116 ANSWERS TO PRACTICAL QUESTIONS 



38. What keeps the earth in motion around the situ ? 



(See Astronomy, p. 22.) 



According to the First Law of Motion, "Every body con- 

 tinues in its state of rest or of uniform motion in a straight 

 line, except in so far as it may be compelled by impressed 

 forces to change that state." 



.7.9. Do we ever see the sun after it sets ? 



(Sec Astronomy, p. 114.) 



The refraction of the atmosphere tends to raise all objects 

 toward the zenith, and, at the horizon, this is no less than 35', 

 or 3' more than the mean diameter of the sun (32'). 



40. When does the earth move the most rapidly in its 

 orbit ? 



(See Astronomy, p. 18.) 

 The earth moves most rapidly in perihelion. 



4:1. Have ive conclusive evidence that any planet is in- 

 habited ? 



(See Astronomy, p. 61; also note, p. 297.) 



May it not be that the same lavish hand that scatters flow- 

 ers and seeds in such profusion (not one in a thousand coming 

 to the perfection and end of its being), sows space with worlds, 

 a few only reaching the full fruition of life ? 



42. When is tivilight the longest? The shortest? Why? 



(See Astronomy, p. 116.) 



Twilight is usually reckoned to last until the sun's depres- 

 sion below the horizon amounts to 18 ; this, however, varies ; 

 in the tropics a depression of 16 or 17 is sufficient to put an 

 end to the phenomenon, but in England a depression of 17 to 

 21 is required. The duration of twilight differs in different 

 latitudes ; it varies also in the same latitude at different sea- 

 sons of the year, and depends, in some measure, on the meteoro- 

 logical condition of the atmosphere. Strictly speaking, in the 

 latitude of Greenwich there is no true night from May 22 to 

 July 21, but constant twilight from sunset to sunrise. Twilight 

 reaches its minimum three weeks before the vernal equinox, 

 and three weeks after the autumnal equinox, when its duration 



