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Sundogs are an optical phenomenon in which a pair of mock suns appear on either side of the sun. These spots form along a halo that appears to encircle the sun. This is an optical phenomenon caused by the refraction of sunlight on flat, hexagonal ice crystals (or diamond dust) found high in the sky in cirrus clouds. When the hexagonal ice crystals are randomly oriented, a halo is observed. In cases where the hexagonal ice crystals are oriented with their flat faces horizontally, sundogs are also observed. This is why halos are seen more often than sundogs. These same bright spots can occur around the moon. Moondogs can appear in a lunar halo, but requires the bright light of either a full moon or nearly-full moon.

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