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 approximately 22° away. They are exactly analogous to sun dogs, but are somewhat rarer because in order to be produced the moon must be bright and therefore full or nearly full. While a moondog may be brightly colored, the lunar halos they form in typically appear colorless to the naked eye because their light is not bright enough to activate the color photoreceptors in humans.