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The brightness of the zenith sky during twilight

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Abstract

A visual photometer is described for measuring the brightness of the zenith sky during twilight. The results of measurements made with it at Bombay on a large number of clear moonless evenings during the period December 1937 to March 1938 are discussed. A red filter RG1 (Schott and Gen.) and a green filter VG1 (Schott and Gen.) were used. With the latter, it was possible to continue observations even when twilight conditions had completely ceased. The variations in brightness agree with a relation proposed by Link, namely,dI/I=β sin θ sec2 θdθ, where I is the brightness of the zenith sky, θ the angular depression of the sun below the horizon and β a constant. If the brightness is due only to primary scattering from the atmosphere illuminated by sunlight, it is possible to connect β with the temperature of the atmospheric layer responsible for the scattering. The difficulty however is to ascertain to what extent secondary scattering is important.

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Authors
  1. M. W. Chiplonkar
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(Communicated by Dr. K. R. Ramanathan)

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Chiplonkar, M.W. The brightness of the zenith sky during twilight. Proc. Indian Acad. Sci. (Math. Sci.) 14, 70–79 (1941). https://doi.org/10.1007/BF03049127

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  • DOI: https://doi.org/10.1007/BF03049127

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