SEEING

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. 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.