A corona is an optical phenomenon consisting of one or more concentric, relatively small-diameter, often faintly coloured rings immediately surrounding the sun or moon, caused by diffraction of light around small water droplets or ice crystals in thin clouds (typically altostratus or altocumulus) — distinct from the much larger-diameter halo (caused by refraction through ice crystals) or the arc-shaped rainbow (caused by refraction and internal reflection in raindrops).
When sunlight or moonlight passes through a thin cloud layer composed of small, relatively uniform water droplets, diffraction around these droplets produces a small-diameter, often...
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