Light pollution
Also called: skyglow, sky glow.
Artificial light scattered into the sky, which brightens it and hides faint stars, galaxies and nebulae.
Streetlights and buildings send light upward, where the air scatters it into a glow over towns that can stretch 100 km. Most people in Europe and North America can no longer see the Milky Way from home. It hurts galaxies and faint nebulae most; planets, the Moon and double stars barely notice.
At the eyepiece
Even a short drive away from town lights helps enormously; so does shielding your eyes from nearby lights. A nebula filter helps emission nebulae, but nothing restores galaxies except a darker sky.
In AstroPilot
Each location's sky darkness comes from the 2025 World Atlas of Artificial Night Sky Brightness.