A typical daytime sky.

Turbulent skies

The sky as seen from an airplane.

Stormy skies

Daytime sky and trees

Clouds made orange by a sunset

Sky at 30,000 feet (9 km) altitude.

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, but this is the result of the air scattering sunlight. [1] 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.

Sky luminance and colors

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.^[1]

Dark Skies

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."

See also

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

References

1. ^ CIE General Sky Standard Defining Luminance DistributionsPDF (710 KiB)

External links

Wikimedia Commons has media related to:

Sky

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