- noun the effect of street or other artificial lighting which makes the sky red or orange at night, and so reduces the visibility of stars
- Effect whereby light from industrial or builtup areas makes astronomy difficult by brightening the night sky. Some US cities have altered their lighting patterns to help astronomy continue, but the usual answer to light pollution is to do astronomy in more remote places. Unlike pollution due to radiation or chemicals, pollution of the electromagnetic spectrum has the pleasing property that a throw of a switch can stop it at once. This means that campaigns against light pollution can succeed rapidly. Low-energy lighting which minimises light sent upwards and places more on the ground where it is needed can allow astronomers to see the stars while people who need light stay happy and lighting bills fall.
- The glare from inefficient outdoor lights, especially around highly populated areas, making it difficult to discern the features of the night sky.