the 

 moon's orbit, alternately advance toward the 

 middle of the disk, and recede from it by a very 

 small quantity. 



This is called the Libration in Latitude, and shews 

 that the moon's axis it not exactly, though near- 

 ly, perpendicular to the plane of her orbit. 



135. A third libration arises from this, that 

 the point to which the moon presents always 

 the same face, is the centre of the earth, round 



which 



