Light pollution is excessive or misdirected artificial light. It comes from a number of sources, from streetlights to external lighting. The majority of light pollution comes from lights either being pointed upwards or the light bouncing off of objects and scattering into the sky. With the main sources being street lights and illuminated buildings it is obvious that light pollution is considerably worse in heavily developed areas.
Excessive light pollution can have some notable effects, one being ‘skyglow’. Skyglow is the name given to the phenomenon of a glow effect that is often seen over populated areas. This glow is caused by the badly directed light shining into the atmosphere then being scattered by the air back down towards the ground.
Light pollution, especially skyglow, is a major irritation to astronomers and other stargazers. This is because it reduces the contrast in the night sky. This contrast reduction makes it much more difficult to see and image stars. Strong light pollution often limits which objects can be seen, with observers only being able to make out the very brightest stars in densely populated areas.
To escape light pollution telescopes are often built far away from populated areas, minimising light pollution and maximising their ability to see the night sky.