152 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 3 



COMPARISON OF THE SOLAR HEATING OF THE EARTH AND THE 



MOON 



On the moon things do not change so rapidly. The moon has no 

 atmosphere, hence no clouds, smoke, or dust. (See pi. 1.) Instead 

 of rotating in 24 hours like our earth, the moon rotates only once in 

 2714 days. Being little more than 2,000 miles in diameter, its cir- 

 cumference is less than 7,000 miles. Hence the moon's surface passes 

 under the sun's rays only about 10 miles per hour. Thus there is 

 opportunity for the moon's illuminated surface to warm up to a 

 practically constant temperature. At the center of the moon's bright 

 face its temperature is found to be 250° F., which is about 40° F. 

 above that of boiling water. Compare this with the earth. Even at 

 the Equator there are few stations where the weather records report 

 temperatures above 112° F., which is 100° below the boiling tem- 

 perature. This is the case even though the sun sends rays equally 

 as intense to the earth as to the moon. The difference in maximum 

 temperatures reached by the two bodies depends partly on the rapid 

 rotation of the earth and partly on its possession of an atmosphere. 



ATMOSPHERIC INFLUENCE ON PLANETARY TEMPERATURE 



Not only do clouds, dust, and smoke produce their effects, but our 

 atmosphere is a sort of ray trap. On cloudless days, sun rays can 

 pass through it readily. They lose only about one-fourth of their 

 heat before reaching the earth. It is not so with the long-wave 

 rays sent outwards by our earth's surface. About three-fourths of 

 them are restrained by the atmosphere. Hence, like a blanket upon 

 a sleeper, the atmosphere must itself be warmed through before the 

 earth's heat given up to the atmosphere by long- wave radiation and 

 by contact can escape to space. With such an effective blanket keep- 

 ing the earth warm, it is not surprising that at night, when the 

 solar heating is all cut off, the earth's temperature does not fall as 

 much as that of the moon. Those who have been on high moun- 

 tains, or in parched deserts, where the atmospheric blanket is so 

 dry as to be less efficient than it is here in the East, know how much 

 more and how much quicker the night air cools than it does near 

 the sea. Indeed, one shivers even in daytime on a high desert 

 mountain the moment he steps into the shade. The following fig- 

 ures [a slide on the screen] exhibit this phenomenon for two places 

 of nearly the same latitude near the Equator and for the moon. 



Average range of temperatures beticeen day and night 



Port au Prince, Haiti 12° F. 



Timbuktu, in the Sahara Desert 27° F. 



On Mars 100° F. 



On the moon 500° F. 



