THE MOON, PLANETS, AND COMETS 261 



different from that used by Adams, arrived at essentially the 

 same conclusion. He wrote to a young German astronomer, 

 Galle, and asked him to search for the suspected unknown 

 planet Galle impatiently waited for night for the stars to 

 come out, and within half an hour after darkness came he 

 began his search and found the unknown body in almost the 

 exact position assigned to it by both Leverrier and Adams. 



Neptune is nearly 3,000,000,000 miles from the earth and 

 can be seen only with telescopic aid. Its distance is so great 

 that more than four hours are required for its light to come to 

 us, though light travels at the rate of 186,000 miles per second 

 or about 10,000,000 miles per minute. Yet it is bound to the 

 remainder of the system by the invisible bonds of gravitation. 



264. Comets. The planets are fixed in dimensions and 

 shape and revolve around the sun in relatively simple orbits. 

 There are other bodies, the comets, which differ in all these 

 respects. They are not often visible and usually for but a 

 short time. They often change their dimensions enormously, 

 and move in most peculiar ways. Until two centuries ago 

 they were regarded by people with superstitious terror and 

 were supposed to foretell disaster. 



The typical comet (fig. .130) is composed of a head whose 

 diameter may range from a few thousand miles to a million 

 miles. Inside of the head there is generally a brighter, star- 

 like nucleus, and streaming out from it in the direction oppo- 

 site from the sun there is usually a tail whose length may be 

 as much as a hundred million miles. 



About four hundred comets have been observed since the 

 invention of the telescope, most of them too faint to be 

 seen without optical aid. Their orbits lie in all sorts of 

 directions, and they are usually very elongated. In fact most 

 of them are so exceedingly elongated that at their remotest 

 points from the sun they are many times the distance of 

 Neptune, and it may be that some of them actually recede 

 to the distance of the stars. 



