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AArtificial light has made cities safer and more convenient at night, but it has also changed the night sky. Light that escapes upwards from streets and buildings is scattered by the atmosphere, producing a glow known as skyglow. A world atlas of artificial sky brightness, published in 2016, estimated that the Milky Way is hidden from more than one third of humanity. The figures were higher in some regions: around 60 per cent of Europeans and nearly 80 per cent of North Americans could not see it.

BThe effects are not only a loss for stargazers, because many animals rely on natural patterns of light and darkness. Sea turtle hatchlings, for example, have long found the ocean by heading towards the brightest horizon, which on an undeveloped beach is the sky above the water. Lights from buildings along the coast can lead them inland instead, where they may be caught by predators, hit by vehicles or become exhausted. One conservation group estimates that around 100,000 hatchlings are misled in this way in Florida each year.

CProtected areas have taken a special interest in the problem. The US National Park Service counts natural darkness among the resources it must protect, alongside landscapes and wildlife. It explains that ecosystems depend on natural cycles of light and dark, and that night skies are part of many cultural and historical traditions. Parks far from large towns can also offer visitors a view of the stars that has become rare for people who live in brightly lit places.

DUnlike many forms of pollution, light pollution can be reduced quickly once a lamp is changed or switched off. DarkSky International and the Illuminating Engineering Society have published five principles for responsible outdoor lighting. Light should be used only when it is useful, directed only where it is needed, kept no brighter than necessary and controlled with devices such as timers or motion sensors. Finally, the principles recommend warm-coloured light, which contains less of the blue light that is more likely to disturb wildlife and brighten the sky.