Greenler, Robert Department of Physics, University of Wisconsin, Madison, Wisconsin.
Last reviewed:June 2020
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Either of two large circles of light surrounding the Sun or Moon that result from the refraction of sunlight by small, hexagonal ice crystals falling slowly through the air. Light passing through the side faces of a hexagonal prism is refracted by an amount that depends on the orientation of the crystal; but a collection of many crystals refracts light passing through two side faces by an average angle of about 22° (Fig. 1). If such crystals tumble randomly as they fall, they will produce the 22° halo, a circle around the Sun with an angular radius of 22°. Rays that pass through a side face and an end face of the prism similarly produce the larger and fainter 46° halo. The halos sometimes have a red inner edge and otherwise appear nearly white.
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