36 LIGHT, VEGETATION AND CHLOROPHYLL 



It has been found that a surface of 1 sq. cm., exposed 

 perpendicularly to the rays of the sun outside the earth's 

 atmosphere, receives a power of 0-135 watts, or 1-94 calories 

 per minute. This power per square centimetre is called the 

 solar constant. 



Is the solar constant really a constant? There is certainly 

 one cause of variation, namely, the distance of our globe 

 from the sun. While the earth is describing in the course of a 

 year its elhptical orbit round the sun situated at one of the 

 foci of the ellipse, the distance changes. We are nearer to the 

 sun at the winter solstice than at the summer solstice. The 

 maximum variation is 3-4 per cent, and the illumination, 

 which is proportional to the inverse of the square of the 

 distance, consequently varies by 6-8 per cent. The solar 

 constant is therefore a mean value. 



Effect of Latitude 



It may seem surprising that we are nearer to the sun in the 

 winter than in the summer, but the resulting difference in 

 illumination is small and does httle to diminish the prepon- 

 derant iniSuence of the height of the sun; the more its rays 

 are inchned, the greater is the area over which the same 

 flux has to be distributed. 



Thus, at Bordeaux, at the medium latitude of 45°, the sun 

 at midday at the equinoxes is at an angle of 45° from the 

 . vertical; at the winter solstice it is at 69° and at the summer 

 solstice at 21°. Consequently, and without taking into account 

 the absorption of the atmosphere which increases the 

 differences still more, the flux of solar radiation for a given 

 horizontal illuminated area is half as great in December, and 

 1 -3 times greater in June, than it is in September and March. 

 There is, therefore, at the confines of the atmosphere at this 

 latitude, 2-6 times more solar radiation in summer than in 

 winter over the same area, so that we can see that the 

 6-8 per cent which depends on the distance of the earth from 

 the sun is trifling. 



At higher latitudes the difference between summer and 



