For most of human history, the stars above were so vivid that the great (0) OBSERVATORIES (OBSERVE) of antiquity could be established almost anywhere. That era is drawing decisively to a close. Artificial light 1POLLUTE now reaches across continents, and for the majority of people alive today the Milky Way has never been directly seen. The measurable 2VISIBLE of the night sky has halved in a generation, and models suggest further sharp losses ahead. The disruption is not confined to stargazing. Many 3NOCTURNE species rely on celestial cues to feed, breed or navigate, and the 4MIGRATE routes of birds in particular are thrown into disarray by urban glare. Professional 5ASTRONOMY have been driven to ever-more-remote sites, and some long-established facilities are effectively obsolete. The 6DARK that remains has accordingly acquired a scientific value it never previously possessed. Much of the offending illumination serves no genuine purpose: poorly directed streetlamps and floodlit car parks contribute to a pattern of genuinely 7WASTE consumption. Targeted 8INTERVENE — shielded fixtures, reduced intensities, and dark-sky ordinances — can restore much of what has been lost, and at markedly lower cost than many environmental remedies.