The new world atlas of artificial night sky brightness

Abstract

Artificial lights raise night sky luminance, creating the most visible effect of light pollution-artificial skyglow. Despite the increasing interest among scientists in fields such as ecology, astronomy, health care, and land-use planning, light pollution lacks a current quantification of its magnitude on a global scale. To overcome this, we present the world atlas of artificial sky luminance, computed with our light pollution propagation software using new high-resolution satellite data and new precision sky brightness measurements. This atlas shows that more than 80% of the world and more than 99% of the U.S. and European populations live under light-polluted skies. The Milky Way is hidden from more than one-third of humanity, including 60% of Europeans and nearly 80% of North Americans. Moreover, 23% of the world's land surfaces between 75°N and 60°S, 88% of Europe, and almost half of the United States experience light-polluted nights.

Keywords: astronomy; environmental protection; light pollution.

Fig. 1. Upward emission functions used to compute the maps.

The polar graph shows the three different light intensity distributions used to compute the three map versions: the Lambertian distribution with a peak toward the zenith (map A; blue), the function with peak intensity at low angles above the horizon plane (map B; green), and the function with peak at intermediate angles, 30° above the horizon plane (map C; yellow). The thick red line shows the overall best-fitting function.

Fig. 2. World map of artificial sky brightness.

The map shows, in twofold increasing steps, the artificial sky brightness as a ratio to the natural sky brightness (assumed to be 174 μcd/m²). Table 1 indicates the meaning of each color level.

Fig. 3. Map of North America’s artificial sky brightness, in twofold increasing steps, as a ratio to the natural sky brightness.

Table 1 indicates the meaning of each color level.

Fig. 4. Map of South America’s artificial sky brightness, in twofold increasing steps, as a ratio to the natural sky brightness.

Table 1 indicates the meaning of each color level.

Fig. 5. Maps of Europe’s artificial sky brightness, in twofold increasing steps, as a ratio to the natural sky brightness (assumed to be 174 μcd/m²).

(A) The map shows the artificial sky brightness in V-band, as in the other maps. (B) The map shows the forecast of the perceived sky brightness for a dark-adapted eye after a transition toward 4000K CCT LED technology, without increasing the photopic flux of currently installed lamps. Table 1 indicates the meaning of each color level.

Fig. 6. Map of Africa’s artificial sky brightness, in twofold increasing steps, as a ratio to the natural sky brightness.

Table 1 indicates the meaning of each color level.

Fig. 7. Map of Asia’s artificial sky brightness, in twofold increasing steps, as a ratio to the natural sky brightness.

Table 1 indicates the meaning of each color level.

Fig. 8. Map of artificial sky brightness for Australia, Indonesia, and New Zealand, in twofold increasing steps, as a ratio to the natural sky brightness.

Table 1 indicates the meaning of each color level.

Fig. 9. Places on Earth farthest from pristine skies and unpolluted zenith skies.

The sky brightness levels used here indicate the following: up to 1% above the natural light (0 to 1.7 μcd/m²; black); from 1 to 8% above the natural light (1.7 to 14 μcd/m²; blue); from 8 to 50% above natural nighttime brightness (14 to 87 μcd/m²; green); from 50% above natural to the level of light under which the Milky Way is no longer visible (87 to 688 μcd/m²; yellow); from Milky Way loss to estimated cone stimulation (688 to 3000 μcd/m²; red); and very high nighttime light intensities, with no dark adaption for human eyes (>3000 μcd/m²; white). The circles indicate the distance toward arriving at large territories (that is, very small islands, such as Ile d’Ouessant, France, are not considered here), toward a sky relatively unpolluted at the zenith (yellow circle), and toward a pristine sky (red circle).

Fig. 10. Map of light pollution’s visual impact on the night sky.

The sky brightness levels are those used in the tables and indicate the following: up to 1% above the natural light (0 to 1.7 μcd/m²; black); from 1 to 8% above the natural light (1.7 to 14 μcd/m²; blue); from 8 to 50% above natural nighttime brightness (14 to 87 μcd/m²; green); from 50% above natural to the level of light under which the Milky Way is no longer visible (87 to 688 μcd/m²; yellow); from Milky Way loss to estimated cone stimulation (688 to 3000 μcd/m²; red); and very high nighttime light intensities, with no dark adaption for human eyes (>3000 μcd/m²; white).

Fig. 11. G20 countries sorted by population exposed to light pollution.

Countries of the G20 group whose populations live under skies polluted by the specified artificial sky brightness. Color ranges are shown on the right and indicate the pollution level (μcd/m²).

Fig. 12. G20 countries sorted by polluted area.

Countries of the G20 group whose area is polluted by the specified artificial sky brightness. Countries are ordered using the area of the three most polluted levels (that is, yellow, red, and white). Different orders may be obtained by choosing different pollution levels. Color ranges are shown on the right and indicate the pollution level (μcd/m²).

Fig. 13. The 20 least polluted countries.

Countries whose populations are exposed to the least light pollution. Color ranges are shown on the right and indicate the pollution level (μcd/m²).

Fig. 14. The 20 most polluted countries.

Countries whose populations are most exposed to light pollution. Color ranges are shown on the right and indicate the pollution level (μcd/m²).

Fig. 15. VIIRS DNB sensitivity.

Fig. 16. Comparisons between sky brightness observations and atlas predictions.

(Left) Contour plot comparing the weighted number of SQM observations to the predictions of the atlas in 0.1 mag_SQM/arcsec² bins. The colors are scaled logarithmically relative to the peak: <0.7 weighted observations (blue), 0.7 to 5 weighted observations (green), 5 to 35.5 weighted observations (red), and 35.5 to 251 weighted observations (black). (Right) Fit residuals (observed minus predicted SQM values) for the complete data set, with a Gaussian fit. Negative values mean that the observer reported a sky brighter than the atlas predicted, whereas positive values mean that the observer reported a sky darker than predicted. The tail of brighter-than-predicted observations is usually either contaminated by nearby light sources or taken in different conditions than assumed (for example, unreported clouds, haze, or fog). The histogram shows the total number of observations, not weighted observations, so some locations contribute to multiple entries.

Fig. 17. Contour plots comparing the weighted number of SQM observations to the predictions of the atlas.

The color scale is logarithmic and scaled relative to the peak value. Observations by citizen scientists tend to be slightly brighter than those made by professionals, with larger tails at very bright values, likely from observations taken too near lamps or under nonideal atmospheric conditions. GaN, Globe at Night; LotN, Loss of the Night app; RASC, Royal Astronomical Society of Canada; IDSP, International Dark Sky Places.

Fig. 18. Histograms showing the residuals (observed minus predicted SQM values) for the whole data set and for each of the different data providers.

Negative values mean that the observer reported a sky brighter than the atlas predicted, whereas positive values mean that the observer reported a sky darker than predicted. The histograms show the total number of observations, not weighted observations, so some locations contribute to multiple entries. Superimposed are the best-fitting Gaussians.

Fig. 19. Comparisons between sky brightness NPS CCD observations and atlas predictions.

(Left) Plots comparing CCD observations to the predictions of the atlas (upper graph, aerosol clarity K = 1; lower graph, K = 0.5). (Right) Fit residuals (observed minus predicted values) for the SQM-based calibration versus CCD NPS observations (upper histogram, K = 1; lower histogram, K = 0.5).