PROPER MOTION. 



which seems to be carried round us by the diurnal motion of the 

 sphere, just as if the stars were attached to it, and merely shared 

 in its apparent motion. 



But the stars, though subject to no motion perceptible to the 

 naked eye, are not absolutely fixed. When the place of a star on 

 the heavens is exactly observed by means of good astronomical 

 instruments, it is found to be subject to a change from month to 

 month and from year to year, small indeed, but still easily observed 

 and certainly ascertained. 



64. It has been demonstrated by Laplace, that a system of 

 bodies, such as the solar system, placed in space and submitted 

 to no other continued force except the reciprocal attractions 

 of the bodies which compose it, must either have its common centre 

 of gravity stationary or in a state of uniform rectilinear motion. 



The chances against the conditions which would render the sun 

 stationary, compared with those which would give it a motion 

 in some direction with some velocity, are so numerous that we 

 may pronounce it to be morally certain that our system is in 

 motion in some determinate direction through the universe. Now, 

 if we suppose the sun attended by the planets to be thus moved 

 through space in any direction, an observer placed on the earth 

 would see the effects of such a motion, as a spectator in a steam- 

 boat moving on a river would perceive his progressive motion on 

 the stream by an apparent motion of the banks in a contrary 

 direction. The observer on the earth would, therefore, detect such 

 a motion of the solar system through space by the apparent motion 

 in the contrary direction with which the stars would be affected. 



65. Such a motion of the solar system would affect different 

 stars differently. All would, it is true, appear to be affected by 

 a contrary motion, but all would not be equally affected. The 

 nearest would appear to have the most perceptible motion, the 

 more remote would be affected in a less degree, and some might, 

 from their extreme distance, be so slightly affected as not to 

 exhibit any apparent change of place, even when examined with 

 the most delicate instruments. To whatever degree each star 

 might be affected, all the changes of position would, however, 

 apparently take place in the same direction. 



The apparent effects would also be exhibited in another manner. 

 The stars in that region of the universe toward which the motion 

 of the system is directed, would appear to recede from each other. 

 The spaces which separate them would seem to be gradually 

 augmented, while, on the contrary, the stars in the opposite 

 quarter would seem to be crowded more closely together, the 

 distances between star and star being gradually diminished. 

 This will be more clearly comprehended by fig. 8. 



7 



