AFor most of human history, the night sky was both a clock and a calendar. The rising of particular constellations helped farmers decide when to sow crops, while sailors used stars to steer across oceans. In many cities today, however, that shared reference point has almost disappeared. Artificial light scatters through the atmosphere and forms a pale dome over urban areas, a phenomenon usually called skyglow. Astronomers were the first group to campaign against it, but they are no longer the only ones concerned.

BEcologists now argue that excessive night lighting changes the behaviour of many species. Migrating birds can become disoriented by brightly lit towers, hatchling turtles may crawl inland towards hotels instead of towards the moonlit sea, and insects are drawn away from feeding and breeding sites. In some regions, researchers have linked lighting near rivers with reduced activity among bats that normally hunt along the water. The problem is not simply the amount of light but also its colour: blue-rich white LEDs scatter widely and can interfere with circadian rhythms.

CPublic safety is often cited as the reason for brighter streets, yet the evidence is more complicated than many people assume. A review by the University of Bristol found that well-designed lighting can reduce some types of night-time accidents, especially at complex junctions, but glare and uneven illumination may make pedestrians less visible. Lighting specialists therefore distinguish between more light and better light. Shielded lamps, lower mounting heights and warmer colour temperatures can improve visibility while reducing skyglow.

DSeveral towns have begun to treat darkness as a resource worth protecting. The island of Sark in the English Channel was named the world's first Dark Sky Island in 2011, while communities in New Zealand, Chile and the American Southwest have adopted ordinances limiting upward light. These rules rarely require a return to darkness everywhere. Instead, they ask businesses and households to place light only where it is needed, when it is needed, and at the lowest useful intensity.

EThe economic case for reform is strong. Light directed into the sky does not make streets safer or shops more inviting; it is simply wasted energy. The International Dark-Sky Association estimates that poorly aimed outdoor lighting costs billions of dollars in electricity each year. Municipal retrofits can therefore pay for themselves surprisingly quickly, especially where old high-pressure sodium lamps are replaced by carefully controlled LEDs. The challenge is to avoid replacing one problem with another by installing lights that are efficient but too bright.

FAdvocates stress that reducing light pollution is not a rejection of modern life. Rather, it is an argument for precision. A city can keep its hospitals, stations and major roads well lit while allowing parks, river corridors and residential streets to become darker after midnight. Some councils have introduced adaptive systems that dim automatically when sensors detect little movement. Early trials suggest that residents often accept dimming once they see that it is gradual and reversible.

GPerhaps the most unexpected benefit is cultural. Dark-sky tourism has grown rapidly, with visitors travelling to remote areas for meteor showers, astrophotography workshops and guided stargazing. For rural communities with few other industries, the night sky can become an economic asset. More importantly, campaigners argue, access to stars gives people a sense of scale that no screen can reproduce. The goal is not to turn every city into a wilderness, but to recover some part of a common inheritance.