ON COMETS. 107 



3. The inclination of its plane to the ecliptic. 



4. The longitude of its node, or the direction of the 



line in which its plane intersects the ecliptic, 

 which is called the line of its nodes. 



5. The longitude of its perihelion, or, which comes to 



the same thing, the angle which the axis of the 

 orbit makes with the line of nodes. 



6. The exact moment when the comet passed through 



its perihelion, or was nearest to the sun. 



7. The direction of its motion (direct or retro- 



grade). 



(15.) It is natural to ask how all these particulars ever 

 can be known ; and to this the answer is By the same 

 system of observation and calculation combined, by which 

 we have come to know the form and dimensions of the 

 orbits of the planets, their times of revolution round the 

 sun, and their situation in space. 



(16.) I believe it was Tycho Brahe, a celebrated Danish 

 astronomer, who first rose to the conception that comets 

 are beyond the moon, and not mere exhalations. The 

 appearance of a great comet in 1577 set him thinking 

 about it, and he was led by his observations and reason- 

 ings on them to a certain knowledge of the fact of its 

 being much more remote than our own satellite ; and he 

 was therefore led to conjecture that the motions of 

 comets had reference rather to the sun as their centre 

 than the earth. The elliptic form of the planetary orbits 

 was not then known, and Tycho accordingly supposed 

 that comets moved about the sun in perfect circles. 

 Borelli, a Neapolitan mathematician, suggested the idea 



