The sky is often defined as the place a person sees when he looks up from the earth. Although almost everyone have 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. The sky is also sometimes defined as the denser gaseous zone of a planet's atmosphere.

On earth, the sky usually looks blue on a sunny day, turns orange and red during sunrise and sunset, and becomes black at night. 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 kites 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. (See also Chinese constellation).

Many cultures ascribed strong religious qualities to Earth's sky, believing it to be the home of the heavens and various deities.

Sky gods/goddesses in different cultures:

Some expressions involving the sky:

  • 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!"

Sky can also refer to: