176 ELEMENTARY SCIENCE 



gain false impressions about the relations as to move- 

 ment between earth and sun. 



During that day, December 22, when the sun is over 

 the tropic of Capricorn, it is evident that the region (cir- 

 cular in outline) around the north pole which remains in 

 darkness throughout the twenty-four hours attains its 

 largest circumference, for on that day the north pole is the 

 farthest within the shadow of earth that it ever gets (see 



Fig. 65). That parallel 

 which throughout that day 

 divides the northern area of 

 darkness from the southern 

 * area of light, the last par- 

 allel toward the north 

 which the sun's rays touch 

 when it is thus in its most 



FIG. 65. Diagram showing where the Southerly position, that 



^ns ffSTJSS parallel forms what we call 

 the tropk f Cancer in the arctic circle. Similarly, 

 the limits of illumination, 



when the sun is at its farthest northern point, mark the 

 antarctic circle. It is a simple problem in geometry to 

 show that the arctic circles must be just as far from the 

 poles as the tropics of Cancer and Capricorn are from 

 the equator. 



Between the tropics and the arctics lie the great tem- 

 perate zones, and it is in these that the seasons are 

 most marked. In them all life responds to the changes 

 of heat and cold, of rain or drought that accompany 

 the seasons, and so closely interwoven are these causes 

 and their effects that we have come to regard the seasonal 

 changes in plant and animal life as parts themselves of 



