What is the Bortle scale, and how much of the night sky have we lost to light pollution?

The Bortle scale is a nine-step ruler for how dark a night sky is, and it helps measure how much dark we have lost: an estimated 83% of the world's people live under light-polluted skies. It measures the brightness of the night sky at a location, from Class 1, the darkest skies available on Earth, to Class 9, inner-city skies. At the higher classes, diffuse sights such as the Milky Way become invisible to the naked eye and fewer stars and planets can be seen at all.

The man who made it was an amateur. John Bortle created the scale and published it in Sky & Telescope magazine in February 2001, so that skywatchers could compare observing sites. He did not simply count the faintest star visible, because that depends on your eyesight: he designed the scale as an alternative to that raw limit.

The cause is light pollution, the alteration of the natural patterns of light and dark resulting from unnecessary and inefficient artificial lighting. Skyglow, the bright haze above cities, scatters light in the atmosphere and reduces the contrast between stars and the sky, making fainter objects much harder to see. Globally, light pollution increased by at least 49% from 1992 to 2017.

You can test your own sky. A Bortle class of four or less is needed to see the Milky Way, and from truly dark skies it is bright enough to cast a shadow. Light does not only hide stars: lights on tall structures can disorient migrating birds. Some places have shown it can be reduced: France cut its light pollution by 33% between 2014 and 2022.