ASTRONOMY. 195 



The Comet of 1811 was remarkable for its beauty. 

 The tail was composed of two diverging beams of 

 faint light, slightly coloured, which made an angle 

 from 15 to 20, and sometimes much more, and 

 were bent outward. The space between was com- 

 paratively obscure. Abrege d'Ast. par DE LAM- 

 BRE, lefon 21. 57* 



203. A Comet remains so short a time in 

 Sight, and describes so small a part of its course 

 within our view, that, from observation alone, 

 without the assistance of hypothesis, we should 

 not be able to determine the nature of its path. 

 The hypothesis most conformable to analogy 

 is, that the Comet moves in an ellipsis round 

 the Sun placed in one of the foci, and that the 

 radius vector from the Sun to the Comet de- 

 scribes areas proportional to the times* 



As the ellipse in which a Comet moves is evidently 

 very ecceatric, it will coincide very nearly with a 

 parabola, at its vertex, or for all the time that a 

 Comet remains in sight. 



If it be supposed that the Comet de- 

 scribes an ellipsis or a parabola, in conformity 

 to the laws of KEPLER, then from three geocen- 

 tric places, known by observation, the orbit may 

 be determined. 



The first solution of this problem was given by NEW- 

 TON, Princip. lib. HI. prop. 41. He calls it Proble- 



ma 



