MOO 



rallaxj their greatest probable error does not exceed twelve seconds. 



MOON, libration of. 



The libration in longitude, or the alternate appearance and disappear- 

 ance of small segments of the moon on the east and west limbs, arises 

 from the uniform angular velocity round her axis, and the variable an- 

 gular velocity in her orbit. It nearly = the equation of the orbit, or 

 about 7 at its maximum. 



The libration in latitude, or the alternate appearance and disappear. 

 ance of the north and south poles, arises from the moon's axis not being 

 perpendicular to the plane of her orbit 



Diurnal libration, or the appearance of a small segment of the western 

 limb at the rising, and of the eastern limb at the setting of the moon, 

 arises from the spectator being situated on the surface, instead of at the 

 centre of the earth. 



MOON, augmentation of diameter of. 



When the moon is at different altitudes above the horizon, it is at dif- 

 ferent distances from the spectator, and therefore there is a change of 

 the apparent diameter. Let the altitude of the moon reckoned from the 

 earth's centre or true altitude ~ a , apparent altitude, or altitude reck- 

 oned from the earth's surface = A ; then increase off diameter = 



in. (* - * A) 



Hor. | diam. X 



a + I 



Hence the following Table : 



Augmentation of moon's semi-diameter, 



181 



