26 



FIRST YEAR SCIENCE 



cerned, these zones are easily separated and bounded by 

 parallels of latitude, yet, on account of other influences, 

 the temperatures of the zones thus bounded are very ir- 

 regular. For instance, at some places like Hammerfest 

 within the frigid zone the average temperature is much 

 higher than at places like Labrador within the temperate 

 zone, so that as regards temperature, the parallels of 

 latitude are uncertain boundaries. This will be more fully 

 discussed later. 



12. The Cause of the Seasons. Since the earth moves 

 around the sun with its axis inclined to the plane of its orbit, 



the hemispheres will at 



MADTM rnniuhv 



different times be in- 

 clined toward and away 

 from the sun. When 

 the northern hemisphere 

 is inclined toward the 

 sun, the rays of the sun 

 cover the north pole 

 continuously for six 

 months, so that at this 

 point there is no night 



Showing rou^the^positions men- for ^ ^ ^ The 



days are longer and the 



nights shorter throughout all the northern hemisphere, 

 and more than this, the rays then fall upon this hemisphere 

 more nearly vertically than during the rest of the year. 



The nearer vertical the rays, the greater the number 

 that fall upon a given area and the greater the amount of 

 heat received by that area. More heat is received in the 

 northern hemisphere not only because the rays fall more 

 nearly vertically but also because the length of the day 

 is increased. The amount of heat received from the sun 

 continues to increase as long as the sun appears to move 



SEPTEMBER EQUINOX 

 THE PATH OF THE EARTH AROUND THE 



SUN. 



