are therefore longer than the nights in the northern hemisphere. The reverse 

 is the case with the southern hemisphere, for there the greater segments of 

 the parallels are dark, and the lesser segments enlightened ; the days are there- 

 fore shorter than the nights. Upon the equator, however, at B, the circle of 

 the earth, is equally divided, and the days and nights are equal. When the 

 south pole leans toward the sun, which it does exactly at the opposite point of 

 the earth's annual orbit, circumstances are reversed : then the days are longer 

 than the nights in the southern hemisphere, and the nights are longer than 

 the days in the northern hemisphere. At the intermediate point of the earth's 

 annual path, figure 7, when the axis assumes a position perpendicular to the 



Fig. 7. 







- 









direction of the sun, then the circle of light and darkness passes through the 

 poles ; all parallels in every part of the earth are equally divided, and there is 

 consequently equal day and night all over the globe. 



In the annexed perspective diagram, fig. 8, these four positions of the earth 

 are exhibited in such a manner as to be clearly intelligible. 



On the day of the 21st of June, the north pole is turned in the direction of 

 the sun ; on the 21st of December, the south pole is turned in that, direction. 

 On the days of the equinoxes, the axis of the earth is at right angles to the 

 direction of the sun, and it is equal day and night everywhere on the earth. 



The annual variation of the position of the sun with reference to the equa- 

 tor, or the changes of its declination, are explained by these motions. The 

 summer solstice the time when the sun's distance from the equator is the 

 greatest takes place when the north pole leans toward the sun ; and the win- 

 ter solstice or the time when the sun's distance south of the equator is great- 

 est takes place when the south pole leans toward the sun. 



In virtue of these motions, it follows that the sun is twice a year vertical at 

 all places between the tropics j.arid at the tropics themselves it is vertical once 

 a year. In all higher latitudes the point at which the sun passes the meridian 

 daily alternately approaches to and recedes from the zenith. From the 21st 

 of December until the 2 1st of June, the point continually approaches the zenith. 

 It comes nearest to the zenith on the 21st of June ; and from that day until the 

 21st of December, it continually recedes from the zenith, and attains its lowest 

 position on the latter day. The difference, therefore, between the meridional 

 altitudes of the sun on the days of the summer and winter solstices at all places 

 will be twice twenty-three degrees and twenty-eight minutes, or forty-six de- 

 grees and fifty-six minutes. In all places beyond the tropics in the northern 

 hemisphere, therefore, the sun rises at noon on the 21st of June, forty-six de- 

 grees and fifty-six minutes higher than it rises on the 21st of December. 

 These are the limits of meridional altitude which determine the influence of 

 the sun in different places. 



