LONG AND SHORT DAYS 



397 



with the position of 



the earth in its or- 

 bit. On March 21 



(Fig. 265), the circle 



of illumination 



passes through the 



north and south 



poles, and half of 



the equator and 



half of every parallel 



of latitude l (Fig. 



266) are alternately 



illuminated and 



shut off from the 



sun, and days and 



nights are equal 



everywhere. On 



June 21, the circle of illumination extends beyond the north 



pole, but does not reach the south pole; people living in the 



northern hemisphere have long days 

 and short nights ; people living in the 

 southern hemisphere have short days 

 and long nights. In this position of 

 the earth, the north pole is constantly 

 illuminated in spite of the rotation of 

 the earth on its axis, and has per- 

 petual day unbroken by night. The 

 south pole, on the other hand, receives 



FIG. 266. Parallels of latitude no illumination and has perpetual 



and meridians of longitude. . , 



night. 



On September 22, the circle of illumination again passes 

 through the poles and day and night are equal everywhere. 

 1 A parallel of latitude is an imaginary circle drawn parallel to the equator. 



FIG. 265. Position of the northern hemisphere 

 during the year. 



