v The Earth a Planet 81 



from north to south, being 1 2 hours each at the equator. 

 The Sun being vertical at noon, 23^ north of the equator, 

 its meridian altitude from the south point of the horizon in 

 the northern hemisphere is equal to the complement of the 

 latitude plus 231. In the southern hemisphere the Sun's 

 greatest altitude is equaljto the complement of the latitude 

 minus 23!. This period is termed the summer solstice^ as 

 the Sun stops in its northern path. It is the middle of the 

 northern summer and of the southern winter. The parallels 

 of 66^- (23^ from the poles) are termed the Arctic and 

 Antarctic Circles, and these are the lowest latitudes in which 

 sunlight or darkness can last for 24 hours at a time. 



123. Autumnal Equinox and Winter Solstice. In 

 three months more it is the autumnal equinox ; the equator 

 comes again into the plane of the Earth's orbit, day and 

 night are equal from pole to pole, and the Sun's meridian 

 altitude is again equal to the complement of the latitude. 

 The Sun is projected on the star-dome in the sign of Libra, 

 and it is the autumn of the northern hemisphere and the 

 spring of the southern. Another period ,of three months 

 brings the Earth into such a position that the equator is 

 23|- N. of the Sun's place in the ecliptic, and con- 

 sequently the Sun is seen vertically overhead at noon from 

 the parallel of 23^ S., which is termed the Tropic of 

 Capricorn after the sign in which the Sun is projected on 

 the star-dome. This is the highest south latitude for a 

 vertical Sun. The Sun is visible everywhere within the 

 antarctic circle, but all within the arctic circle is in day- 

 long darkness. . In all parts of the southern hemisphere 

 the Sun's meridian altitude above the north point of the 

 horizon is 23* greater than the complement of the latitude ; 

 in the northern hemisphere it is 23^- less, and the days 

 grow shorter and the nights longer from south to north, 

 day and night being equal on the equator. This is the 

 winter solstice, midwinter in the northern hemisphere and 

 midsummer in the southern. 



124. Altitude of the Sun. The altitude of the Sun 

 and duration of daylight are described above for a globe 

 without an atmosphere. On account of refraction ( 150) 



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