THE EARTH AS A PLANET 3 



in an eclipse the stars begin to appear as in the evening. 

 Astronomers are all agreed that if it were not for the 

 greater brilliancy of the sun we should see the heavens 

 full of stars all the time. 



. In the northern hemisphere the stars, except those at 

 the north, which seem to go around in a circle, appear to 

 rise in the east and to set in the west, just as the sun does. 

 If we observe the stars which rise to the east, southeast, 

 and northeast of us, we shall find that these are above the 

 horizon for different lengths of time. 



The ancients noticed these facts, and explained them 

 by saying that the earth was at the center of a hollow 

 sphere, upon the inner surface of which were the stars, 

 and that this sphere was continually revolving about the 

 earth and also slightly changing its position in respect to 

 the earth. We of the present day know that it is the 

 earth that is turning around on an imaginary axis, and 

 also gradually changing its position in relation to the 

 stars. We also know that this axis, if extended far 

 enough, would almost strike a star in the center of the 

 northern heavens, which we call the North Star. The 

 points on the surface of the earth through which the 

 axis passes are called the poles. 



2, The Earth as one of the Planets. If we carefully ob- 

 serve the bright points which appear in the sky at night, 

 we shall see that almost all of them shine with a twinkling 

 light. There are, however, three of the brightest which 

 give a steady light like that of the moon. When the po- 

 sitions of these three bodies are carefully observed for 

 some time, it will be seen that they are continually chang- 

 ing their places among the stars, whereas the positions of 

 the stars do not appear to change relatively to one another. 



One of these three brightest points has a reddish brown 

 color and has been named Mars, from the Roman god of 



