TRANSLATORY MOTION OP THE SOLAR SYSTEM. 135 



of gravity of the whole solar system in universal space. Its 

 velocity i's such* that, according to Bessel, the relative motion 

 of the Sun, and that of 61 Cygni, is not less in 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 measurement, 

 and the advancement recently made in the art of observing, 

 did not cause our advance towards remote stars to be per- 

 ceptible, like an approximation to the objects of a distant 

 shore in apparent motion. The proper motion of the star 61 

 Cygni, for instance, is so considerable, that it has amounted 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 towards 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 

 globe, 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 portion of 

 the annual motion of the fixed stars due to the translation of 

 the whole solar system in iiniversal space, and to the true 

 proper motion of the stars. The difficult problem of numeri- 

 cally separating these two elements, the true and the apparent 

 motion, has been effected by the careful study of the direction 

 of the motion of certain individual stars, and by the consider- 

 ation of the fact that, if all the stars were in a state of absolute 

 rest, they would appear perspectively to recede from the point 

 in space towards which the Sun was directing its course. 

 But the ultimate result of this investigrtion, confirmed by the 

 calculus of probabilities, is that our solar system and the stars 

 both change their places in space. According to the admir- 



* Bessel, in Schum. Jahrb.fur 1839, s. 51 ; probably four millions 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 in her orbit round 

 the Sun. 



