352 GENERAL SCIENCE 



the results of a lessened insolation and of the ever increasing 

 losses of heat by reflection as the angle of obliquity increases. 

 The daily path of the sun rises continually higher in the 

 heavens from December to June, the sun rising farther and 

 farther north, crossing the meridian higher and higher up 

 from the south point of horizon, and setting farther and 

 farther north. This makes an ever enlarging diurnal circle 



b' 



FIG. 116. Diurnal circles of the sun at different seasons: axa', at summer 

 solstice (June 21); bzb', winter solstice (December 21); EyW, vernal and 

 autumnal equinoxes (March 21, and September 21). dd' is the diurnal circle 

 of a star which for an observer in northern latitudes never rises nor sets, i.e., 

 a star always above horizon. 



above the horizon, and longer and longer daytimes. Upon 

 these changes in obliquity of the sun's rays, and upon the 

 duration of the periods of sunlight, depends the changes of 

 seasons. 



These changes are scarcely considered from day to day, 

 and less frequently are they given their real significance in 

 terms of the revolution of the earth about the sun. When 

 one looks out toward the sun at any time, it is difficult to 

 realize that the distance between the earth and the sun is 

 approximately the enormous value of 93,000,000 miles. 

 So distant is the sun from the earth that of the heat given off 

 by the sun only about one two-billionth part is received by the 



