Brightness and color of the sky during the 2017 solar eclipse

Direct solar illumination disappears in the umbra when the Moon's shadow creates a total solar eclipse. However, the sky above an observer in the umbra is not completely dark because of light that scatters into the umbra from the penumbra (partial eclipse region) and beyond. We show that a simple 2nd-order scattering model reproduces the brightness and color within a factor of 2 relative to measurements made with a radiometrically calibrated all-sky imaging system at the total solar eclipse of 21 August 2017 observed in Rexburg, Idaho USA. The model includes a first scattering point outside the umbra and a 2nd scattering point at the center of the umbra that redirects the light downward to the observer. The simulations show that the primary zenith skylight at the center of the umbra arises from light whose first scattering point is near an altitude of 10 km, the first scattering creates an orangish ring of light symmetrically around the horizon up to approximately 10 degrees elevation, and the second scattering creates zenith skylight that is reduced by approximately four orders of magnitude from daylight and that has a slightly higher blue-red ratio than the daylight before and after the eclipse.