1 72 ELEMENTARY SCIENCE 



So this clew alone does not take us far. However, 

 in considering it, you may have noted that the figure 

 indicates that the axis of the earth is inclined. That is, as it 

 spins, its axis does not intercept its orbit at right angles, 

 but at an acute angle, and the position of the axis is the same 

 throughout the year. Here is another clew we should 

 use, for evidently this fact must have a great effect upon 

 the way in which the earth's surface intercepts the sun's rays, 

 and, consequently, upon the distribution there of solar 

 light and heat. 



Thus we have three great geometrical facts to consider 

 in studying the distribution of solar effects upon the earth. 

 These are its revolution, its rotation, and the inclination of 

 its axis. Evidently, mathematics has an important part 

 in the study of nature. The problem we are now consider- 

 ing is largely a problem in geometry, and the whole science 

 of the heavenly bodies (astronomy) is largely a matter of 

 mathematics. 



Effects of the Inclination of Earth's Axis. Let us re- 

 member that, in spite of the complication of our problems 

 by this newly observed fact, the fact that one-half of the earth 

 is always in the sunshine remains the same. The inclina- 

 tion of the axis simply results in such shifting about of 

 this sunlit area that we are sure to be puzzled by it unless 

 we think it out carefully. 



Now you know that the sun at noon in summer is much 

 higher in the sky than it is at noon in winter; you know 

 that on a winter's noon the sun stands considerably south 

 of the zenith, by which we mean the point in the heavens 

 directly above your head wherever you happen to be 

 (see Figs. 62 and 63). This evidently indicates that in our 



