Figure 1.

An analysis of the celestial light available for orientation on a moonless night. (a) Linear images (pixel values proportional to absolute spectral photon radiance measured in each channel) of the sky in November 2015, oriented so that magnetic north is to the left and west at the top. Areas below 20° elevation were removed, and images were filtered with a Gaussian filter of 2°, 4°, 8° and 16° half-width. (b) Spectral photon radiance along the Milky Way in November 2015 (red line) and February 2016 (blue line). The maximum at southern elevations of approximately 30° and 45°, in November and February, respectively, corresponds to the ‘bright spot’ consisting of the constellation Crux and the Southern Pleiades. (c) Michelson contrast between points of the same elevation in the southern versus the northern arm of the Milky Way. Colours as in (b).