Sky

View of the night sky in July from Earth
The day's blue sky, clouds and the Moon

The sky is an unobstructed view upward from the surface of the Earth. It includes the atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from outer space.

In the field of astronomy, the sky is also called the celestial sphere. This is an abstract sphere, concentric to the Earth, on which the Sun, Moon, planets, and stars appear to be drifting. The celestial sphere is conventionally divided into designated areas called constellations.

Usually, the term sky informally refers to a perspective from the Earth's surface; however, the meaning and usage can vary. An observer on the surface of the Earth can see a small part of the sky, which resembles a dome (sometimes called the sky bowl) appearing flatter during the day than at night.[1] In some cases, such as in discussing the weather, the sky refers to only the lower, denser layers of the atmosphere.

The daytime sky appears blue because air molecules scatter shorter wavelengths of sunlight more than longer ones (redder light).[2][3][4][5] The night sky appears to be a mostly dark surface or region spangled with stars. The Sun and sometimes the Moon are visible in the daytime sky unless obscured by clouds. At night, the Moon, planets, and stars are similarly visible in the sky.

Some of the natural phenomena seen in the sky are clouds, rainbows, and aurorae. Lightning and precipitation are also visible in the sky. Certain birds and insects, as well as human inventions like aircraft and kites, can fly in the sky. Due to human activities, smog during the day and light pollution during the night are often seen above large cities.

  1. ^ Baird, J. C.; Wagner, M. (1982). "The moon illusion: I. How high is the sky?". Journal of Experimental Psychology: General. 111 (3): 296–303. doi:10.1037/0096-3445.111.3.296. PMID 6215460.
  2. ^ 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. 17: 223–33. Bibcode:1868RSPS...17..223T. doi:10.1098/rspl.1868.0033. JSTOR 112380. S2CID 121593427.
  3. ^ Lord Rayleigh (June 1871). "On the scattering of light by small particles". Philosophical Magazine. 41 (275): 447–51.
  4. ^ Watson, J. G. (June 2002). "Visibility: Science and Regulation". J. Air & Waste Manage. Assoc. 52 (6): 628–713. Bibcode:2002JAWMA..52..628W. doi:10.1080/10473289.2002.10470813. PMID 12074426. S2CID 1078961.
  5. ^ Gibbs, Philip (May 1997). "Why is the sky Blue?". Usenet Physics FAQ. Archived from the original on 2 November 2015. Retrieved 11 December 2012.

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