SOS Astronomy. [Lecture 19. 



star by the naked eye ; but its moons, or satel- 

 lites, can only be seen by a good telescope. 



Besides these, there are other bodies attached 

 to our system, which, although their orbits are 

 singularly eccentric, have yet many things in 

 common with those which we have been describ- 

 ing ; they are called COMETS. 



They are not luminous of themselves, but, 

 like the planets, are opake bodies, shining only 

 by the light of the sun, which they reflect to- 

 wards us. All the comets revolve round the 

 sun in a manner peculiar to themselves, that is, 

 in elliptical orbits exceedingly long and eccentric, 

 yet regulated by laws similar to those of the pla- 

 nets themselves, each describing equal areas in 

 equal times, about the sun as a centre of force. 

 On this principle astronomers have attempted to 

 calculate the period of their return, and in one 

 case at least with success, since it is generally 

 agreed that the comet which appeared in 1759 is 

 the same which was observed in 1531, 1607, and 

 1682. Its periodical revolution is therefore com- 

 pleted in about 76 years, and it may be conse- 

 quently expected again in the year 1835. 



Some of the comets move from West to East, 

 like the planets, while others proceed in a con- 

 trary direction from East to West, and in the 

 contrary order of the signs of x the zodiac. Some 

 pass nearly in the line of the ecliptic, and some 

 almost perpendicular to it. These orbits being ex- 



