This remarkable visual phenomenon is described here by the Dutch astronomer Marcel Minnaert (1893-1970) in his wonderful book `Light and Colour in the Open Air'. The book was reprinted in 1993 under the title `Light and Color in the Outdoors' (Springer 1993). Get a copy -- though his advice about looking at the Sun does not conform to modern Health and Safety practice. It is only safe to look at the Sun through shade 13 or 14 arc-welder's glass or metalised filters specially designed for viewing the Sun.
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