THE CAUSE OF THE SEASONS 



31 



then the shortest and the days in the southern hemisphere 

 the longest. From this point until March 21, when the sun 

 is again vertical over the equator, the inclination of the north 

 pole away from the sun decreases. The days when the 

 sun is over the equator are called the autumnal (Sept. 23) 

 and vernal (March 21) equinoxes, since the days and nights 

 are then of equal length all over the earth. 



The greater heat- 

 ing of the hemisphere 

 at one part of the 

 year than at another 

 gives us the changes 

 which we call the 

 Since the 



A LAPLANDER'S HUT 



Made of thick sod to retain heat in 

 the frigid zone. 



seasons. 



change in the length 



of the day and in the 



direction of the sun's 



rays is very small 



within the tropics, the 



change in the amount 



of heat received is 



very slight, so that 



in this region there 



is almost no change of seasons. But at the poles, where for 



six months there is continuous night and for six months 



continuous day, the change of seasons is exceedingly great. 



At middle latitudes the changes, though marked, are not 



excessive. 



There are then two causes which combine to give us our 

 change of seasons : the revolution of the earth around the 

 sun. and the inclination of the earth's axis to the plane of 

 its orbit. 



