240 HUMPHREYS." EARTH LIGHT 



all parts of the sky, invariable, and continuous. It will also be 

 assumed that whatever the size of meteoric masses, (doubtless 

 the vast majority are but minute grains) their light producing 

 efficiency, or ratio of luminous to total energy, is the same as 

 that of the sun. 



With these assumptions it is possible to compute, from known 

 data, the rate at which meteoric material must be picked up to 

 produce the observed amount of "earth light," as follows: 



"Earth light" per 10 square degrees = star of first magnitude. 



Full moon = star of —11.77 magnitude = 120,000 stars of 

 first magnitude. 



Area full moon = 0.2 square degree. 



Hence, brightness full moon = 6 X 10 6 brightness of "earth 

 light." 



But the brightness of the full moon is equal to that of a white- 

 mat surface illuminated by a 1200 candle-power light at one 

 meter's distance, 6 or, in symbols, 1200 m.c. (meter candles). 



Hence brightness of earth light = 2x 10~ 4 meter candles. 



Now normal zenith sunshine = 10 5 m.c., 1 or is 5 X 10 8 times 

 brighter than "earth light," and consequently delivers 25 XlO 7 

 times as much energy per square centimeter as would be radiated 

 from both sides combined of a self-luminous shell equivalent 

 in brightness to "earth light." 



Hence, since the solar constant is about 1.92 calories per square 

 centimeter per minute, the total energy used, according to the 

 above assumptions, in the production of ' 'earth light" is 



1 92 



4irR 2 X '- calories per minute, 



25 X 10 7 



in which R is the radius of the earth in centimeters, or 



27 X 10 15 ergs per second, roughly. 



Let this energy be supplied by M grammes of matter moving 



with the average velocity of meteors, or 42 kilometers per second, 



then 



i MV 2 = 27 X 10 



2 



15 



or M = 3 X 10 roughly. 



6 Circular of the Bureau of Standards, 28: 7. 1911. 



