Sky radiance during a total solar eclipse: a theoretical model.

This paper describes a radiative transfer model for estimating the brightness or radiance of the sky during a total solar eclipse. The model is approximate; it only considers sunlight that diffuses into the umbra by first- and second-order scattering processes. It nevertheless correctly predicts the major observed features of the eclipsed sky including reddening of the horizon and lowering of zenith radiance over the normal day value by approximately 4 orders of magnitude. The calculated absolute zenith radiance at blue wavelength is about 20% lower than observed during the 1973 African solar eclipse. The model predicts, and observations confirm, that the zenith has the highest blue-red ratio (color temperature) and lowest brightness of any place in the sky during totality.