Moon dog
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A paraselene, or moon dog, is a brightening in the clouds caused by the refraction of moonlight in high-atmosphere cirrus clouds. It can appear on either side or completely around the moon in the form of a halo. They are most typical at approximately 22 degrees. It is the nighttime equivalent of a sun dog, or parhelion, caused by the sun in the same manner. The only types of clouds that can form moon dogs are cirrus and cirrostratus clouds. However, low-level ice crystals suspended in the air can form moon dogs also. Moon dogs are rarer than sun dogs, because in order to produce moon dogs or other types of halos, the moon must be bright and therefore full or near full.
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