IN DESCRIPTIVE ASTRONOMY. 115 



have not yet attained that degree of density which must neces- 

 sarily precede the formation of a solid surface. They are, 

 therefore, now in a geologic age similar to that in which the 

 earth existed before its crust had become solidified. (See Schel- 

 len's Spectrum Analysis, p. 337.) 



SO. Hoiv many titties does the moon turn on her axis 



each year ? 



(See Astronomy, p. 123.) 



The moon turns on her axis once each month. 



31. Can you explain the different signs used in the 



almanac ? 



(See "Astronomical Signs" in the Dictionary.) 



32. Shoiv how the moon is a prophecy of the earth's 

 future. 



The moon is a worn-out globe, and presents the same ap- 

 pearance that the earth will probably offer ages hence. 



33. Does the sun really rise and set ? 



(See Astronomy, pp. 14, 87.) 



No. This is only an optical illusion, being an illustration of 

 our tendency to transfer motion. 



34. Are the bright portions of the noon mountains or 

 plains ? 



The lofty portions, or mountains, of the moon reflect the 

 light to the earth most strongly, and hence appear the bright- 

 est. The deep valleys, lying in shadow, look dark. 



35. Which of the heavenly bodies are self-luminous ? 



(See above, Question 29 ; also Astronomy, note, p. 163.) 

 Jupiter and Saturn probably emit light, at least from the 

 brighter spots of their surface. Read Newcomb's Astronomy, p. 

 342. 



36. Why is not a solar eclipse visible over the whole 



earth ? 



(See Astronomy, p. 140.) 



37. What is meant by the "mean distance" of a 

 planet ? 



The "mean distance" is the average distance. 



