CHAPTER SEVENTEEN 



COMETS 



IGNORANT persons have sometimes been terrified by 

 the appearance of a comet, thinking it portended some 

 great calamity, perhaps the end of the world. Those 

 who view the heavens with telescopes have no fear 

 of comets doing any harm, for they see one or more 

 nearly every year, and sometimes more than one may 

 be visible at a time. Only a few of the comets that 

 astronomers study ever become bright enough to attract 

 the attention of other persons. These comets become 

 brighter and brighter night after night as they draw 

 nearer to the sun. They do not strike the sun but pass 

 sometimes very near it and, after going part way around 

 it, go off into space again, gradually becoming fainter 

 and fainter until they are no longer visible to the naked 

 eye and finally not even with a telescope. The whole 

 period of visibility is usually not many months, but in 

 some cases it is more than a year. 



The tail of a comet. Comets have a different shape 

 from the other heavenly bodies and differ greatly even 

 among themselves. Bright comets have a tail, which 

 is somewhat curved and millions of miles in length. 

 This is always directed away from the sun, as if the latter 

 in some way repelled the particles of which the comet 

 consists. After the comet in its motion has passed 

 as near the sun as its orbit permits and is receding from 

 the sun, the tail goes ahead instead of following, as it 

 does when the comet is approaching the sun. In a few 

 instances the length of a comet's tail has been greater 

 than the distance from the earth to the sun. 



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