118 DISSERTATION SECOND. [parti. 



SECTION IV. 



ASTRONOMY 



1. Ancient Astronomy. 



It has already been remarked, that the ancients made 

 more considerable advances in astronomy than in almost any 

 other of the physical sciences. They applied themselves 

 diligently to observe the heavens, and employed mathema- 

 tical reasoning to connect together the insulated facts, which 

 are the only objects of direct observation. The astrono- 

 mer discovers nothing by help of his instruments, but that, 

 at a given instant, a certain luminous point has a particular 

 position in the heavens.- The application of mathematicks, 

 and particularly of spherical trigonometry, enables him to 

 trace out the precise tract of this luminous spot ; to disco- 

 ver the rate of its motion, whether varied or uniform, and 

 thus to resolve the first great problem which the science of 

 astronomy involves, viz. to express the positions of the 

 heavenly bodies, relatively to a given plane in functions of 

 the time. The problem thus generally enunciated, compre- 

 hends all that is usually called by the name of descriptive 

 or mathematical astronomy. 



The explanation of the celestial motions, which natural- 

 ly occurred to those who began the study of the heavens, 

 was, that the stars are so many luminous points fixed in the 

 surface of a sphere, having the earth in its centre, and re- 

 vohing on an axis passing through that centre in the space 

 of twenty-four hours. When it was observed that all the 

 stars did not partake of this diurnal motion in the same de- 

 gree, but that some were carried slowly towards the east, 

 and that their paths estimated in that direction, after cer- 

 tain intervals of time, returned into themselves, il was be- 

 lieved that they were fixed in the surfaces of spheres, 



