[1] Step outside in almost any town tonight and look up, and you will probably see a milky orange haze where the stars ought to be. This glow, which scientists call skyglow, is the most familiar form of light pollution, and it now hangs over four-fifths of the world's population. It is created when artificial light bounces off dust and moisture in the air and is scattered back down to earth, smothering all but the very brightest stars. For most city children, a sky thick with stars is something they have never seen at all, and a few are frightened the first time they glimpse the crowded heavens from a dark hillside. Yet a growing movement of astronomers, doctors and ordinary residents is arguing that the darkness we have lost can be reclaimed, and at surprisingly little cost.

[2] The problem has crept up on us almost unnoticed. As towns spread outward through the twentieth century, so did their lighting, and much of it was poorly aimed. Each new road, car park and shop front added its own glare to the sky, and no one thought to add it all up. Old-fashioned lamps scattered their beams in every direction, throwing as much light sideways and upward into the sky as downward onto the pavement where it was actually needed. Brighter is not the same as safer, campaigners point out: a harsh, badly placed lamp creates deep shadows and dazzles the eye, so that a pedestrian may in fact see less, not more. Each wasted beam also burns electricity that nobody ever uses, a small extravagance repeated millions of times across a single country.

[3] The consequences reach far beyond a spoiled view of the heavens. Many animals depend on natural darkness in ways we are only beginning to understand. Newly hatched sea turtles, which for millions of years have crawled toward the faint shimmer of the sea, now stumble inland toward the brighter glare of a coastal road and perish. Migrating birds, navigating by the moon and stars, are drawn off course by floodlit buildings and circle them until they drop from exhaustion. Even insects, the quiet foundation of countless food chains, gather and die around lamps in numbers that alarm the scientists who count them. Because so many other creatures feed on those insects, the loss ripples outward through the whole web of life.

[4] Humans, it turns out, are not immune either. Our bodies evolved to expect a clear rhythm of bright day and genuine night, and that rhythm quietly governs when we feel sleepy and when we feel alert. Bright artificial light late in the evening, doctors warn, can disturb sleep and leave people tired and irritable the following day. Some researchers suspect that the effects run deeper still, reaching into our long-term health, though the picture is not yet fully understood. What is clear is that a glowing bedroom window, lit all night by a street lamp outside, is not the restful darkness our ancestors knew.

[5] The encouraging news is that light pollution, almost alone among environmental problems, can be undone in an instant. Unlike carbon already in the air or plastic already in the ocean, both of which linger for centuries, a badly aimed light simply stops polluting the moment it is switched off or pointed downward. The remedies are mostly cheap and practical: fitting lamps with simple hoods so they shine only where they are needed, choosing warmer colours that scatter less, and dimming or switching off lights in the small hours when the streets are empty. Several towns that have done so report lower electricity bills and no rise in crime or accidents whatsoever, which has begun to win over even the most cautious councils.

[6] Around the world, communities are beginning to act. Some regions have been declared official dark-sky reserves, where lighting is strictly controlled so that visitors can once again see the faint band of the Milky Way arching overhead. Such places are discovering an unexpected reward: tourists will travel a long way, and pay handsomely, simply to stand under a truly dark sky, bringing welcome money to remote villages. For the campaigners, this is the heart of their message. The night sky is not gone forever; it is merely hidden behind a switch, and that switch, with a little will, can be turned the other way.