LIGHT AND VEGETATION 43 



At two seasons of the year, the equinoxes, the length of the 

 day is also twelve hours over the whole surface of the globe. 

 In all other cases, the days and the nights are unequal. 



Our globe is constantly bathed in sunUght over one half 

 of its surface, while the other half is in shadow; as a result 

 of the diurnal rotation of the earth about its axis, each point 

 on its surface, except those in the polar regions, passes suc- 

 cessively in twenty-four hours through the light and dark 

 spaces, thus creating the alternation of day and night at that 

 point (Fig. I, 9). 



The speed of rotation is uniform ; the length of day and 

 night at one point depends on what fraction of its circular 

 path is illuminated. As a result, all the places situated on the 

 same parallel — or, v/hich is equivalent, at the same latitude — 

 have days of the same length. 



If the boundary of the shadow passed through the poles, 

 which happens only at the equinoxes, it would divide all the 

 parallels into two equal parts and the days would everywhere 

 be equal to the nights. But the axis through the poles makes 

 an angle with the plane of separation of the shadow and the 

 light which varies in the course of the year and reaches its 

 maximum value, 23 J°, at the time of the solstices. 



Let us consider the season of the summer solstice. The 

 North Pole is in the illuminated hemisphere at 23J° from the 

 circle of separation. The day there is continuous; it is the same 

 for all the parallels of latitude situated in the neighbourhood 

 of the Pole, including the one whJch is separated from it by 

 23 J° and which is called the Arctic Circle. All the polar region 

 within this circle performs its daily rotation without passing 

 into the shadow so that there the sun does not set. 



Round the South Pole, the region within the Antarctic 

 Circle remains, on the contrary, constantly in shadow and 

 there the sun does not rise. 



Between these two polar circles, the lengths of day and 

 night vary progressively and are equal at the equator. 



It remains to be explained why the Earth's axis makes a 

 variable angle with the plane of separation of shadow and 



