THE EARTH AND ITS NEIGHBORS 409 



the sky at night and the part which is illuminated by the 

 sun is turned toward us and reflects the sun's light to us 

 the same as the wall of a building will reflect light. When 

 all of the moon which is illuminated by the sun can be 

 seen, the moon is said to be full. When only one-half of 

 the illuminated part can be seen, it is called first quarter 

 during the light of the moon and last quarter during the 

 dark of the moon. The period known as " light of the 

 moon" lasts 14 days, extending from "new moon" 



DIAGRAM OF AN ECLIPSE 



Showing why an object casts a shadow of two intensities 

 when in the presence of a large luminous body like the sun. 



to "full moon," during which time the visible illuminated 

 part is increasing in size. The period known as "dark of 

 the moon" also lasts 14 days, during which time the 

 visible illuminated part is decreasing in size. 



284. Eclipse of Sun and Moon. The earth and moon, 

 like all opaque objects, cast a shadow in the direction 

 opposite the sun. Since the sun is very large, being 100 

 times the diameter of the earth and 400 times the diameter 

 of the moon, the shadows of the earth and moon come to 

 a point and are cone-shaped. The average length of the 

 earth's shadow is 856,000 miles and that of the moon is 

 232,000 miles. When the moon happens to pass through 

 the earth's shadow at night, a part or all of it is invisible 



