When they look at sky brightness—what they call the sky background—they use a unit of brightness per area of the sky: how much light is spread out over a given parcel of celestial real estate.

The light pollution in these areas makes the entire sky appear light grey in colour, and the Milky Way is effectively invisible. A 2016 study estimated that even on the clearest night, one third ...

When you look up at the night sky, can you see the stars? If you live in a large city or near one, the answer is probably no. The culprit is not just clouds and weather — it's light pollution.

The first panel, representing a light-polluted night sky, only shows the few brightest stars. Each panel shows progressively more and fainter stars, representing darker and darker skies.