ASfRONOMY. 127 



was less formerly than it is at present. This in- 

 crease of the mean motion is called the Moon's 

 Acceleration. It amounts to 9" in the present age; 

 and the effect of it on the moon's place increases 

 as the squares of the time. 



125. The period of the phases, or the time 

 from full moon to full moon, is greater than the 

 tropical revolution, as being the time required 

 to describe 360, with an angular velocity that 

 is equal to the difference between the angular 

 velocity of the moon, and the angular velocity 

 of the sun : this gives the period of the phases, 

 or what is called the Lunar Month, equal to 



29^.530588 = 29 d 12 



See BIOT, torn. n. p. 8^8. Also WOODHOUSE, p. 305, 

 The above is LA PLACE'S determination, 



Moon's Orbit. 



126. It may be found, from observations of 

 the Moon's apparent diameter, as was done in 

 the case of the Sun, that the orbit in which she 

 moves is nearly an ellipsis, having the earth in 

 its focus, about which the radius vector drawn 



to 



