Density of sources in the secondary data set of Gaia DR1: a) all 1141 million sources in the secondary data set; b) the 685 million sources in common with the IGSL; c) the 456 million new sources. These maps use an Aitoff projection in equatorial (ICRS) coordinates, with origin α = δ = 0 at the centre and α increasing from right to left. Mean densities are shown for sources in cells of about 0.84 deg².