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A moon dog or moondog (scientific name paraselene, plural paraselenae, i.e. "beside the moon") is a relatively rare bright circular spot on a lunar halo caused by the refraction of moonlight by hexagonal-plate-shaped ice crystals in cirrus or cirrostratus clouds. Moondogs appear to the left and right of the moon 22° or more distant. They are exactly analogous to sun dogs, but are rarer because to be produced the moon must be bright and therefore full or nearly full. Moondogs show little color to the unaided eye because their light is not bright enough to activate the cone cells of humans' eyes.

See also

• Sun dog
• Halo (optical phenomenon)
• Moon ring