Sky

The sky is the unobstructed view upward from the surface of the Earth, taking in both the atmosphere and, beyond it, outer space. During the day it usually looks blue because air molecules scatter shorter wavelengths of sunlight, mainly blue light, more strongly than longer wavelengths through a process called Rayleigh scattering, with the blue deepest overhead and paler near the horizon where light passes through far more air. At night the sky becomes a mostly dark field showing stars, the Moon, and the planets, and during twilight it can pass through reds, oranges, and purples as sunlight crosses an increasingly thick slice of atmosphere near the horizon. Human activity has begun to change how the sky looks and is seen, with smog dulling it by day and light pollution washing out stars by night over cities.