180 



EARTH AND HEAVENLY BODIES 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



What heavenly bodies make up our solar system? 



In ancient times people thought that the earth made 

 up the greater part of the universe. Even today it is 

 hard for us to understand the true nature of our earth 

 and its movements. 



If we could transport ourselves far enough out into 

 space we should see the earth as it really is a huge 

 object floating in space. We should notice that it is 

 moving at a very rapid speed, eighteen and one-half 

 miles per second or 66,600 miles per hour. If we con- 

 tinued to watch the earth long enough we should see 

 that it is revolving around the sun. The path in which 

 the earth moves around the sun is called the earth's 

 orbit. The earth's orbit is six hundred million miles 

 long, and 365*4 days are required for the earth to get 

 completely around it. 



From our position out in space we should notice 

 many other bodies revolving around the sun. We now 



From Astronomy by Moulton. By permission of the Macmillan Company, 

 publishers. 



FIG. 296. RELATIVE DIMENSIONS OF THE PLANETS AND THE SUN 



know that the earth is only one member of the sun's 

 family. Circling in orbits around the sun are nine ma- 

 jor planets, besides smaller planetoids, meteors, and 

 comets. All these bodies with the sun make up the 

 solar system. See Figure 298. 



The sun is the dominant member of the solar sys- 

 tem. Its mass is seven hundred times greater than the 

 mass of all the members of its family combined. The 

 planets, comets, and meteors are held in their orbits 

 by the great gravitational attraction of the sun. 



What is the nature of our sun? Our sun is a star. 

 To us it looks much larger than the stars we see at 

 night, but in reality it is just a medium-sized star. Our 

 sun looks much larger because it is only about 

 93,000,000 miles away from us while the next nearest 

 star is about 25,000,000,000,000 miles away. Compared 

 to the size of the earth, however, the sun is enormous. 

 The diameter of the sun is 865,000 miles as compared 

 with 8,000 for the earth, and its volume is 1,000,000 

 times that of the earth. If the sun were represented by 

 a basketball the earth would be about the size of a pea. 

 If the earth with the moon revolving around it could 



be placed at the center of the sun, the orbit of the 

 moon would be only about half way to the surface 

 of the sun. 



If we look at the sun through a smoked glass it ap- 

 pears as a great, bright disk, but through a telescope 

 interesting features appear on its surface. Figure 297 

 is a photograph of the sun. Notice the dark markings 

 at various places. These are called sun spots. Excep- 

 tionally large sun spots can be seen through a smoked 

 glass without the aid of a telescope. Astronomers by 

 constant watching have found that sun spot activity 



Courtesy Mount Wilson Observatory 



FIG. 297. A PHOTOGRAPH OF THE SUN 



Note the sun spots. 



varies. In some years hardly a spot can be seen. Then 

 the spots appear more often. In about five years from 

 the time only a few spots are visible on the face of 

 the sun, many sun spots develop. After this the sun 

 spots gradually disappear. This fading away and ap- 

 pearance again of great sun spots requires a period of 

 about eleven years. During the time of great sun spot 

 activity interesting things happen on the earth. Com- 

 pass needles are affected. Electrical energy liberated 

 by the sun spots interferes with our radio reception 

 and our communication by telephone and telegraph. 

 Brilliant displays of the aurora borealis (northern 

 lights) are common. Some scientists think that there 

 may also be a connection between weather phenomena 

 on the earth and sun spot activity which, if under- 



