PART IV. THE EAETH IN RELATION 

 TO OTHER ASTRONOMICAL BODIES 



CHAPTER XX 

 THE MOON, PLANETS, AND COMETS 



247. Questions for Discussion. 1. Does the moon shine by its own 

 light? Does the sun? Does the earth? 2. What causes the water to 

 rise from oceans and lakes, thus maintaining the supply which flows 

 down again and provides water power? 3. What is the source of the 

 energy held in the fuel from which heat and electricity may be pro- 

 duced? 4. Is the sun the source of all our energy? 5. Is the sun the 

 source of energy for all the stars and planets ? 6. Why does the moon 

 change its appearance from evening to evening? 7. What causes the 

 change of seasons on the earth? 8. Is the moonlight which comes to 

 us from the moon or from the sun ? 9. Since both earth and moon re- 

 volve about the sun, how do you account for the fact that the moon is 

 sometimes visible from the earth and sometimes not ? 10. How could 

 you make a simple telescope so as to study the moon? 11. What are 

 the reasons for thinking that the moon is not inhabited and does not 

 produce crops of plants as does the earth? 12. What is an eclipse of 

 the moon? 13. What arguments are presented to show that Mars is 

 inhabited? 



248. Introductory statement. In Part III we discussed 

 the use of energy upon the earth, and the sun as our source 

 of radiant energy. It will be profitable at this point to con- 

 sider the relation of the earth to the sun, moon, stars, and 

 planets. We may gain a notion of the relative insignifi- 

 cance of the size of the earth and of the men who live upon 

 it Notwithstanding our smallness as parts of the universe, 

 we have a right to dignify ourselves by recalling that it is 



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