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Why are many observatories located on mountaintops?

Almost all of the world's finest ground-based observatories are located on mountains, for a variety of reasons. First and foremost, starlight appears less distorted in the thin atmosphere on mountaintops. (Space-based telescopes such as Hubble and Spitzer Space Telescope circumvent the disturbing effects of the atmosphere by flying above it.)

At high altitudes, there is less atmosphere to absorb infrared energy, which reveals details about some of the coldest objects in the universe, such as clouds of gas and dust and the disks of dust that give birth to planets.

Mountaintops also have unobstructed views of the horizon in all directions. Lastly, most cities and towns -- with their accompanying light pollution -- are situated in valleys and plains, so remote mountaintops are among the last places on Earth to find the dark skies so sought after by astronomers.