JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. II, MAY 19, 1912 • No. 10 



ASTRONOMY. — On ''earth light," or the brightness, exclusive of 

 starlight, of the midnight sky. W. J. Humphreys. To appear 

 in the Astrophysical Journal. 



Investigations begun some years ago by Newcomb, 1 and con- 

 tinued by Yntema, 2 Fabry, 3 Abbot 4 and others, have conclusively 

 shown that there is more light in the midnight sky than can be 

 accounted for by the stars alone. It is not due to nebulae or. 

 any other constant source since its brightness varies from night 

 to night and even during the same night ; nor can it be caused by 

 anything entirely outside the atmosphere, since it increases in 

 brightness as the horizon is approached. 



It has been suggested by Yntema that it may be due, at least 

 in part, to a permanent Aurora, and indeed this is highly proba- 

 ble from the fact that the green ' 'auroral line,"X5770, may be 

 seen on almost any dark clear night in any part of the sky. 5 



But there is another possible source of sky light, possibly 

 of the "permanent aurora" itself, that deserves consideration; 

 namely, the bombardment of the outer atmosphere by material 

 of meteoric origin. So far as such a bombardment produces 

 light at all it must be through a considerable depth of the rarer 

 portion of the atmosphere, and therefore it should appear brighter 

 as the zenith distance is increased. 



For simplicity of numerical calculations it will be assumed 

 that "earth light" is both constant and uniform — the same over 



1 Astrophysical Journal, 14: 297. 1901. 



2 On the Brightness of the Sky and the Total Amount of Starlight. Gronigen: 

 Gebroeders Hoitsema. 1909. 



3 Astrophysical Journal, 31: 394. 1910. 



4 Annual Report Smithsonian Institution, 1911, p. 64. 

 6 Campbell, Astrophysical Journal, 2: 162. 1895. 



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