THE NON-LIVING WORLD 181 



The planets, like the earth, revolve on their axes while 

 they go round the sun, and their satellites revolve round 

 the planets they attend more slowly than such planets 

 revolve on their own axes. An exception, however, 

 o'ccurs in the case of the planet Mars, one of the 

 satellites of which circulates round it in less than a 

 third of the time that planet takes to revolve on its own 

 axis. 



Since the earth's orbit is elliptical, our globe must be 

 nearer the sun at one time (in one part of its path) than 

 at another. It is furthest from the sun during our 

 summer and nearest in winter, but its axis slants in such 

 a manner that its north pole inclines towards the sun in 

 summer and away from it in winter. Therefore the 

 earth's northern hemisphere receives the sun's rays most 

 directly when it is furthest from it, and least so when it 

 is nearest to it. This is why our summer season is the 

 warmest, while the same period is the coldest for the 

 globe's southern hemisphere. In spring and summer 

 the condition is intermediate, hence the four " seasons." 



During each spring and autumn, our obliquely inclined 

 globe is for a short time in such a position that the 

 illuminated half of its surface is situated symmetrically 

 as regards the poles. This is the period of the equinoxes, 

 or of equal night and day all over the globe. 



As the northern pole becomes more and more inclined 

 towards the sun, its daylight is prolonged till (e.g. North 

 of North Cape) the sun never sets, while in winter it 

 never rises. The opposite condition of course obtains 

 at the Antarctic, or south pole. 



The circuit described by the earth in its path round 

 the sun is constantly changing to a small degree. It 

 alternately approximates to, and diverges further from, 

 a truly circular path. It is evident that when its degree 



