Moon dog

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This article is about the optical phenomenon. For other uses, see Moondog (disambiguation).
A halo around the Moon and a pair of moon dogs over the observatory at the University of Alberta in Edmonton, Canada.

A moon dog, moondog, or mock moon,[1] (scientific name paraselene,[1] plural paraselenae, meaning "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 as ~22 degree radius halo around the moon (see: wiki on 22 degree halo), halo size to half degree size of moon.[2] They are exactly analogous to sun dogs, but are rarer because to be produced the moon must be bright, about quarter moon or more. Moondogs show little color to the unaided eye because their light is not bright enough to activate the cone cells of the human eye.

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  1. ^ a b "Definition of paraselene". Collins English Dictionary. 
  2. ^ Lisle, Jason. The Stargazer's Guide to the Night Sky. p. 83.