THE EARTH AS A PLANET 183 



the sun would thus be directly overhead as the earth turns 

 on its axis. 



State the relative lengths of the path of the observer 

 through the illuminated and the unilluminated regions, and 

 of the day and night periods for the observer. 



ROTATION COMBINED WITH REVOLUTION, AND 

 CHANGES IN SEASONS 



Upon a crayon box on the table support horizontally an ellipse that 

 has been cut from heavy cardboard. Its major axis (long diameter) 

 should be at least two feet, and its sides so slightly flattened that it is 

 very nearly a circle. Let its circumference represent the earth's path 

 around the sun (the earth's orbit). Into a hole a little at one side of 

 the center (and toward one "end") crowd a marble nearly halfway 

 through to represent the position of the sun in the earth's orbit. 



1. What is meant (a) by an ellipse; (6) by the revolution 

 of a body as distinguished from rotation ? Around this 

 elliptical circumference carry several times the sphere that 

 represents the earth, always observing, however, these two 

 directions : 



1. Have the sphere tipped northward so that its axis 

 makes an angle of approximately 23J with a perpendicular 

 to the cardboard. (This tips the axis about one fourth way 

 over from a vertical position.) 



II. Keep the direction of the axis, no matter where the 

 sphere is placed, at all times parallel to this first direction, 

 This makes it at all times point towards the North Star in 

 the heavens. 



2. Adjust the ellipse so that when 4 the sphere is placed at 

 the end of the major axis nearer the marble, the north pole 

 of the sphere (earth) is tipped away from the marble (sun). 

 Then state : 



