Sky
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Sky".

Rays of light shining through clouds near the Washington Monument in Washington D.C.

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 because of the air's scattering of sunlight.¹²³⁴ 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 the scattering of sunlight, which results in a blue color perceived by the human eye. On a sunny day Rayleigh Scattering gives the sky a blue gradient — dark in the zenith, light near the horizon. Light that comes in from overhead encounters 1/38th of the air mass that light coming along a horizon path encounters. So, fewer particles scatter the zenith sunbeam, and therefore the light remains a darker blue.⁵

The sky can turn a multitude of colors such as red, orange and yellow (especially near sunset or 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 modelling sky luminance under weather conditions ranging from clear sky to overcast.⁶

Dark Skies

Main article: Dark-sky movement

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

• Air
• Diffuse sky radiation
• Sky brightness
• skygazing

References

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. doi:10.1098/rspl.1868.0033, http://links.jstor.org/sici?sici=0370-1662%281868%2F1869%2917%3C223%3AOTBCOT%3E2.0.CO%3B2-X&size=LARGE. 
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, http://scholar.google.com/scholar?hl=en&lr=&q=cache:aulPiqN6uTUJ:www.awma.org/journal/pdfs/2002/6/Crit_Review.pdf+. Retrieved on 19 April 2007. 
4. ^ Why is the sky Blue?
5. ^ Why is the sky bluer on top than at the horizon
6. ^ eSim 2008 (May 20th - 22nd, 2008) 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?