In his 1965 book, Starlight Nights, amateur astronomer and renowned comet finder Leslie Peltier recalled looking down at Los Angeles from Mount Wilson Observatory and noting the city lights encroaching on the mountainside. He was also concerned about the reflection of sunlight off growing fields of space debris. He wrote, “I realize that I have been witness to mankind’s latest pollution in the name of progress—the contamination of the skies.”
Today humanity’s ability to enjoy the night sky is increasingly threatened. Sky glow, which is caused by artificial lighting that is directed upward or reflected and scattered by aerosols in the atmosphere, is altering ambient light levels and obscuring once-visible astronomical objects. In 2016 computations with light propagation software showed that more than 80% of the world population, including 99% of people in the US and Europe, lives under light-polluted skies, where anthropogenic sources alter natural light levels. Less than two-thirds of the world’s population can see the Milky Way in the night sky.