450 SPECIAL RESULTS IN THE URANOLOGICAL 



lastly nebulae, so far as these are not remote and very dense 

 clusters of stars. 



The transition from the sidereal portion of Uranology, 

 from the heaven of the fixed stars, to our solar system, is 

 only the transition from the universal to the particular. In 

 the class of double stars, self-luminous cosmical bodies move 

 round a common centre of gravity : in our solar system, 

 which is composed of very heterogeneous elements, dark 

 cosmical bodies revolve around a self-luminous one, or rather 

 round a common centre of gravity which is sometimes 

 within and sometimes without the circumference of the 

 central body. The several members of the solar domain are 

 more dissimilar in their nature than for many centuries there 

 had been reason to suppose. They divide themselves into 

 primary planets, and secondary ones or satellites, the pri- 

 mary planets having among them a group in which the orbits 

 intersect each other ; an unascertained number of comets ; 

 the ring of the zodiacal light ; and, with great probabi- 

 lity, the periodic meteor-asteroids. 



It still remains to state expressly the three great laws 

 discovered by Kepler, in their actual application to the mo- 

 tions of the planets. First law : Every path of a planetary 

 body is an ellipse, having the Sun in one of its foci. Second 

 law : Every planetary body describes round the Sun equal 

 areas in equal times. Third law : the squares of the periodic 

 times of revolution of two planets are to each other as the 

 cubes of their mean distances. The second of these laws is 

 sometimes called the first, because it was discovered earlier 

 than the others. (Kepler, Astronomia nova, seu Physica 

 coelestis, tradita commentariis de motibus stellse Martis, ex 



