PHYSICAL ASTRONOMY. 551 



feet, upon the whole, is to diminish the Moon's 

 gravity, by one-half of the said quantity, or 



The disturbing force in the direction of the radius 

 vector may be more conveniently expressed ; for 

 if F be the force by which the Earth attracts the 

 Moon, P the periodical time of the Earth's revo- 

 lution round the Sun, and P' that of the Moon 

 round the Earth, it has been already shewn, that 



? ^: F : : -L- 

 - 



is therefore nearly of the Moon's gravi- 



ty, and the mean disturbing force = 



OOQ 



251. From the diminution of her gravity by a 

 358th part, the Moon describes her orbit at a 

 greater distance from the Earth, with a less an- 

 gular velocity, and in a longer time, than if she 

 were urged to the Earth by her gravity alone. 



If the solar action were to cease, the Moon would 

 come nearer to the Earth, and would describe her 



orbit 



