THE SURFACE OF THE EARTH. 



orbit. Its motion in that case is in a direction contrary to that 

 of the hands of a clock, or to that of a common screw when turned 

 so as to cause it to move inwards or forwards. 



"While the globe thus moves round the sun, its axis keeps con- 

 stantly the same direction, so that in any one position it is 



21st Dtv. 



21st Jur 



Feb. 



A-pril. 



parallel to the direction which it had in any other position. On 

 the 21st of June, as shown to the right of the diagram, the 

 northern extremity of the axis, or the north pole, leans towards 

 the sun through an angle of 23^. If we suppose a parallel of 

 latitude to be described at that distance from the pole, all places 

 within that parallel will receive the light of the sun, and the 

 rotation of the globe on its axis will not throw any of these places 

 upon the dark side of the earth. It follows, therefore, that on 

 the 21st of June the sun does not set at all to any place above 

 that parallel, and there is consequently twenty-four hours day. 



By referring to the position of the earth, on the extreme left 

 of the figure, which it has on the 21st of December, it will be seen 

 that then the north pole is inclined from the sun, just as much as 

 it was inclined towards it on the 21st of June, and that the same 

 places, bounded by the parallel of latitude 23| from the pole, 

 .which on the 21st of June lay altogether on the enlightened side 

 of the globe, and enjoyed twenty-four hours day, now lie on 

 the dark side, and have twenty-four hours night. As the sun 

 never sets to such places on the 21st of June, so it never rises 

 to them on the 21st of December. 



In the positions successively assumed by the earth, in July and 

 August, the north pole is less and less inclined towards the sun, 

 and the part around it, which has no sunset, is limited by a less 

 and less parallel of latitude. On the 21st of September, the equator 

 being presented to the sun, there is no part around the pole 

 172 



