288 ROUND THE YEAR 



THE SHORTEST DAY OF THE YEAR. 



Dec. 21. The sun rises in London at 8 h. 6 m., 

 and sets at 3 h. 5 1 m. To-day there is less than eight 

 hours of full daylight, more than sixteen hours of 

 night and twilight. Even at noon the sun has little 

 power. His rays strike us at a low angle, 15 only. 

 On the longest day he attains a height of 62, more 

 than four times as high as at noon on the shortest 

 day. 



The sunshine is not only brief but faint, because of 

 the small elevation of the sun at noon. Take any 

 definite part of the earth's surface, such as a particular 

 field. If that were turned full towards the sun it would 

 receive the greatest possible number of rays ; if turned 

 edgewise, it would receive none ; for any intermediate 

 position it would receive more or fewer according to 

 its inclination, the amount being proportional to the 

 sine of the angle. At 62 it would receive '883, at 1 5 

 only -259 of the full amount, which is taken as unity. 



When the sun is low, his rays pass very obliquely 

 through the earth's atmosphere, and much light and 

 heat are absorbed. If the depth of the atmosphere is 

 taken at unity, a ray passing through it at 62 from 

 the horizontal plane will be 1*133 l n g 3 '864 long 

 at 15. 



The shortest day is not on an average of years the 

 coldest day, nor the longest day the hottest We 

 have to take into account the effect of foregoing 

 temperature. In summer the surface of the earth 



