126 ANSWERS TO PRACTICAL QUESTIONS 



" Where the air is unusually full of condensed vapor, as 

 occurs in polar regions, the twilight is greatly lengthened ; 

 where the air is unusually dry, as occurs in the tropics, twilight 

 is said sometimes to be shortened to fifteen minutes." 



37. Name the causes which make our summer longer 

 than winter. 



(See Astronomy, p. 102.) 



38. Why is not total darkness produced when a dense 

 cloud passes between us and the sun ? 



(See Astronomy, p. 117.) 



39. Why does the time of the tide vary each day ? 



(See Astronomy, p. 148.) 



40. Why is an annular longer than a total eclipse ? 



(See Astronomy, p. 140.) 



4:1. Why is it colder in winter than in summer ? 



(See Astronomy, pp. 97, 98.) 



42. Do the solar spots affect our iveather ? 



(See Astronomy, p. 48.) 



43. Can the moon be eclipsed in the day-time ? 



(See Astronomy, p. 114.) 



44. Why are the sidereal days of uniform length ? 



Because of the almost absolute uniformity of the earth's 

 rotation. 



45. Why are not the solar days of uniform length ? 



(See Astronomy, p. 266.) 



46. What do the moon 9 s phases prove ? 



The moon's phases prove that she is spherical, and shines 

 by the reflected light of the sun. 



47. Why do the sun and moon appear flattened when 



near the horizon ? 



(See Astronomy, p. 115.) 



48. How many stars can we see with the naked eye ? 



No one sees more than 6,000, and few more than 4,000 stars. 



