ON THE SOLAR SYSTEM. 405 



The Georgian planet, discovered by Dr. Herschel * in 1780, sometimes 



also called Herschel, and sometimes Uranus, revolves in 83| years, at a 



distance from the sun equal to 19 times that of the earth. Its diameter is 



a t little more than 4 times that of the earth, and the weight of bodies at its 



aw face a little less than here. Notwithstanding its dimensions are by no 



/means comparatively small, it appears to us as a star of the sixth or seventh 



/ magnitude, and is seldom seen by the naked eye. Its orbit approaches 



.very near to the ecliptic ; its disc is said to be somewhat flattened, and it is 



supposed to revolve with considerable rapidity. 



These ten [eleven] planetary bodies are the only ones hitherto discovered 

 ^'i \ch have any title to be considered as primary planets, that is, as bodies 

 reviving round the sun, in orbits so nearly circular, as to remain always 

 within the reach of our observation. It has been conjectured that the 

 number of planets may in reality be much greater, that not only many 

 small and perhaps invisible bodies may be revolving in the intervals of the 

 planets with which we are acquainted, but that larger bodies also may 

 belong to our system, which never approach within such a distance as to be 

 seen by us. Some have even bestowed names, borrowed from the ancient 

 mythology, on these imaginary planets ; but the idea of such an appropria- 

 tion of terms is rather to be regarded as belonging to the regions of poetical 

 fiction than to those of solid philosophy. 



The largest and the most remote of the primary planets have their 

 attendant satellites, or secondary planets, accompanying them in their 

 respective revolutions round the sun, and moving, at the same time, in 

 subordinate orbits, round the primary planets. The earth is attended by 

 the moon, Jupiter by four moons or satellites, Saturn by seven, besides his 

 ring, and the Georgian planet by six moons. All these satellites move in 

 the direct order of the signs, and in planes not very remote from the eclip- 

 tic, excepting those of the Georgian planet, which revolve in planes nearly 

 perpendicular to the ecliptic. Each of these planets thus becomes the cen- 

 tral luminary of a little system of its own, in which the motions and the 

 periods observe the same general laws as prevail in the solar system at large. 

 Of the 28 primary and secondary planets, we are indebted to Dr. Herschel 

 for the knowledge of 9 ; the Georgian planet, with its six satellites,t and 

 the two innermost moons of Saturn. 



The motions of some of these satellites, in particular of those of Jupiter 

 and of the moon, are of considerable importance for the assistance they 

 afford us in determinations of time, and of the relative situations of places. 

 They are subjected to considerable irregularities, but the united labours ol 

 various astronomers have enabled us to calculate all their motions with the 

 greatest accuracy. 



The moon performs a complete sidereal revolution in 27 days 7| hours, 



* Account of a Comet, Ph. Tr. 1781, Ixxi. 492. Herschel, Ph. Tr. 1783, p. 1. 

 Bode, Von dem neu Entdeckten Plan. Berl. 1784. Lexell, do. 4to, Petersb. Wurm, 

 Gotha, 1791. Robison, Ed. Tr. i. 305. 



f An Account of the Discovery of the Satellites of the Georgian Planet, Ph. Tr. 

 1787, p. 125 ; 1788, p. 364 ; 1798, p. 47. Account of the Discovery of a Sixth and 

 Seventh Satellite of the Planet Saturn, Ph. Tr. 1790, p. 1 , 427. 



