110 ANSWERS TO PRACTICAL QUESTIONS 



21. W7iy is the angle which the celestial equator makes 

 with the horizon constant ? 



(See Astronomy, p. 29.) 



The celestial equator is perpendicular to the axis of the 

 heavens, and hence all parts of it make the same angle with 

 the celestial axis and with the horizon. 



I 98 1. Would the earth rise and set to a Lunarian ? 



(See Astronomy, p. 134.) 



The earth would not rise or set, as the moon does with us, 

 but would merely oscillate to and fro through a few degrees. 

 A Lunarian would see the earth constantly in the sky, under- 

 going all the phases the moon presents to the earth. But when 

 it is full moon to us, it is new earth on the moon. During the 

 first and last quarters, the changes would occur during the day- 

 time ; during the second and third, in the night. The rapid 

 rotation of the earth, repeated fifteen times during a lunar 

 night, must greatly diversify the appearance of the earth. See 

 Olmstead's Letters on Astronomy, p. 180. 



2. Could there be a transit of Jupiter ? 



(See Astronomy, p. 67.) 



No. Jupiter is a superior planet. 



3. IVliy does Mars' inner-moon rise in the west ? 



(See Astronomy, note on p. 153.) 



This satellite performs a revolution in its orbit in less than 

 half the time that Mars revolves on its axis. In consequence, 

 to the inhabitants of Mars, it would seem to rise in the west 

 and set in the east. The revolution of the moon around the 

 earth and of the earth on its axis, are both from west to east ; 

 but, the latter revolution being the more rapid, the apparent 

 diurnal motion of the moon is from east to west. In the case 

 of the inner satellite of Mars, however, this is reversed, and it 

 therefore appears to move in the actual direction of its orbital 

 motion. The rapidity of its phases is also equally remarkable. 

 It is less than two hours from new moon to first quarter. New- 

 comb and Holden's Astronomy, p. 339. 



