

AV DESCRIPTIVE ASTRONOMY. 125 



utes a day, and no other group of stars exhibits to the naked 

 eye an observation of time so easily made. How often have 

 we heard our guides exclaim in the savannas of Venezuela, or 

 in the desert extending from Lima to Truxillo, "Midnight is 

 past, the Cross begins to bend ! " How often those words re- 

 minded us of that affecting scene, where Paul and Virginia, 

 seated near the sources of the river of Lataniers, conversed 

 together for the last time, and where the old man, at the sight 

 of the Southern Cross, warns them that it is time to separate. 

 HUMBOLDT. 



31. Why does not the earth move with equal velocity in 

 all parts of its orbit ? 



Because at perihelion it is nearer the sun than when in 

 aphelion, and hence the attraction is stronger. 



32. How many Jovian-years old are you ? 



A Jovian-year equals 11.86 earth-years. 



33. Why is the sky blue ? 



The blue light of the firmament is light reflected by solid 

 particles generally of aqueous vapor in the air. It is notice- 

 able that early in the morning and late at night, when the 

 sun's rays fall obliquely upon the atmosphere, they are polar- 

 ized by reflection. The reflected light is blue ; the transmitted 

 light of the sky is orange or red. (Read Tyndall's Light, p. 

 152.) 



34. At ivhat season of the year does CJiristmas occur in 



Australia ? 



(See Astronomy, Art. vii., p. 98.) 



35. Wliat causes the apparent movement of the sun 

 nortJi and south ? 



(See Astronomy, p. 95.) 



It is caused by the fact that the axis of the earth is inclined 

 to the plane of the ecliptic, while the earth is revolving about 

 the sun. 



36. On what part of the earth is the twilight tlie long- 

 est? Tlie shortest? 



(See Astronomy, p. 116 ; Manual, p. 116.) 



