LECTURE XX. 



ASTRONOMY. 



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THE SECONDARY PLANETS. 



THE Secondary Planets are those which per-- 

 form their revolution round other planets, which 

 themselves make their revolutions round the sun. 

 They are reckoned eighteen in number, viz. the 

 moon, the four satellites of Jupiter, the seven sa- 

 tellites of Saturn, and the six satellites of Uranus. 



I shall first speak of the moon ; since, from 

 her proximity to the earth, we have a better op- 

 portunity of observing her motions and phaeno- 

 mena, than we have of the other secondary pla- 

 nets. 



The apparent diameter of the moon, if seen 

 at the same distance from the earth as the sun, 

 would be little more than four seconds. Whence 

 we may conclude that her diameter is at least 

 390 times less than that of the sun. The moon's 

 diameter is about iiths that of the earth, or about 

 2170 miles. The whole bulk of the moon is 

 about Jr of that of the Earth. 



The moon being much nearer to the earth than 

 the planets are, and having an apparent diameter 

 of more than half a degree, has been known ever 



