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. [cite journal |last=Tyndall |first=John |authorlink=John Tyndall |year=1868 |month=December |title=On the Blue Colour of the Sky, the Polarization of Skylight, and on the Polarization of Light by Cloudy Matter Generally |journal=Proceedings of the Royal Society of London |volume=17 |pages=pp. 223–233 |url=http://links.jstor.org/sici?sici=0370-1662%281868%2F1869%2917%3C223%3AOTBCOT%3E2.0.CO%3B2-X&size=LARGE |doi=10.1098/rspl.1868.0033 ] [ cite journal |last=Rayleigh |first=Lord|authorlink=Lord Rayleigh |year=1871 |month=June |title=On the scattering of light by small particles |journal=Philosophical Magazine |volume=41, 275 |pages=pp. 447–451 ] [ cite journal |last=Watson |first=JG |year=2002 |month=June |title=Visibility: Science and Regulation |journal=J. Air & Waste Manage. Assoc |volume=52 |pages=pp. 628–713 |url=http://scholar.google.com/scholar?hl=en&lr=&q=cache:aulPiqN6uTUJ:www.awma.org/journal/pdfs/2002/6/Crit_Review.pdf+ |accessdate = 2007-04-19 ] [ [http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html Why is the sky Blue? ] ] 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.

ky luminance and colors

The light from the sky is a result of the scattering of sunlight, which results in a light 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 an air mass 1/38th of the mass of the air for a sunbeam coming along a horizon path. So, fewer particles scatter the zenith sunbeam, and therefore the light remains a darker blue. [ [http://weatherquesting.com/bluer-on-top.htm Why is the sky bluer on top than at the horizon] ]

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. [PDFlink| [http://www.esim.ca/2002/documents/Proceedings/other2.pdf eSim 2008 (May 20th - 22nd, 2008) General Sky Standard Defining Luminance Distributions] |710 KiB ]

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 [http://www.darksky.org/ International Dark Sky Association] (IDA) and supported by organizations in many countries such as [http://www.rasnz.org.nz/darkskies/index.htm The Royal Astronomical Society of New Zealand - RASNZ] and [http://www.sky.org.nz 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.

ee also

*Air
*Diffuse sky radiation
*Sky brightness
*Skygazing

References

External links

* [http://www.rayching.co.nz/sky.htm Day Sky Images]
* [http://www.astronomy.net.nz/ Night Sky Images]
* [http://www.hanifworld.com/Sky.htm Sky Photo Gallery]
* [http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html Why is the sky blue?]