134 CELESTIAL PHENOMENA. 



of the tail, which streams from it to a distance of many 

 millions of miles. Thus the central forces are the main* 

 taining as well as the constituent forces of the system . 



Our Sun, viewed in relation to all the bodies so various 

 in magnitude and density which revolve around it, may be 

 regarded as at rest, although it revolves around the common 

 center of gravity of the whole system, which center, in conse- 

 quence of the varying position of the planets, falls sometimes 

 within and sometimes without the body of the Sun itself. Alto- 

 gether distinct in its nature is the movement of translation of 

 the Sun, the progressive motion of the center of gravity of 

 the whole solar system in universal space, which is supposed 

 to take place with such prodigious velocity, that, according 

 to Bessel( 101 ), the relative motion of the Sun and the star 

 61 Cygni amounts, in a single day, to no less than 3336000 

 . miles. "We should be unconscious of the change of place 

 of the solar system, were it not that, by the perfection of 

 our astronomical instruments, and by improved methods 

 of observation, we are enabled to note our progress by refe- 

 rence to distant stars, as in a vessel we estimate its speed 

 by the apparent motion of objects on shore. The proper 

 motion of 61 Cygiri is, nevertheless, so considerable, as to 

 produce a displacement of a whole degree in 700 years. 



We can measure the amount of changes in the relative 

 positions of the stars to one another, in their proper 

 motions as these changes are called, with far greater 

 certainty than we can explain their cause. After de- 

 ducting all that depends on the precession of the equi- 

 , noxes, and the nutation of the Earth's axis consequent 

 upon the influence of the Sun and Moon on the sphe- 

 roidal form of the earth, -all that results from the 



