Why are telescopes often built far from cities and their lights?

Telescopes are often built far from cities and their lights to avoid light pollution, which is the unwanted or excessive artificial light that interferes with astronomical observations.12

City lights create a phenomenon known as "skyglow," where upward-directed light is scattered and reflected by particles in the atmosphere. This effect significantly reduces the visibility of stars and other faint celestial objects, making it difficult for both professional and amateur astronomers to observe the night sky. Even lights from relatively small towns can cause significant degradation of the night sky for observers many kilometers away.2

Beyond avoiding light pollution, astronomers select remote, high-altitude sites for other critical reasons:

• Atmospheric Turbulence: Telescopes are often placed on high mountains to minimize atmospheric turbulence, which causes images to flutter, dance, or become diffuse.34
• Atmospheric Transparency: High-altitude locations often have lower humidity, which is essential because water vapor and other gases in the atmosphere can absorb radiation, particularly in the infrared region of the spectrum.3

By choosing isolated, high-altitude locations, astronomers can achieve the best possible "seeing"—the sharpness of a telescopic image—and maximize the clarity of their observations.34