seeing

/see"ing/, conj.
1. in view of the fact that; considering; inasmuch as.
n.
2. the act of a person who sees.
3. the sense of sight.
[1495-1505; SEE1 + -ING2]

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      in astronomy, sharpness of a telescopic image. Seeing is dependent upon the degree of turbulence in the Earth's atmosphere for a given telescope. Scintillation, the “twinkling” of stars to the unaided eye, is a commonly known result of turbulence in the higher reaches of the atmosphere. Poor seeing in telescopes is more a result of turbulence in the lower atmosphere. This turbulence sets a limit on the features that a telescope can resolve.

      Turbulence, whether in the upper or lower atmosphere, generates unstable regions of varying densities, diminishing the atmosphere's ability to allow a beam of light to pass straight through with unchanging intensity. When the light from a celestial object is moved about in a rapid and random fashion by atmospheric turbulence, the image formed by a small telescope flutters and dances about. In a larger telescope, the distortions are magnified and the image becomes more diffuse. Seeing is expressed in units of seconds of arc, or 1/3,600 of a degree. The best observatory sites have seeing between 0.5 and 1 second of arc.

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Universalium. 2010.

Synonyms:

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