text: After sunset the civil twilight sets in, and ends when the Sun drops more than 6° below the horizon. This is followed by the nautical twilight, when the Sun reaches heights of −6° and −12°, after which comes the astronomical twilight defined as the period from −12° to −18°. When the Sun drops more than 18° below the horizon, the sky generally attains its minimum brightness.
  Several sources can be identified as the source of the intrinsic brightness of the sky, namely airglow, indirect scattering of sunlight, scattering of starlight, and artificial light pollution.
  Visual presentation
  Depending on local sky cloud cover, pollution, humidity, and light pollution levels, the stars visible to the unaided naked eye appear as hundreds, thousands or tens of thousands of white pinpoints of light in an otherwise near black sky together with some faint nebulae or clouds of light. In ancient times the stars were often assumed to be equidistant on a dome above the Earth because they are much too far away for stereopsis to offer any depth cues. Visible stars range in color from blue (hot) to red (cold), but with such small points of faint light, most look white because they stimulate the rod cells without triggering the cone cells. If it is particularly dark and a particularly faint celestial object is of interest, averted vision may be helpful.