44 LIGHT, VEGETATION AND CHLOROPHYLL 



light. This is a result of the revolution of our planet round the 

 sun; the ellipse described during this revolution is in a plane 

 to which the axis of rotation of the earth is not perpendicular 

 but is inclined at an angle of 23^° from the normal. 



The movement of the earth has been compared to that 

 of a top which spins rapidly on its point (this represents the 

 diurnal rotation) wliile its point describes circles on the 

 ground (this represents the annual revolution) ; the axis of 

 the top is also inclined, but here the analogy ceases, for the 

 axis of the earth remains always parallel to the same fixed 

 direction, while that of the top describes a cone. 



Because its axis retains this fixed direction in space, the 

 earth turns its north pole towards the sun at the summer 

 solstice; then, six months later, when it is at the opposite 

 point of its orbit, it turns its south pole towards the sun. At the 

 equinoxes, when it is half way between these two positions, 

 the two poles are at equal distance from the sun and they 

 are situated on the contour of the shadow. 



This geometric description takes into account, not only 

 the unequal length of the days, but also the different inchna- 

 tions of the sun above the horizon at the different seasons, 

 which, as we have already seen, cause variations in the solar 

 illumination received on a horizontal surface. 



The table below gives the length of the longest day and of 

 the shortest day for different northern latitudes not included 

 within the Arctic Circle. 



