214 



PHYSIOGRAPHY 



since the equator is always bisected by the circle of illumination. 

 Days and nights are not always equal in any other latitude^ unless 



Fig. 207. Diagram to illustrate the effect of inclination of the earth's axis 

 on the length of day and night. In the figure, more than half of every 

 parallel of the northern hemisphere is illuminated. The days in the 

 northern hemisphere are therefore more than twelve hours long, since 

 the half of each parallel is the measure of 180 of longitude, and 180 of 

 longitude corresponds to twelve hours of time. Similarly less than half 

 of every parallel of the southern hemisphere is illuminated, and the days 

 are therefore less than twelve hours long. 



Fig. 208. The relation of the earth to the sun's rays at a time six months 

 later than that represented in Fig. 207. The conditions of day and 

 night in the hemispheres are reversed. 



at the poles, where there is one day of six months and one night of 

 six months. 



Apparent motion of the sun. The effect of the inclination of 

 the axis of the earth is to make the sun appear to move north and 

 south once during each revolution of the earth about the sun. The 



