THE EARTH'S NEAREST NEIGHBORS 39 



sometimes it is crescent-shaped. At certain times it is 

 invisible. When it is on the other side of the earth from 

 where we are, of course we do not see it. 



The moon's light is reflected sunlight, and the sun shines 

 upon half of the moon all the time, except during a lunar 

 eclipse, that is, an eclipse of the moon. When the moon 

 is in the same direction from us as the sun, the sun's light 

 falls upon the half of the moon turned away from us, and 

 the side toward us is dark and hence invisible. 



Moon 



FIG. 11. ROTATION AND REVOLUTION OF THE MOON 



1. What quarters of the moon are turned toward the earth in position 

 1 shown at the left side of the diagram? 2. What quarters are lighted? 

 Why? 3. What part of the moon's revolution is .completed between posi- 

 tions 1 and 2 (at the bottom)? 4. What part of its rotation? 5. Name the 

 quarters of the moon turned toward the earth in position 2. 6. Are they 

 both lighted? Why? 



In a day or two, as the moon changes its direction from 

 us with relation to the sun, light strikes a small part of the 

 half turned toward us and gives the crescent or "new 

 moon," seen in the western sky soon after sunset. As the 

 moon continues to move eastward around the earth, it is 

 seen farther from the western horizon each evening, a larger 

 portion of the half toward us is lighted, and the crescent 

 changes gradually to a half -disk. This is called "first quar- 



