PRESSURE OF LIGHT IN ASTRONOMY 77 



the sun, it is moving to some extent with the 

 stream of momentum issuing from the sun, and so 

 is receiving less momentum each second and ex- 

 periencing less sunlight pressure than if it were at 

 rest or were moving in a circular orbit. We may 

 regard these forces, due to excess and defect of 



momentum received, as extra forces superposed 

 on the inverse square force. At such a point as P 

 the extra force is along SP and is a resistance 

 to the shortening of SP, and at such a point as Q 

 the extra force is along QS and is a resistance to the 

 lengthening of SQ. The net result is a resistance 

 to the change of distance from the sun, a tendency to 

 make the elliptic orbit less elliptic and more circular. 



