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Spiral Evolution of Twilight - The Four Bands

In general, twilight is atmospheric scattered sunlight that is visible when the Sun is below the horizon. In a dedicated article, D.Lynch et al. describe a series of visual antidusk observations made with the Sun moving below the observer's horizon in "very clear or cloudless" conditions. They name four main visual components of the antidusk:
• the blue “upper sky” (US) extending downwards from the observer's zenith;
• the reddish “Cintura di Venere” (BV);
• the "blue band" (BB) which is located below the Belt of Venus and is part of the shadow of the Earth;
• the “horizon band” (HB) a band of sky between the blue band and the horizon, which, although located further into the earth's shadow, is slightly brighter than the blue band.