Sky
Sky, the atmosphere and space as seen from the earth. The clear sky is blue during daylight hours because sunlight, which contains all colors, is scattered by the molecules of air as it strikes the upper atmosphere. Blue light is scattered the most and illuminates the whole sky. At night the sky is black since no sunlight is striking the visible portion of the atmosphere.
The sun and stars appear to move across the sky and are commonly described as doing so; actually, this apparent motion is caused by the movements of the earth.