Sky

The sky is (roughly) the 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, insects, 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 a deep blue surface, but this is the result of the air scattering sunlight. [1] 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.

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.

Sky luminance and colors

The colour of the sky is a result of diffuse sky radiation and the fact that air is actually a very transparent blue color^[1]. 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.

Sky luminance distribution models have been recommended by CIE (the International Lighting Commission) for the design of daylighting schemes. Recent developments relate to “all sky models” for modelling sky luminance under weather conditions ranging from clear sky to overcast sky^[2].

Sky in religion

Many cultures ascribed strong religious qualities to Earth's sky, believing it to be the home of the heavens and various deities. Among the Chinese, in certain situations, sky means god. For example, people may say "They became friends after several fights: this is the idea of the sky" or "Why does the sky always fool me?" Some religion and art historians believe that in Christianity, in particular, the connection between heaven and sky arose because of a medieval painting technique, where different places were drawn separately on the same picture. Heaven, a different and more important world, was usually painted above the earthly world and was separated from it by clouds.

Deities of the sky

Ouranos/Uranus, Zeus/Jupiter (Greek/Roman mythology)
Shu, Nut, Horus (Egyptian mythology)
Indra (Hindu scripture)
Cabaguil (Maya mythology)
Torngasoak (Inuit mythology)
Anu (Sumerian mythology)
Ashanti (Ashanti mythology)
Denka (Dinka mythology)
Altjira, Baiame (Aboriginal mythology)
Sin (Haida mythology)
Shanga (Yoruba mythology)
Gamab (Khoikhoi mythology)
Ukko (Finnish mythology)

Expressions

A pie in the sky is something that is wanted very badly but is unobtainable.
When someone's assumptions or premises concerning a particular subject seem to be radically different from everything you know and recognize about the world, you wonder what color the sky is in their world.
Similarly, because the blue hue of the sky during daytime is such a universally observed and understood phenomenon, people say "as sure as the sky is blue" to mean that something is an indisputable fact.
Blue skies are evoked as a symbol for good times, optimism and emotional peace.
To be sky-clad means to be in the nude.
Similarly, to go skying is a euphemism for stripping or streaking.
To show that there are no limits on what we can discover or accomplish, you can say, "The sky's the limit!"
"Blue sky thinking" means coming up with a host of ideas without being restricted to what is practical (similar to brainstorming); in particular, an optimistic assumption is made that there will be no obstacles or opposition to implementation of the ideas.
"The sky is falling" is a common expression that is used when any sort of disaster is occurring. This came from the fable Chicken Little

References

External links

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[1] The sky is blue because air is blue

[2] CIE General Sky Standard Defining Luminance Distributions

[1]

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