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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 be defined precisely. The concept of the sky, as it is applied here on Earth, can be generalized to mean the space seen when one looks upward from the surface of any planet (see Skies of other planets). The sky is also sometimes defined as the denser gaseous zone of a planet's atmosphere.

On earth, the sky usually looks blue (due to Rayleigh scattering) on a sunny day. It turns orange and red during sunrise and sunset, and becomes black at night. See Diffuse sky radiation for more detail. Some of the things people see in the sky are clouds, rainbows, and aurorae. Lightning can also be seen in the sky during storms. People also say that birds, airplanes, and kitess fly in the sky.

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.

Many cultures ascribed strong religious qualities to Earth's sky, believing it to be the home of the heavens and various deities. Some religion and art historians believe that in Christianity, in particular, the connection between heaven and sky arose because of medieval painting technique, where different places where drawn on the same picture separated. Heaven, a different world, was usually painted above earthly world (being the more imporant one) and was separated using clouds. This meaning of "parallel" worlds was lost on less subtle folk, who thought heaven to be literally above the clouds.

Sky gods/goddesses in different cultures:

• Uranus, Zeus (Greek mythology)
• Shu, Nuit, Horus (Egyptian mythology)
• Cabaguil (Maya mythology)
• Torngasoak (Inuit mythology)
• Anu (Goidelic mythology)
• Ashanti (Ashanti mythology)
• Denka (Dinka mythology)
• Altjira, Baiame (Aboriginal mythology)
• Sin (Haida mythology)
• Shanga (Yoruba mythology)
• Gamab (Khoikhoi mythology)

• A pie in the sky is something that is wanted very badly but is unobtainable.
• When someone's assumptions and premises about everything he speaks about seem to radically different from everything you know and recognize about the world, you wonder what color the sky is in his 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.

• A rock band: see Sky (band)
• A British children's television series of the 1970s: see Sky (series)
• Sky Television (later British Sky Broadcasting) and its digital service Sky Digital.