On the basis of both spaceborne measurements (Polarization and Directionality of the Earth's Reflectances (POLDER)) and radiative transfer modeling, it is shown that the polarization of reflected solar light (300--900 nm) at the top of the atmosphere is most often perpendicular and less often parallel to the plane of single scattering. In terms of the elements of the Stokes vector, ∣U∣ is typically much smaller than ∣Q∣ when using the single-scattering plane as reference. When using any other plane, U(λ) ≈ Q(λ) tan 2χss, with χss being the angle of the plane of polarization with the reference plane in the case of single Rayleigh scattering. Exceptions to these findings are noted, and an error analysis of U(λ) ≈ Q(λ) tan 2χss is presented. A simplified polarization calibration algorithm for spaceborne spectrometers like the Spectrometer for Atmospheric Cartography (SCIAMACHY) is presented using the U-Q relation studied in this paper. It is shown to be just as accurate but less sensitive to measurement error than the current operational algorithm.