THE EARTH'S NEAREST NEIGHBORS 45 



image is brighter. Secondly, by its form the lens directs the 

 light in such a way that the image is magnified. 



The largest lenses used in telescopes in this country are the 

 lens forty inches in diameter in the Yerkes telescope of the 

 University of Chicago, and one of thirty-six inches in 

 the Lick telescope of the University of California. Many 

 colleges have telescopes with lenses from eight to twenty 

 inches in diameter, and the great universities and national 

 observatories have much larger ones. The largest telescope 

 in the United States Naval Observatory at Washington has 

 a twenty-six-inch lens. 



Questions on Fig. 14. 1. What kind of eclipse does this picture represent? 

 2. Considering only the motion of the moon around the earth, in which 

 direction would the shadow spot on the earth travel? 3. Does it travel 

 more or less rapidly because the earth moves in the same direction? 4. If 

 the moon were fixed and the earth rotated, as it does, in what direction 

 would the shadow spot move? 



EXERCISES 



1. How may any observer distinguish the planets from stars? 

 Could it be done in one observation? Why? 



2. Which is more like a mirror, the surface of land or of water? 

 Does the planet earth, then, reflect much or little light? 



3. Why is it more reasonable to talk of the possibility of life upon 

 Mars than upon any other planet? 



4. Which receives more light and heat from the sun, Mars or the 

 earth? Give the reason. 



5. How many rotations does the earth make while the moon is 

 making one? 



6. What portion of the moon is lighted by the sun at one time? 

 From what body does the moon receive light upon one half only? 



7. Why does the moon have day and night? How long does day- 

 light last at any one place on the moon? 



8. In what direction is the full moon seen at sunset? 



9. In what part of the sky, and at what time of the day, is the 

 new moon first visible? What is its form? 



10. Why is not the moon visible from the earth at the time of a 

 solar eclipse? 



