ASTRONOMY. 



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been entirely discovered by Herschel. It cannot 

 be seen without a telescope, but it does not require 

 a powerful one. The satellites cannot be seen 

 without the most powerful telescopes. 



Comets. 



Besides these planets already mentioned, there 

 are some other bodies which revolve round the sun, 

 called comets. They move in very eccentric ellipses, 

 and their periods of revolution are so long, and 

 imperfectly known, that few are ever observed 

 twice. They are only seen by us when they are 

 in that part of their orbit which is nearest to the 

 sun, and then they move so fast, they soon become 

 again invisible to us. The number of comets is 

 also unknown ; numbers of small ones have been 

 discovered by telescopes. Their distances are in- 

 conceivably great, and most of them move entirely 

 beyond the planetary orbits; though some have 

 descended below Mars. Their appearances are 

 very different. Some resemble only a faint vapour; 

 others have a nucleus or solid part in the middle. 

 When they approach the sun, they put forth the 

 appearance of a beard or tail of luminous matter, 

 which is sometimes of astonishing length. These 

 tails are always directed from the sun. 



With respect to the real nature and use of the 

 comets in the system, we are entirely unacquainted. 



Fixed Stars. 



The fixed stars are so called, because they are 

 observed not to change their places in the heavens, 



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