THE EARTH 



11 



is Sirius the " Dog Star," which when visible is the 

 brightest star in the whole sky. 



The change in the position of stars which can be ob- 

 served from hour to hour is caused by the rotation of 

 the earth on its 

 axis, and the 

 change in position 

 apparent from 

 month to month 

 is caused by the 

 revolution of the 

 earth around the 

 sun. The appar- 

 ent motion of the 

 stars is always 

 westward, because 

 the earth's motion 

 on its axis and 

 around the sun is 

 from west to east. 



The Moon. - - The earth has one satellite, the moon, 

 which revolves around it once a month (27.32 days) 

 and accompanies it through space in its journey around 

 the sun. The diameter of the moon is about 2163 

 miles and its distance from the earth is about 240,000 

 miles. Although we see the moon as a very bright 

 object at night for a part of every month, yet it has 

 no light of its own but shines entirely by reflected 

 light from the sun (Figure 9). It has a rough, barren, 

 rocky surface and as far as is known has no air or 

 water upon it. While the moon makes one journey 

 around the earth, it rotates only once on its axis/ 

 Because the moon's period of rotation is the same as 



FIG. 8. Constellations in the Northern Sky. 



