The zenith skylight intensity was measured, with a resolution of 10 Å, over the wavelength range from 5200 Å to 6400 Å during a total solar eclipse at Hermon, Maine. The intensity was found to change by about two orders of magnitude in the 2-min period before totality and reached a minimum during totality of 19.5 kR/Å at 5200 Å. The spectral distribution remained that of the day sky until the sun was more than 99.8% obscured. During totality, the shorter wavelengths were enhanced, indicating a shift to the blue in sky color. Comparisons with an independent measurement from an aircraft show that the intensity scale height of the secondary scattered component, predominating at totality, is significantly less than that of the day sky. The measurements are compared with the day and twilight sky.
William E. Sharp, John W. F. Lloyd, and S. M. Silverman, "Zenith Skylight Intensity and Color during the Total Solar Eclipse of 20 July 1963," Appl. Opt. 5, 787-792 (1966)