Atlas Technical Reference
Data processing and scale by David Lorenz (University of Wisconsin-Madison)
21.99
21.93
21.89
21.81
21.69
21.51
21.25
20.91
20.49
20.02
19.50
18.95
18.38
17.80
Light Pollution Zone
The zones correspond to the colors on the atlas shown above. Increasing the zone number by 1 (e.g., from 2a to 3a) corresponds to a factor of 3 increase in artificial light pollution. This scale allows astronomers to quickly identify the quality of the sky for observing or imaging.
Light Pollution Index (LPI)
The LPI is the ratio between artificial sky brightness and the natural sky. An LPI of 1.0 means artificial light is equal to natural brightness. When LPI is less than 1, natural sources like stars are the dominant light source. In cities, the LPI often exceeds 30.0, meaning artificial glow is 30 times stronger than the stars.
Magnitudes per Square Arc-second
This is a scientific unit measuring the brightness of a tiny patch of sky. Astronomers use a reverse scale: the higher the number, the darker the sky. A pristine natural sky is roughly 22.0. As light pollution gets worse, this number drops because the background sky is becoming brighter and “washing out” the stars.
Scientific Methodology
This atlas is a 2024 re-calculation of the World Atlas of Artificial Night Sky Brightness. It uses NOAA VIIRS satellite data to model how light from ground sources scatters through the atmosphere. This map simulates brightness at “Zenith” (straight up) and provides an objective physical measurement of sky quality.