![]() The maps were calibrated to match the time of satellite overpass, at around 1 a.m. Maps were produced to show the zenith artificial sky brightness in twofold increasing steps as a ratio to the natural sky brightness ( Figs. In fact, as previously demonstrated ( 7, 8), the sky brightness outside cities is dominated by the component of the light escaping at low angles above the horizon plane, exactly where the fit upward function differs most from the pure Lambertian distribution. The fact that the main component of the upward flux was found to be the Lambertian one does not mean that the reflected light is the origin of the main component of the artificial sky brightness. This component presumably originates from poorly shielded luminaires. The fit suggests that, in addition to the Lambertian distribution resulting from surface reflections, low-angle upward emissions are an important component of light emission from cities. Factors other than the actual light intensity distribution may influence its shape (for example, atmospheric transparency that is higher or lower than that assumed by the model). This upward function is not meant to be considered a “real” or “best” upward function but is simply the function that produces the best statistical fit to the entire observational data set. ![]() Using the maximum likelihood fit described in Materials and Methods, we found an average upward emission function that best fits the whole data set (red curve in Fig. To overcome this, we present the world atlas of artificial sky luminance, which was obtained with our dedicated light pollution propagation software using the new calibrated, high–dynamic range, high-resolution data from the Visible Infrared Imaging Radiometer Suite (VIIRS) Day/Night Band (DNB), new precision charge-coupled device (CCD) brightness measurements, and a new database of Sky Quality Meter (SQM) measurements. Although researchers from disparate fields are now interested in light pollution, its magnitude is poorly known on a global scale because measurements are sporadically distributed across the globe. Light pollution is no longer only a matter for professional astronomers ( 3, 4). Even small increases in sky brightness degrade this experience. ![]() In addition to hindering ground-based optical astronomical observations, the artificial brightening of the night sky represents a profound alteration of a fundamental human experience-the opportunity for each person to view and ponder the night sky. Artificial light scattered in the atmosphere raises night sky luminance, creating the most visible negative effect of light pollution-artificial skyglow. During moonless nights, the luminance of the clear sky background far from the Milky Way and zodiacal light is about 22 magnitude per square arcsecond (mag/arcsec 2) in the Johnson-Cousins V-band ( 2), equivalent to 1.7 × 10 −4 cd/m 2. Natural lighting levels are governed by natural celestial sources, mainly the Moon, natural atmospheric emission (airglow), the stars and the Milky Way, and zodiacal light. Light pollution is the alteration of night natural lighting levels caused by anthropogenic sources of light ( 1).
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