CHAPTER XI. 



GRAVITATIONAL ASTRONOMY IN THE l8TH CENTURY. 



"Astronomy, considered in the most general way, is a great problem 

 of mechanics, the arbitrary data of which are the elements of the 

 celestial movements ; its solution depends both on the accuracy of 

 observations and on the perfection of analysis." 



LAPLACE, Preface to the Mecanique Celeste. 



228. THE solar system, as it was known at the beginning 

 of the 1 8th century, contained 18 recognised members: 

 the sun, six planets, ten satellites (one belonging to the 

 earth, four to Jupiter, and five to Saturn), and Saturn's 

 ring. 



Comets were known to have come on many occasions 

 into the region of space occupied by the solar system, and 

 there were reasons to believe that one of them at least 

 (chapter x., 200) was a regular visitor; they were, how- 

 ever, scarcely regarded as belonging to the solar system, 

 and their action (if any) on its members was ignored, a 

 neglect which subsequent investigation has completely 

 justified. Many thousands of fixed stars had also been 

 observed, and their places on the celestial sphere determined ; 

 they were known to be at very great though unknown 

 distances from the solar system, and their influence on it 

 was regarded as insensible. 



The motions of the 18 members of the solar system were 

 tolerably well known ; their actual distances from one 

 another had been roughly estimated, while the proportions 

 between most of the distances were known with considerable 

 accuracy. Apart from the entirely anomalous ring of 

 Saturn, which may for the present be left out of considera- 

 tion, most of the bodies of the system were known from 



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