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When seen from altitude, as here from an airplane, the sky's color varies from pale to dark at elevations approaching the zenith

Turbulent skies

The sky's zenith appears centered in this daytime photograph taken looking up though trees

Clouds made orange by a sunset

The sky is the part of the atmosphere or of outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight the sky of Earth has the appearance of a deep blue surface, as the result of the air's scattering of sunlight.^[1][2][3][4] The sky is 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.

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 and precipitation can also be seen in the sky during storms. On Earth, birds, insects, aircraft, and kites are often considered to fly in the sky. 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.

The light from the sky is a result of diffuse sky radiation which results in a light blue color being perceived by the human eye. 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 can turn a multitude of colors such as purple (especially near sunset and sunrise) and black at night. Scattering effects also partially polarize light from the sky.

Sky luminance distribution models have been recommended by the International Commission on Illumination (CIE) for the design of daylighting schemes. Recent developments relate to “all sky models” for modeling sky luminance under weather conditions ranging from clear sky to overcast sky.^[5]

Dark Skies is the name usually given to the campaign to reduce and eventually eliminate light pollution from as much of the planet as possible. The campaign is led by the International Dark Sky Association (IDA) and supported by organizations in many countries such as The Royal Astronomical Society of New Zealand - RASNZ and Dark Sky Taonga. Light pollution is defined by the IDA as; "Any adverse effect of artificial light including sky glow, glare, light trespass, light clutter, decreased visibility at night, and energy waste.

• Diffuse sky radiation — Why the sky is blue.
• Skygazing
• Air

1. ^ Tyndall, John (December 1868). "On the Blue Colour of the Sky, the Polarization of Skylight, and on the Polarization of Light by Cloudy Matter Generally". Proceedings of the Royal Society of London 17: pp. 223-233. 
2. ^ Rayleigh, Lord (June 1871). "On the scattering of light by small particles". Philosophical Magazine 41, 275: pp. 447-451. 
3. ^ Watson, JG (June 2002). "Visibility: Science and Regulation". J. Air & Waste Manage. Assoc 52: pp. 628-713. Retrieved on 2007-04-19. 
4. ^ http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html
5. ^ CIE General Sky Standard Defining Luminance DistributionsPDF (710 KiB)

Wikimedia Commons has media related to:

Sky

• Day Sky Images
• Night Sky Images
• Sky Photo Gallery
• Why is the sky blue?