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A moon dog (or moondog) or mock moon, also called a paraselene (plural paraselenae) in meteorology, is an atmospheric optical phenomenon that consists of a bright spot to one or both sides of the Moon. They are exactly analogous to sun dogs.
A member of the halo family, moon dogs are caused by the refraction of moonlight by hexagonal-plate-shaped ice crystals in or cirrostratus clouds. They typically appear as a pair of faint patches of light, at Angular distance to the left and right of the Moon, and at the same altitude above the horizon as the Moon. They may also appear alongside 22° halos.
Moon dogs are rarer than sun dogs because the Moon must be bright, about quarter moon or more, for the moon dogs to be observed. Moon dogs show little color to the unaided human eye because their light is not bright enough to activate the eye's .
The colors of moon dogs, when discernible, follow the same pattern as Sun dog: red nearest the Moon, with outer parts fading through orange and blue. However, the colors are usually much less vivid, often blending into the surrounding white halo or into the parhelic circle, if present. Because of this, many moon dogs appear as bright, whitish patches rather than rainbow-colored spots.
The same Ice crystal that produce moon dogs can also create other halo phenomena, such as the circumzenithal arc and the 22° halo. These features often occur together in the sky, and the 22° halo can seem to link the two moon dogs in a circular arc around the Moon. As the Moon rises higher, the angle of refraction through the crystals changes, and the moon dogs appear to shift slightly outward from the 22° halo while remaining at the Moon’s altitude.
The word “dog” in this context has Old English roots, “docga,” and was used to describe a powerful breed of canine; its exact origins remain uncertain. “Moon” comes from ancient words in various languages that originally denoted the month or the heavenly body, with Greek “selēnē” linked to light and brightness.
The “moon dog” phenomenon itself is a type of atmospheric optical event, also called a paraselene, where bright spots appear on either side of the Moon due to the refraction of moonlight by hexagonal Ice crystal in Cirrus cloud. This atmospheric effect is analogous to the “sun dog” phenomenon, which has a similarly obscure etymology, with “dog” possibly relating to tracking or following the sun, or connected to older Nordic mythology about wolves (solhunde) hunting the sun or moon.
In summary, the name “moon dog” combines the celestial element “moon” with the metaphorical “dog” that follows or bays at it, capturing both the visual phenomenon and folkloric naming traditions rooted in English and older Indo-European linguistic elements.
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