286 



THE SEASONS CLIMATE AND HEALTH 



309. Length of Day and Its Effect upon the Heating Power 

 of the Sun. The days and nights are always equal in length 

 at the equator, 12 hours each. The poles of the earth have 

 six months day and six months night. Between the equator 

 and the poles, the length of day and night is constantly 

 changing. At latitude 40 north, the days vary in length 

 from about nine hours on December 22 to 15 hours on June 

 21. The farther north we go the longer the summer day 

 becomes till we reach the north pole when the day is six months 

 in length. Just why this is so is easily shown by experiment. 



Exercise 68. To Show Why the Length of Day Varies at Different 



Places on the Earth's Surface 



(a) Use an orange or a small schoolroom globe for this experiment, 

 and perform it in the evening or in a darkened room. Place a lighted 

 lamp upon the table and hold the orange a few feet from it. If you 

 use an orange, let the stem and the bloom scars of the orange be the 

 two poles and draw a line about the orange to represent the equator. 



FIG. 216. The length of day. 



Let the north pole be tilted toward the lamp 23^. Now note as 

 carefully as possible the position of the dividing line between the 

 lighted and unlighted surfaces of the sphere. Draw a pencil line 

 around the sphere to mark this line. If we now mark the 40th parallel 

 of latitude on the sphere, we shall find that about ^^ of it was lighted. 

 This means that, if we revolve the sphere upon its axis, that any point 

 upon the 40th parallel would be in the light during i% 4 of a revolu- 

 tion. Therefore, the day at the 40th parallel is 15 hours in length 

 on June 21, or at the SUMMER SOLSTICE. At this same time the pole 

 is constantly in the light, and just % of the equator is lighted. 



