What is 'light pollution'?

. Light pollution is a term used by astronomers. . When using an optical telescope to view the light from distant: planets; stars; nebulae; and galaxies; . ….. the telescope would ideally be sited in a totally dark region so that the only light entering the telescope would be the light originating from those distant objects. . In practice many amateur astronomers, observing the night sky, find that light gets into thier telescope from earth bound street lamps, motor vehicle headlamps, and illuminated windows in surrounding buildings.
. ….. It is these Earth bound (terrestrial) light sources that constitute their “light pollution” - making it difficult, or often impossible, to observe the lower intensity light sources that are likely to be their main objects of interest. . Serious professional / research astronomers will therefore chose to site their more powerful optical telescope as far away from such terrestrial light sources as possible, thus reducing the light pollution.
. Far away from the cities and similar human habitation, the only nocturnal light pollution is likely to be the light from sources like moonlight being scattered from clouds and atmospheric dust particles. . For this reason, the best terrestrial optical telesopes are usually built on the top of high mountains in the middle of remote arid (moisture / cloud free) deserts. Thus avoiding most light pollution, due to the near total absence of unregulated human light sources, clouds, dust particles, etc.. . The Hubble Space Telescope has been placed in orbit, outside our atmosphere, above all human habitation, clouds, dust and similar atmospheric particles to avoid as much light pollution as possible thus giving optical images of unprecendented clarity and sensitivity to distant low intensity light sources. .

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