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    Re: A halo-like rainbow round the Sun: some sextant measurements.
    From: Frank Reed
    Date: 2017 May 17, 07:40 -0700

    Tony Oz, regarding the angles for a common rainbow and a sundog, you wrote:
    "I expected this halo would be the same."

    It may be helpful to remember that these originate from very different refraction phenomena. Ordinary rainbows occur when sunlight refracts and reflects within more or less spherical drops of water. They are seen when the Sun emerges while rain is still falling on the opposite side of the sky. The key element here is liquid water. But as you noted, your sundog was created by cirrus clouds which consist entirely of tiny ice crystals (easily wafted about by air currents creating the filamentous appearance of cirrus clouds), and in the case of sundogs the ice crystals are supposed to be rather long in one dimension. A great old book on these phenomena and a thousand other fascinating things is Minnaert's "Light and Color...". The original was Dutch. There are two English translations --one older and available cheap; the other newer, rather better, and relatively expensive.

    Frank Reed

       
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