THE EARTH AS A PLANET 355 



4. Distinguish between rotation and revolution for the earth. What is 

 meant by the orbit of the earth? 



5. How much nearer the sun is the earth in our winter season than in summer 

 time? What is true of the relative lengths of the times of revolution of 

 the earth, i.e., of the lengths of the years? What does this argue of the 

 averages of the earth's velocity in its orbit, and of the extent (length) of 

 that orbit year by year? 



6. Why are those circles of the earth known as the Tropics located 23^ 

 on either side of the earth's equator rather than some other distance? 



7. Account for eclipses (a) of the moon; (b) of the sun. 



8. The diameter of the earth is 8000 miles; of the sun 880,000 miles. How 

 many times greater volume has the sun 1 ? 



9. What is the cause of the varying obliquity of the sun's rays as found in the 

 lesson on Solar Heating? 



THE MOON, AND ITS PHENOMENA 



It is in the changing phases of the moon, and in its varying 

 angular distances from the sun, that we can actually see the 

 revolution of one celestial body about another. The moon 

 is only 240,000 miles from the earth, a small distance in the 

 scale of the celestial sphere. In its revolution about the 

 earth the moon comes in between earth and sun (though 

 seldom in a direct line), and moves on away from the sun 

 farther and farther as measured in degrees along the curved 

 background of the sky. When it is on the side of the earth 

 opposite the sun, and 180 from it, an observer on the earth 

 sees the side of the moon that is illuminated. Then as the 

 moon continues in its path around the earth it approaches 

 closer and closer to the sun as seen from the earth till once 

 more it is on the same side as the sun. The dark side of the 

 moon is then toward the earth. The moon has no light (or 

 heat) of its own. 



There is an abiding satisfaction in being able definitely 

 and quickly to grasp the relationship of sun, moon, and 

 earth in space as the moon's form and its position relative 

 to the sun are noted. It makes an excellent preparation for 



1 The volumes of spheres are to each other as the cubes of their diameters. 



