The Midnight Sun. 



99 



earth's equator, is termed) forms the horizon. The hea- 

 venly bodies, such as those stars which are visible at all, 

 never rise or set, but may be observed during the whole 

 of their apparent revolution, caused by the real rotation of 

 the earth on its axis, their distance from the horizon never 

 varying, and their motion being in circles parallel to the hori- 

 zon. Stars below the equator, that is, all the stars of the 

 southern half of the celestial vault, are never visible, while 

 those in the northern half never disappear. The sun, for six 

 months in the year, when his position is below the equator, or 

 he has south declination, never rises above the horizon ; while 

 during the other six months, having north declination, he 

 never sets, but moves round in a series of circles nearly parallel 

 to the horizon, or, strictly speaking, in a spiral, first ascending, 

 till on the longest day he attains an altitude of 23 J degs. ; and 

 then descending, till lost to view about the 21st of September. 

 If the observer depart from either the pole or the equator, in 

 the first case the pole will sink from its position over his head ; 

 and in the latter, the pole towards which he is travelling will 

 rise. Wherever he may stop, the pole will be the same number 

 of degrees above the horizon as the observer must use to ex- 

 press his latitude. To illustrate such a position Fig. 3 is 

 drawn. The diagram repre- .? 



sents the north pole elevated 

 60 degs. above the horizon, 

 showingthat tobethe northern 

 latitude of the place, and 

 here, or indeed at any other 

 latitude, except degs. and 

 90 degs., all the heavenly 

 bodies rise and set obliquely, 

 their diurnal paths making 

 with the horizon angles equal 

 to the co-latitude, that is, the 

 difference between the latitude 

 and 90 degs. ; in this case the 

 co -latitude equals 30 degs. 

 A heavenly body on the equator will have its diurnal path half 

 above and half below the horizon, and if the body so situated 

 be the sun, the days and nights will be equal to that place. 

 Those bodies to the south of the equator will, in the latitude of 

 the figure, have the greater part of their diurnal path below 

 the horizon, and the smaller part above, as shown at I ; on the 

 contrary, those to the north of the equator have the greater 

 part of their daily path above the horizon, and the smaller 

 below, as at II. Now the sun varies his distance from the 

 equator, ranging about 23| degs. on either side of it. When 

 vol. v. — NO. II. I 





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