Astronomers carry out their observations of the heavens at observatories. In most cases observatories are located high up a mountain above the cloud base, where the air is clearer and cleaner. Where possible, they are located far away from cities so that they do not suffer from light pollution, or skies brightened by the presence of city lights. Most observatories have a collection of instruments, mainly light telescopes. These are housed in dome-roofed buildings: the domes open at night to expose the telescopes to the heavens. The telescopes are fitted with motor drives so that they can be moved to follow the motion of the stars, as they arc across the heavens. Among the world's leading observatories are Palomar and Mt Wilson Observatories near Los Angeles, California; Kitt Peak Observatory, Arizona; Mauna Kea Observatory, Hawaii; Cerro Tololo Observatory, Chile; Royal Greenwich Observatory, England; Roque de los Muchachos Observatory, Canary Islands; Siding Spring Observatory, New South Wales.
