Fractional polarization Q/I and U/I of the earthshine measured for highland (top) and mare regions (bottom) for the four different filters B,V,R, and I (left to right). The solid curves are q_maxsin² fits to the data. The error bars give the statistical 1σ noise Δ_noise of the data, whereas the mare I band data at phase angle 109.5° are additionally affected by a substantial systematic offset Δ_syst > 0.5%. The dots in the V panel for the mare region indicate the measurements of Dollfus (1957) and a corresponding q_maxsin² fit (dashed line) is also given.