4 ASTRONOMY. 



a tail, it must necessarily always present the same 

 side to the sun. Some comets show no trace of a 

 tail, while others acquire one near their perihelium. 

 The comet of 1816 was supposed to deviate 72 

 from the ecliptic. 



Comets distinguished from Planets. 



Comets are distinguished from planets by revolving 

 in much more eccentric orbits ; their periods of re- 

 volution are very different from one century to 

 another ; their distance and proximity to the sun are 

 so great that they must be at one time congealed, 

 and at another melted, if they resemble earthly sub- 

 stances. 



The little Planets, or Asteroids. 



Diameter of Ceres (or the Piazzi) 161 miles.* 

 Ditto of Pallas (or the Gibers) 100 



The astronomer Piazzi had the merit of discover- 

 ing a new planet, and was the first to observe several 

 comets, while his minute classification of the stars has 

 been the cause that Pallas, Juno, and Vesta have been 

 discovered. With regard to these Asteroids, as they 

 are all between the orbits of Mars and Jupiter, and 

 similar in their mean movements, a speculation has 

 been advanced among the scientific, that they are the 

 component parts of a ruined planet. 



Precession of the Equinoxes. 



The sun, which was in the Ram in the time of the 

 Argonauts, is now in the Bull. 



This motion is called the " Precession of the 

 Equinoxes," because by it the time and place of 



* Ceres is about the size of Ireland, which would make a 

 similar appearance, were it dug out of the ocean, and projected 

 into the air. 



