September 1953 L E T T E R S T O T H E E D I T O R 805

Brightness and Color of the Twilight Sky EDWARD V. ASHBURN

Michelson Laboratory, U. S. Naval Ordnance Test Station, Inyokcrn, China Lake, California

(Received May 22, 1953)

IN a recent paper Hulburt' has given an interesting discussion of the brightness and color of the twilight sky and their

relation to the density of the atmosphere. Hulburt's intepretation is complete and adequate in some features. It appears that the following items would make it possible to strengthen the incom­plete portions of the interpretation.

1. In reference 1 Hulburt gives the results of his computations of the color of the zenith sky at twihght and states "no quantita­tive measurements of the colors of the twilight sky are available with which to make comparison." This statement is surprising since Bullrich,2 Dufay and Gauzit,3 Gauzit and Grandmontagne,4

Grandmontagne,5 Gruner,6 Reesinck,7 and Ashburn,8 among others, have published quantitative data on the colors of the twilight sky. In particular, Bullrich2 cites Siedentopf's data on the color temperature of the twilight sky. These measurements show that the color temperature varies from approximately 6500°K to 17 901°K during the course of twilight. Hulburt did not present the results of his calculations in suflScient detail to allow a comparison to be made with the numerous measurements. A direct comparison would be of interest and perhaps support Hulburt's interpretation.

2. Hulburt's discussion of the sky brightness is based upon measurements made by Koomen et al.9 and Hulburt.10 These measurements covered the entire visual band. Such measurements do not permit making the distinction between a small variation in intensity in the wavelength band where the spectral sensitivity of the receiver is high and a large variation in intensity in a wave­length band where the spectral sensitivity of the receiver is low. It appears, therefore, that Hulburt's computations of the variation of the zenith-sky brightness with solar-zenith distance should be

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compared to those measurements that were made using narrow wavelength pass bands.

3. Koomen et al.9 measured the polarization of the zenith sky at twilight for a wide spectral band. Robley11 has shown that the polarization varies from approximately 90 percent in the red to 60 percent in the blue when the solar-zenith angle is 90°. Hulburt's conclusions on the relative importance of primary scattering were based, in part, on the measurements of Koomen9 et al. Robley's11

results indicate that Hulburt's conclusions should be amplified to cover the variation of the polarization with wavelength.

1 E. O. Hulburt, J. Opt. Soc. Am. 43. 113-118 (1953). 2 K. BuUrich. Ber. deut. Wetterdienstes, U. S. Zone No. 4 (1950). 3 J. Dufay and J. Gauzit, Ann. astrophys. 9, 135-1942 (1946). 4 J. Gauzit and R. Grandmontagne, Publications de l'Observatoire de Lyon, Tome III, Fascicule 10. 5 R. Grandmontagne. Compt. rend. 207, 1436-1438 (1938). 6 P. Gruner, Gerlands Beitr. Geophys. 1-2, 202-207 (1935). 7 J. J. M. Reesinck, Pliysica's Grav. 11, 61-77 (1944). 8 E. V. Ashburn, J. Geopliys. Research 57, 85-93 (1952). 9 M. J. Koomen, C. Lock, D. M. Packer, R. Scolnik, R. Tousey, and E. O. Hulburt. 10 E. O. Hulburt, J. Opt. Soc. Am. 28, 227-236 (1938). 11 R. Robley, Ann. géophys. 6, 191-211 (1950).