PHYSICAL ASTRONOMYc 



Density of the Sun, = 1 



- of the Earth, = 3.9393 



- of Jupiter, = 0.8601 



- of Saturn, = 0.4951 

 - of Uranus, = 1.1376 



246. The immoveable point to which the pla- 

 nets gravitate, is not the centre of the Sun, but 

 the centre of gravity of the solar system. 



From the equality of action and re-action, the gravi- 

 tation of the planets to the Sun must be accompa- 

 nied by the gravitation of the Sun to the planetSj, 

 so that the quantity of the motion of the former, 

 estimated in any direction, must be equal to that 

 of all the latter estimated in the opposite. The 

 Sun, therefore, moves in an orbit, about the only 

 point of which the condition cannot be disturbed 

 by the mutual action of the surrounding bodies, 

 viz. the centre of gravity of the whole. 



If there were only one planet, the Sun and that pla- 

 net would describe similar conic sections, of which 

 their common centre of gravity would be one of 

 the foci ; their distances from that point being al- 

 ways inversely as their masses. If there be a num- 

 ber of planets, the path of the Sun will become a 

 more complicated curve, but will be such as to 

 furnish a centrifugal force in respect of each pla- 

 net, just able to counteract the gravitation toward 

 it. 



VOL. II. Q 24?. The 



