THE EARTH'S PLACE IN THE UNIVERSE 23 



There is no star within millions of millions of miles of the 

 solar system, and except an occasional comet or meteor, 

 nothing ever enters the vast space between the solar system 

 and the stars. 



13. The Sun. By far the largest and most important 

 body in the solar system is the sun. Its diameter is more 

 than one hundred times that of the earth. The distance 

 from the center of the sun to its surface is nearly twice the 

 distance from the earth to the moon. 



The interior of the sun is thought to be like a heavy, 

 white-hot liquid, but the outer portions, which we see, are 

 known to be intensely heated gases. The sun is composed 

 of the same elements as the earth, but the condition of these 

 elements is so different that they can be recognized only by 

 means of an instrument for the study of light, called the 

 spectroscope. % It is not known just what makes the sun hot, 

 but it is known that it is not a burning body as some 

 people have supposed. The condition of the sun is like 

 that of the stars. It is so near us that, by looking through 

 smoked or colored glass, we are able to see its shape and 

 apparent size. It looks about the size of the full moon, 

 but is really very many times as large. 



14. Day and Night. We learned when we were very 

 young that the earth rotates on its axis, once in twenty- 

 four hours. It is this rotation which makes it seem as if 

 the sun rises in the east, passes across the sky, and then 

 sinks below the western horizon. Half of the earth is 

 turned toward the sun and hence is lighted at one 

 time; in that half it is day, while in the unlighted half it 

 is night. At the moment the sun is rising at a given place, 

 there are 180 degrees of unlighted earth to the west of that 

 place and 180 degrees of daylight to the east. The light is 

 constantly advancing toward the west and retreating from 

 the east. Places where the sun is setting are half-way around 

 the earth from places where it is rising. 



