pollution Light pollution is an ever-increasing source of sky brightness in urbanized areas. In densely populated areas that do not have stringent light pollution control, the entire night sky is regularly 5 to 50 times brighter than it would be if all lights were switched off, and very often the influence of light pollution is far greater than natural sources (including moonlight). With urbanization and light pollution, one third of humanity, and the majority of those in developed countries, cannot see the Milky Way. Twilight When the sun has just set, the brightness of the sky decreases rapidly, thereby enabling us to see the airglow that is caused from such high altitudes that they are still fully sunlit until the sun drops more than about 12° below the horizon. During this time, yellow emissions from the sodium layer and red emissions from the 630 nm oxygen lines are dominant, and contribute to the purplish color sometimes seen during civil and nautical twilight. After the sun has also set for these altitudes at the end of nautical twilight, the intensity of light emanating from earlier mentioned lines decreases, until the oxygen-green remains as the dominant source. When astronomical darkness has set in, the green 557.7 nm oxygen line is dominant, and atmospheric scattering of starlight occurs. Differential refraction causes different parts of the spectrum to dominate, producing a golden hour and a blue hour. Relative contributions The following table gives the relative and absolute contributions to night sky brightness at zenith on a perfectly dark night at middle latitudes without moonlight and in the absence of any light pollution. (The S10 unit is defined as the surface brightness of a star whose V-magnitude is 10 and whose light is smeared over one square degree, or 27.78 mag arcsec−2.) The total sky brightness in zenith is therefore ~220 S10 or 21.9 mag/arcsec² in the V-band. Note that the contributions from Airglow and Zodiacal light vary with the time of year, the solar cycle, and the observer's latitude roughly as follows: where S is the solar 10.7 cm flux in MJy, and various sinusoidally between 0.8 and 2.0 with the 11-year solar cycle, yielding an upper contribution of ~270 S10 at solar maximum. The intensity of zodiacal light depends on the ecliptic latitude and longitude of the point in the sky being observed relative to that of the sun. At ecliptic longitudes differing