The relation between celestial colour gradients and the position of the sun, with regard to the sun compass.

Colour gradients along the sky caused by atmospheric scattering were measured on sunny days. It is concluded that whereas the shape of the spectral intensity distribution in the short wavelength range is stable, the distribution at longer wavelengths depends on the direction of measurement. We expressed these relative intensity differences as a spectral contrast. This contrast plotted as a function of angular difference with respect to the position of the sun establishes a smooth gradient. We suggest that the pigeon's UV sensitivity is part of a colour processing system, which is well adapted to employ these gradients in order to derive the sun's position.