Sky

The sky is often defined as the place a person sees when he or she looks up from the earth. Although almost everyone has seen it, the sky is hard to define precisely. The concept of the sky, as it is applied here on Earth, can be generalized to mean the hemisphere above the horizon, or above the visible surface of the planet and objects attached to it. It can be seen when one looks upward from the surface of a planet (see Skies of other planets). The sky is also sometimes defined as the denser gaseous zone of a planet's atmosphere. Birds, airplanes, and kites are often considered to fly in the sky.

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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).

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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 in. The celestial sphere is divided into regions called constellations.

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See skies of other planets for descriptions of the skies of various planets and moons in the solar system.

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Horizon: :For other uses of horizon, see Horizon (disambiguation)...

Planet: A planet in common parlance is a large object in orbit around a star that is not a star itself. The name comes from the Greek term πλανήτης, planētēs, meaning "wanderer", as ancient astronomers noted how certain lights moved across the sky in relatio...

Skies of other planets: REDIRECT Extraterrestrial skies...

Sky related Images and Photos (experimental)

Sky Blue	Sky Spirit	Wilco: Sky Blue Sky	Wilco: Sky Blue Sky
Wilco: Sky Blue Sky CD	Wilco: Sky Blue Sky CD	Red Sky	Crowded Sky, The (DVD)
Blue Sky	Sky Blue c.1940	Topaz Sky I	Aspen Sky