18 THE OPEN SKY 



Comets. Sometimes comets appear in the sky and 

 excite the greatest wonder. They usually have a very bright 

 spot as the nucleus of a head, which shades gradually into 

 a less luminous tail that streams across the sky for millions 

 of miles. Some of the comets travel in great orbits around 

 the sun and appear at regular intervals. They may be 

 considered as part of the solar system. Others have ap- 

 peared once and then have disappeared, never to return. 

 Halley's comet is probably the best known of all the comets. 

 It takes about 75 years to make a trip around its orbit 

 and was last seen in 1910. It was named after the English 

 astronomer Halley because, by mathematical calculations, 

 he traced its history to almost the beginning of the Chris- 

 tian era, and prophesied correctly the year of its next 

 return. 



SUMMARY 



The sun is more than 100 times greater than the earth in 

 diameter and in circumference, and more than a million times 

 greater in volume. It appears as a tremendous ball of flame, 

 and is the source of the earth's heat and light. 



The few steady-shining points of light in the evening sky 

 which are constantly changing their positions among the 

 stars are planets. These, like the earth, revolve in regular 

 orbits about the sun as a center. Each of the myriads of 

 twinkling stars is a sun, shining by its own light. There is 

 reason to believe that many of these suns have planets re- 

 volving about them. The nearest of the stars is thousands 

 of billions of miles away, and the distances of remote stars 

 from the earth are immeasurable. The ancients thought 

 that the earth was the center of the universe and that the 

 heavenly bodies revolved about it, but we know that the 



