4 OUR PHYSICAL WORLD 



for all industrial activities, still our little earth receives only a 

 minute fraction of the power the sun is continually giving off, 

 for the sun is radiating its energy, light, heat, and chemical 

 influence in all directions. The earth is only a tiny sphere some 

 8,000 miles in diameter, nearly 93,000,000 miles from the sun. 

 It, therefore, is hardly more than a speck compared with the 

 sphere 186,000,000 miles in diameter which the sun fills with its 

 energy. In fact, it is estimated that all the planets intercept 

 only about one hundred millionth part of the sun's flood of power 

 which is constantly pouring out into space. 



Astronomers calculate that the sun gives off every hour as 

 much radiant energy as would be produced by the burning in that 

 time of a layer of hard coal 25 feet thick covering its entire surface. 

 This is equivalent to 140,000 horse-power for every square yard 

 of the sun's surface. If all the coal in Pennsylvania were mined 

 and then burned in one second it would not produce as much 

 energy as the earth receives from the sun in the same time. 

 Such figures are almost beyond comprehension. It is well-nigh 

 impossible to form any idea of the temperatures of the sun. 

 It is believed that the outer radiating portion registers about 

 10,000 F., while the temperatures of the inner portions probably 

 range above 50,000. 



But how can the sun remain so hot when it is spending its 

 energy at such a profligate rate? It seems probable that one 

 main source of its heat is the constant contraction that occurs 

 in it. We know that when a body takes up heat it expands. A 

 familiar example is the expansion of the mercury in the thermom- 

 eter bulb as it gets hotter, which causes the mercury to rise in 

 the tube. (See also experiment 94 in the Field and Laboratory 

 Guide in Physical Nature-Study.) The reverse is also true, that 

 when a body contracts it liberates heat. The sun is so very large 

 that it is estimated it need only contract 250 feet in diameter a 

 year to produce the energy it radiates into space. This is so 

 slight an amount as to be immeasurable from the earth, except 

 after the lapse of thousands of years. Quite probably, too, there 



