We love our electric lights. In fact, if you want to get a good sense of the population density of any first-world city, just wait for the sun to go down. 8.7 million people inhabit Greater London, and as this image from January of 2017 clearly illustrates, they all own at least one high-intensity light bulb.
Star gazing in these conditions can be frustrating or even impossible, thanks to a phenomenon known as skyglow. Light scattered by the atmosphere above our cities serves as a perpetual night light. Sleep deprivation induced by this relentless exposure has become a significant public health problem in many urban areas.
A grassroots effort known as the dark-sky movement works to raise awareness of, if not shed new light on, the impact of light pollution on people and wildlife.
Learn more: http://www.darksky.org/light-pollution/