148 



GENERAL SCIENCE 



The obliquity of the sun's rays from its rising to setting 

 changes from 90 in the morning to a minimum value when 

 on meridian (solar noon), and then to 90 again when it sets. 

 It will be found by continued observations that this daily 

 path varies in points of rising and setting, and in meridian 

 altitude (number of degrees up from the south point of 

 horizon at noontime). Changes in the diurnal circles of the 

 sun relative to our horizon are the result of the revolution of 

 the earth about the sun. Any complete understanding of 

 these changes requires the study of Astronomy. 



FIG. 51. Relation of solar heating to altitude of the sun. An illustration 

 of the teaching that the more nearly vertical the sun's rays the more restricted 

 the area heated by a solar beam of any given cross-section, and the more 

 intense the heating effect. 



It is not at all difficult to imagine, especially during the 

 month of June every year, that if the sun continued after 

 June 21 to rise farther and farther north of east, and to set 

 farther and farther north of west, there would come a time 

 when the sun neither set nor rose. Its diurnal circles would 

 remain continuously above the horizon. We are taught in 

 astronomical geography that an observer within 23^ of the 

 poles of the earth (inside the Polar Circles) may witness the 

 sun continuously above the horizon for days at a time; that 

 at the poles this time of continuous sunlight (absence of night- 

 time) must be somewhat more than six months in duration. 



