TEMPERATURE OF THE AIR 



237 



When they are vertical farther south than 15 S., points on the 

 parallel of 75 N. do not see the sun. When the sun's rays are 

 vertical in latitude 15 N. (B) or in any latitude farther north, no 



- -ymv- ^c; 



n \ \ \ 



30 



Fig. 213. Diagram to illustrate seasons in latitude 75. When the sun's rays 

 are vertical at C, the circle of illumination is represented by the line 90 

 -90. The half of each parallel of 75 is then illuminated, and days and 

 nights on that parallel are therefore equal. The same is true of all other 

 latitudes. When the sun's rays are vertical at B, in latitude 15 N., the 

 circle of illumination is represented by b 6, the whole of the parallel of 

 75 N. is illuminated, and daylight is continuous throughout the twenty- 

 four hours. No part of the parallel of 75 S. is illuminated at this time 

 and on that parallel darkness is continuous. When the sun is vertical 

 at A, in latitude 233^ N., the circle of illumination is represented by 

 a a. While the sun appears to move from position B to position A and 

 back again to B, the parallel of 75 N. is continuously illuminated, while 

 the parallel of 75 S. at the same time is continuously in darkness. When 

 the sun appears to move from the position where its rays are vertical at 

 B to the position where its rays are vertical at D, a part of each parallel 

 of 75 is illuminated, and during this time, therefore, there is light and 

 darkness in the course of the twenty-four hours. When the sun's rays 

 are vertical between B and C, more than half of the parallel of 75 N. is 

 illuminated, and less than half of the parallel of 75 S. When the sun 

 is vertical at C the half of each parallel of 75 (and of all other parallels) 

 is illuminated, and days and nights are equal everywhere. The conditions 

 in latitude 75 S., while the sun appears to move from C to E and back 

 to C, may be worked out by the student. 



point on the parallel of 75 N. will be in darkness during any part 

 of the twenty-four-hour day. When the sun's rays are vertical 

 in any latitude between 15 S. and 15 N., a part of the parallel 



