CELESTIAL MEASURINGS AND WEIGHINGS. 203 



(28.) But, besides this, the two stars in question are 

 remarkable in another way. They are both conspicuous 

 DOUBLE STARS, and have been long watched with especial 



a 



o 



e 



Fig. i. 



interest by reason of the two individuals of which each 

 respectively consists standing to each other in the rela- 

 tion of sun and planet, or planet and satellite. Not only 

 do they keep close company with each other in their 

 journey through space by a common proper motion; but 

 while so journeying together they revolve about each 

 other or about their common centre of gravity in regular 

 elliptic orbits, under the influence of that very same law 

 of gravitation which retains the planets in their circula- 

 tion round the sun. They are, therefore, at the same 

 distance from us, and therefore both equally and simi- 

 larly apparently displaced by parallax; so that each of 

 them, microscopically watched, appears to describe the 

 same minute annual parallactic ellipse above spoken of. 

 This must not be confounded with the elliptic orbit the 

 two stars describe about each other. That is on a far 

 larger scale, and requires many years for its completion. 



