Darkest Skies: Where You Can Still See the Milky Way. A Sky Glow Model of the World and the United Kingdom from VIIRS Night Lights, Checked Against Citizen Sky Meter Readings

Artificial light scattered in the atmosphere brightens the night sky and hides the Milky Way from much of humanity. I build a simple, open sky glow model from the Earth Observation Group's annual VIIRS night lights for 2023, read for all 246 countries from WorldPop's harmonised 3 arc second files, and use it to map artificial zenith brightness for the world at 2.5 arc minutes and for the United Kingdom at 30 arc seconds. Each lit cell's light spreads with a power of distance; the exponent and the scale are fitted to 5,775 Globe at Night Sky Quality Meter readings from 2019 to 2025, and in spatial cross-validation the model misses a typical reading by 0.49 magnitudes per square arcsecond (correlation 0.79). With WorldPop's 2023 population and the thresholds of the 2016 World Atlas of Artificial Night Sky Brightness, 52% of the world's people live where artificial glow hides the Milky Way, against 36% in the atlas for 2014; the two agree closely on the ranking of 148 countries (Spearman's rho = 0.94). In the United Kingdom the share is 85% of people but only 20% of land. The darkest areas are the Scottish islands and Highlands; the nearest Milky Way sky is 35 km from central London. Radiance seen from space fell 13.5% over Great Britain between 2015 and 2023, a change that the sensor's blindness to blue LED light probably exaggerates. Interactive version: https://maps.tristanhotham.com/dark-skies/. Data and code: https://github.com/Tristan-Hotham/dark-skies. Blog post: https://blog.tristanhotham.com/2026/10/28/dark-skies/. Text, figures and derived data: CC BY 4.0, except dark_sky_places.csv (from English Wikipedia, CC BY-SA 4.0). Code: MIT. Night lights and population: WorldPop (CC BY 4.0), from Earth Observation Group VIIRS VNL data. Sky meter readings: Globe at Night (CC BY 4.0). Places: OS Open Names (Open Government Licence v3) and GeoNames (CC BY 4.0). Boundaries: Natural Earth (public domain) and ONS (Open Government Licence v3).