Pollution, pollution, pollution

This satellite view of the continental United States at night shows the light pollution produced by outdoor lighting.  The glow from such lighting makes faint stars and galaxies invisible and forces astronomers to place their telescopes as far from population centers as possible.  Even so, some observatories are threatened by spreading light pollution.

Because astronomical telescopes are so sensitive, we often find observatories located on mountaintops far from cities.  The air above high mountaintops is thin, so it is more transparent, the sky is darker, and the stars are brighter.  Also, the winds blow smoothly over some mountains, so there is little turbulence to ruin the seeing.  In addition, mountaintops located far from cities suffer less from light pollution, the brightening of the night sky by light scattered from artificial outdoor lighting.