THE EARTH AS A PLANET 181 



angular distance of any visible object from the direction 

 (plane) of the cardboard as you turn more and more; (6) 

 what is found true of the relative distances apart at differ- 

 ent times of any two objects in sight ; (c) how far around one 

 must turn that an object shall apparently have moved 90 

 degrees from the plane of the cardboard. 



4. Repeat the above but with cardboards extending out- 

 wards from both sides of the face. State (a) when an 

 object may be said to "set" ; (b) how far around one must 

 turn that an object which has once " set" may "rise" again ; 

 (c) the significance of saying that an object thus coming into 

 view, and then disappearing behind the cardboard horizon, 

 has an "apparent motion" ; (d) where the real motion exists. 



5. What do you mean by the east horizon of any ob- 

 server? By the west horizon? What prevents our seeing 

 the sun when it 



is below the west 

 horizon ? 



Instead of think- 

 ing of the sun as 

 moving down be- 

 low the west hori- 



. , ,, FIG. 79. The horizon at sea. 



zon at nightfall, 



think of the sun as standing still. Account for the so-called 



"setting" of the sun. What does "sunrise" really mean? 



6. Use a sphere with a heavy wire axis to represent the 

 earth, and by means of a thumb-tack fix a spot on the sphere 

 that represents your position as an observer in the northern 

 hemisphere. Let this same tack hold in place for an horizon 

 plane a circular cardboard sufficiently large to have the 



