SEASONS AND CLIMATE. 



which has continual day, all places being twelve hours exposed 

 to the sun, and twelve hours removed from it. 



After the 21st of September, the north pole begins to incline 

 from the sun, and in October, a small portion around it is entirely 

 deprived of the sun's light. This portion, thus involved in con- 

 tinual night, constantly increases in extent until the 21st of 

 December, when it extends to the parallel of latitude already 

 mentioned. After the 21st of December, the north pole is less 

 and less inclined from the sun, so that the portion involved in. 

 continual night is less in January, and still less in February, 

 and on the 21st of March, as on the 21st of September, there are 

 twelve hours day and twelve hours night to all places. 



After the 2 1st of March, the north pole begins again to lean 

 towards the sun, and a portion of the earth around it enjoys 

 continual day, this portion incrsasing in extent during the 

 months of April and May, and attaining its greatest magnitude 

 on the 21st ol Juce, when its extent, as already explained, is 

 a circle 23^ irom the pole. 



171. The parallel which is 23 irom the pole, is 66| from 

 the equator, and consequently has the latitude north of 66^. 



This parallel of latitude is called the Arctic or Polar Circle, 

 and the polar region included by it is called the Frigid 

 Zone. 



All the circumstances, which are here explained, respecting the 

 north polar region, circumscribed by the arctic circle, will be 

 equally applicable to the corresponding region, circumscribed by 

 a circle 23| from the south pole. Such a circle is called the 

 South Polar or Antarctic Circle, and the polar region circum- 

 scribed by it is the Southern Frigid Zone. 



172. Although the diurnal and nocturnal phenomena of the two 

 frigid zones, northern and southern, are precisely the same, they 

 are not simultaneous, those which are identical being produced 

 at opposite epochs of the year, as will be rendered evident by 

 examining attentively the several positions of the illuminated and 

 dark hemispheres of the globe in the successive months, in 

 fig. 12. When the entire north polar circle is enlightened on 

 the 21st of June, the entire south polar circle is dark. There is 

 continual day in the one, and continual night in the other. On 

 the contrary, when the entire north polar circle is dark on the 

 21st of December, the entire south polar circle is enlightened. 

 There is continual night in the one, and continual day in the 

 other. 



The phenomena on the 21st of June, therefore, in the northern 

 frigid zone, are identical with those of the 21st of December in 

 the southern frigid zone, and vice versd. 



173 



