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A moon dog (or moondog) or mock moon, also called a paraselene (plural paraselenae) in
meteorology Meteorology is the scientific study of the Earth's atmosphere and short-term atmospheric phenomena (i.e. weather), with a focus on weather forecasting. It has applications in the military, aviation, energy production, transport, agricultur ...
, is an atmospheric optical phenomenon that consists of a bright spot to one or both sides of the
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
. They are exactly analogous to sun dogs. A member of the halo family, moon dogs are caused by the
refraction In physics, refraction is the redirection of a wave as it passes from one transmission medium, medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commo ...
of moonlight by hexagonal-plate-shaped
ice crystals Ice crystals are solid water (known as ice) in crystal structure, symmetrical shapes including hexagonal crystal family, hexagonal columns, hexagonal plates, and dendrite (crystal), dendritic crystals. Ice crystals are responsible for various at ...
in cirrus clouds or
cirrostratus cloud Cirrostratus () is a high-altitude, very thin, and generally uniform stratiform genus-type of cloud. It is composed of ice crystals, which are particles of frozen water. Cirrostratus is difficult to see and can produce halos. These optical ...
s. They typically appear as a pair of faint patches of light, at around 22° to the left and right of the Moon, and at the same
altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...
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
cone cell Cone cells or cones are photoreceptor cells in the retina of the vertebrate eye. Cones are active in daylight conditions and enable photopic vision, as opposed to rod cells, which are active in dim light and enable scotopic vision. Most v ...
s.


See also

*
Halo (optical phenomenon) A halo () is an optical phenomenon produced by light (typically from the Sun or Moon) interacting with ice crystals suspended in Earth's atmosphere, the atmosphere. Halos can have many forms, ranging from colored or white rings to arcs and spo ...
* Circumhorizontal arc * Circumzenithal arc *
Gegenschein Gegenschein (; ; ) or counterglow is a faintly bright spot in the night sky centered at the antisolar point. The backscatter of sunlight by interplanetary dust causes this optical phenomenon, being a zodiacal light and part of its zodiacal light ...
* Zodiacal light


References

Atmospheric optical phenomena {{Meteorology-stub