PHYSICAL ASTRONOMY. 63 



r 



no farther than to involve the simple power of > 



the investigation would lead to nothing, as a 

 would disappear from the equation ; but if it be 



r* 

 carried to the terms that involve , an equation 



a 



may be found from which a can be determined. 

 Dr STEWAE/?, therefore, having first compared the 

 mean disturbing force with the gravity of the 

 Moon to the Earth, from the motion of the ap- 

 sides, proceeded to determine the ratio of the 

 former force to the force that retains the Earth in 

 its orbit, by carrying the approximation to the 

 terms that involved the square of the Sun's dis- 

 tance, and from that computed the Sun's parallax 

 at 6".9. Sun's Distance, prop. 10. 



This value of the parallax is no doubt too small ; it 

 is only an inconsiderable part of the disturbing 

 force of the Sun, into which his distance enters as 

 an element, and therefore any deduction founded 

 on it must be liable to error. 



MAYER has also sought to determine the Sun's pa- 

 rallax from one of the lunar equations, as deduced 

 from the solution of the problem of the three bodies. 

 The co-efficient of this equation involves the Sun's 

 parallax, and gives it equal to 7 ".8, and MAYER 

 thinks that the error cannot exceed a 24-th part. 

 Theoria Lunae, 51. 



263. The 



