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  Antitiwilight Colors And Belt Of Venus Caption
  Antitiwilight Colors And Belt Of Venus
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  Generally speaking, twilight is sunlight scattered by the atmosphere that is visible when the Sun is below the horizon. In a dedicated paper, D.Lynch et al. describe a series of visual observations of the antitwilight made with the Sun moving beneath the observer horizon under “very clear or cloud-free” conditions. They name four principal visual components of the antitwilight:
  •the blue “upper sky” (US) which extends downward from observer zenith;
  •the reddish “Belt of Venus” (BV);
  •the “blue band” (BB) which lies below the Belt of Venus and is part of the Earth’s shadow;
  •the “horizon band” (HB) a band of sky lying between the blue band and the horizon, which, though it lies farther into Earth’s shadow, is slightly brighter than the blue band (not present in this image).
  The colors’ variation in the result of several physical phenomena such as: Rayleigh scattering, molecular absorption, aerosol scattering, multiple scattering and refraction.
  The blue color of the upper sky would seem to be explainable by Rayleigh scattering alone but in fact, some of its color is also the result of ozone absorption. It goes from greyish to blue as far as we move from the BV to the zenith.
  The BV’s pink color is a consequence of the yellowish or reddish low Sun illuminating air molecules. At the same time, the scattered light’s color is modified by Rayleigh scattering adding blue to it. As is well known, red and blue together produce purple and pink is bright purple.
  The BB rich blue color is a consequence of both multiple Rayleigh scattering and optical depth effects. When the solar altitude is less than about 15°, the BB appears on the lower and thicker layers of the atmosphere. Thus, multiple scattering is important here. Rayleigh scattering modifies white sunlight by 1∕λ4 , and with each successive scattering, the light becomes progressively bluer. Being adjacent to the pink BV, t