The sky is (roughly) that part of the atmosphere or of outer space visible from the surface of the earth (or any other planet; see Skies of other planets). Birds, airplanes, and kites are often considered to fly in the sky. It is difficult to define precisely for several reasons. During daylight the sky has the appearance of an opaque blue surface, but this is the result of the air scattering sunlight. *There is no "blue object" above the earth in any normal sense, so it is hard to say what object the sky is. The sky is thus sometimes defined as the denser gaseous zone of a planet's atmosphere. At night the sky has the appearance of a black surface or region scattered with stars. But if we then say that the sky is the entire visible universe, it would not be the same thing we see during the day. The color of the sky is a result of diffuse sky radiation. On a sunny day the Earth sky usually looks as a blue gradient — dark in the zenith, light near the horizon (due to Rayleigh scattering). It turns orange and red during sunrise and sunset, and becomes black at night.
During the day the Sun can be seen in the sky, unless covered by clouds. In the night sky (and to some extent during the day) the Moon, planets and stars are visible in the sky. Some of the natural phenomena seen in the sky are clouds, rainbows, and aurorae. Lightning can also be seen in the sky during storms. As a result of human activities, smog during the day and light radiance during the night are often seen above large cities (see also light pollution).
In the field of astronomy, the sky is also called the celestial sphere. This is an imaginary dome where the sun, stars, planets, and the moon are seen to be travelling. The celestial sphere is divided into regions called constellations.
See skies of other planets for descriptions of the skies of various planets and moons in the solar system.
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