HOW THE SUN WARMS THE EARTH ABBOT 151 



INFLUENCE OF OBLIQUITY 



The effect of sun rays in producing boat must always depend on 

 the obliquity of the receiving surface to the vnys received. Figure 2 

 illustrates this point. The rays whose front have the width ab must 



FiGiRB '~. — Influence of obliquity by radiation. 



evidently produce a greater intensity of heating effect on the sur- 

 face cd at right angles to the beam than on the surfaces ef or gh^ 

 where the same amount of radiation is spread over large areas. Ex- 

 periment: I can illustrate this readily. (See fig. 1.) Note how the 

 warmth of the thermopile, as measured on the screen, diminishes 

 when I rotate it gradually from being at right angles until it reaches 

 parallelism with the rays it receives from the heated lamp. Let us 

 apply this principle of response to obliquity to our thought of the 

 warming of the earth by sun rays. We see, as in figure 3, that 



MORN - WINTER ~ 



NCCN - SUMMER 



FiGLRE 3. — Solar heating of the earth influenced liy obliquity. 



the intensity of the solar heating is greatest at noon and greatest in 

 summer, because at these times the obliquity is least. This is the 

 reason why, in the United States, July is warmer than January, 

 even though the sun is 3,000,000 miles farther away in July than it 

 is in January. Returning to consideration of the effect of rapid ro- 

 tation in preventing the surface of the earth from attaining con- 

 stancy of temperature under solar heating, we now see that, owing 

 to change of obliquity of solar rays, the intensity of the earth's 

 heating continuallj^ changes from morning to night. Darting along 

 at 1,000 miles per hour, three times as fast as a racing airplane, the 

 earth's surface never has time to reach a constant temperature under 

 solar heating. 



