LECTURE XIX. 

 ASTRONOMY. 



THE PRIMARY PLANETS ; THE MODE OF CALCU- 

 LATING THEIR DISTANCES, &C. 



THE planets, I have already intimated, are 

 opaque bodies, very nearly spherical, and we 

 have reason to believe much like the earth. 

 They are not luminous of themselves ; and be- 

 come visible only by reflecting the light which 

 they receive from the sun. Kepler discovered 

 some of the principal laws by which the motions 

 of the planets are governed. He was the first 

 that demonstrated, by calculations equally diffi- 

 cult and laborious, that they must revolve in 

 elliptical, and not in circular orbits. He calcu- 

 lated by the observations of Tycho, the distance 

 of Mars from the Sun in different parts of his 

 orbit, and proved that it could not possibly be 

 adjusted to the circumference of a circle. New- 

 ton showed afterwards, by the theory of attrac- 

 tion, that the curve which a planet describes 

 would be strictly an ellipsis, of which the central 

 star (or sun) occupies one of the foci, were it not 

 for the slight irregularities occasioned by the at- 



