iKANSLAlORY MOTION OF THE SCLAft SYSTEM. 145 



orbits at a distance 44 times greater than that of Uranus ; 

 no.y, in those comets whose nucleus appears to us, from its 

 inconsiderable mass, like a mere passing cosmical cloud, the 

 Sun exercises its attractive force on the outermost parts of the 

 emanations radiating from the tail over a space of many mill- 

 ions of miles. Central forces, therefore, at once constitute and 

 maintain the system. 



Our Sun may be considered as at rest when compared to all 

 the large and small, dense and almost vaporous cosm'.cal bodies 

 that appertain to and revolve around it ; but it actually ro- 

 tates round the common center of gravity of the whole sys- 

 tem, which occasionally falls within itself, that is to say, re- 

 mains within the material circumference of the Sun, what- 

 ever changes may be assumed by the positions of the planets. 

 A very different phenomenon is that presented by the trans- 

 latory motion of the Sun, tliat is, the progressive motion oi" 

 the center of gravity of the whole solar system in universal 

 space. Its velocity is such* that, according to Bessel, the 

 relative xnotion of the Sun, and that of 61 Cygni, is not less 

 m one day than 3,336,000 geographical miles. This change 

 of the entire solar system would remain unknown to us, if the 

 admirable exactness of our astronomical instruments of meas- 

 urement, and the advancement recently made in the art of 

 observing, did not cause our advance toward remote stars to 

 be perceptible, like an approximation to the objects of a dis- 

 tant shore in apparent motion. The proper motion of the star 

 61 Cygni, for instance, is so considerable, that it has amount- 

 ed to a whole degree in the course of 700 years. 



The amount or quantity of these alterations in the fixed 

 stars (that is to say, the changes in the relative position of 

 self-luminous stars toward each other), can be determined 

 with a greater degree of certainty than we are able to attach 

 to the genetic explanation of the phenomenon. After taking 

 into consideration what is due to the precession of the equi- 

 noxes, and the nutation of the earth's axis produced by the 

 action of the Sun and Moon on the spheroidal figure of our 

 fflobe, and what may be ascribed to the transmission of light; 

 that is to say, to its aberration, and to the parallax formed by 

 the diametrically opposite position of the Earth in its course 

 round the Sun, we still find that there is a residual portioc 



* Bessel, in Schum., Jahrb.fur 1839, s. 51 ; probably four milliona 

 of miles daily, in a relative velocity of at the least 3,336,000 miles or 

 more than double the velocity of revolution of the Earth iu her oi'jit 

 round the Sua. 

 Vol. I — G 



