THE STELLAR UNIVERSE. 



371 



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 

 the 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 

 the universe in any direction, an observer placed on the earth would observe 

 the effects of such a motion, as a spectator in a steamboat 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 there- 

 fore 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. 



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 per- 

 ceptible 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 di- 

 rected 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 the annexed diagram. 



Fig. 22. 



Let the line S S' represent the direction of the motion of the system, and 

 let S and S' represent its positions at any two epochs. At S, the stars ABC 

 would be separated by intervals measured by the angles A S B, and B S C, 

 while at S' they would appear separated by the lesser angles A S' B, and 

 B S 7 C. Seen from S', the stars ABC would seem to be closer together 

 than they were when seen from S. For like reasons the stars a b c, toward 

 which the system is here supposed to ;ncve, would seem to be closer together 

 when seen from S, than when seen from S x . Thus, in the quarter of the 

 heavens toward which the system is moving, the stars might be expected to 

 separate gradually, while in the opposite quarter they would become more 

 condensed. In all the intermediate parts of the heavens they would be affected 

 by a motion contrary to that of the solar system. Such in general would be 

 the effects of a progressive motion of our system. 



