FORM, MOTIONS, LATITUDE, AND LONGITUDE 215 



effect on the earth is illustrated by Fig. 209. That is, the revolu- 

 tion of the earth about the sun, while it rotates on an axis inclined 

 toward the plane of its orbit, makes the sun appear to move from 

 a place where its rays are vertical 23j/2 (nearly) north of the 

 equator (direction S, Fig. 209), to a place where they are vertical 

 23^2 (nearly) south of the equator (direction W), and back again 

 in one year. 1 The result, so far as the earth is concerned, is as if 



Sp&A 



Fig. 209. The inclination of the earth's axis, as it revolves about the sun, 

 makes the sun appear to travel north and south. The sun is vertical at 

 the equator on the 21st of March (Sp), then appears to move northward 

 until it is vertical 23^ north of the equator (S); it then appears to move 

 southward until it is vertical again at the equator (A), and then 23^ 

 south of the equator (W) ; it then appears to move north until it is ver- 

 tical at the equator. These changes are accomplished in the course of 

 one year. 



the sun moved from /S, which corresponds to the time of the summer 

 solstice, to Sp & A, which corresponds to the time of the autumn 

 equinox, then to W, which corresponds to the time of the winter 

 solstice, then back again to Sp & A, which corresponds to the spring 

 equinox, and finally to S, while the earth is making one circuit 

 about the sun. 



When the sun is vertical at points north of the equator, the 

 days are longer than the nights in the northern hemisphere, and 

 the sun's rays strike the surface in the northern hemisphere more 

 nearly vertically than they do in the southern hemisphere. When 



1 The inclination of the earth's axis is not quite constant. Its present 

 inclination (1907) is 23 27' 5". 



