= Solution
<Atmospheric extinction> changes the amplitude of the incoming light through absorption and scattering. Its <wavelength> dependence alters the measured <optical spectrum> and colors; clouds and aerosols introduce additional time dependence. Scattered moonlight and atmospheric emission increase <sky brightness>.
<Atmospheric refraction> changes the apparent position of the source. Its wavelength dependence, <atmospheric dispersion>, spreads a broadband image toward the zenith. The mean <refractive index> gradient therefore affects direction even in the absence of small-scale turbulence.
<Atmospheric turbulence> produces rapidly varying <optical path lengths>, distorting the phase and curvature of a nominally plane <wavefront>. Different pupil regions acquire different phase delays, producing <astronomical seeing>, image motion, and short-exposure <speckle patterns>. Different lines of sight sample different fluctuations, giving <anisoplanatism>.
Propagation through the fluctuating medium also changes the intensity through <atmospheric scintillation>, the familiar twinkling of stars. A phase-only <adaptive optics> correction can reduce atmospheric phase errors but does not remove absorption, all scintillation, or the atmospheric emission background.
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