TABLE 8. DATES OF THE BEGINNING AND DURATION (IN DAYS) OF ARCTIC SEASONS 



These simple schemes are somewhat modified by refraction, which elevates the sun above 

 the horizon, and the twilight phenomena of either the civil or astronomical state.* 



The change of astronomical conditions is determined not only by the earth's rotation around 

 the sun (upon which the seasons of the year depend) and around its own axis (upon which the time of 

 day depends), but also by the change of distance from the earth to the sun. 



It is assumed that the solar heat reaching the upper limits of the atmosphere equals 1.94 

 g-cal per min. per cm^ of the surface perpendicular to the sun's rays. This magnitude is called 

 the solar constant and it varies somewhat with time in connection with the variation of the amoimt 

 of sunspots and with the variation of distance to the sun. Thus, during the years that are charac- 

 terized by the maximum number of sunspots, when the sun's surface is most active, the solar con- 

 stant is approximately 2 per cent greater than during the years that are characterized by the min- 

 imum number of sunspots. When the earth is nearest to the sun (about January 1, perigee) the 

 constant is 1.07 times greater than when the earth is farthest from the sun (about July 1, apogee). 



There are incident and diffuse radiations. The incident radiation is associated with direct 

 sunbeams, the diffuse radiation with sunbeams that are reflected in the atmosphere and that strike 

 objects from all sides. 



The incident and diffuse radiations add up to the total solar radiation. 



The intensity of solar radiation (as well as the solar constant) is measured in g-cal per min. 

 per cm2. 



♦Civil twilight lasts from the moment the upper rim of the sun disappears to the moment the 

 sun reaches 7° below the horizon. This is a conditional concept. It is assumed that the illumi- 

 nation during civil twilight is sufficient for reading In the open. 



Astronomical twilight lasts from the sun's disappearance to the moment the sun reaches 17' 

 below the horizon and the stars of the 6th magnitude can be distinguished by eye . 



26 



