A moon dog (or moondog) or mock moon, also called a paraselene (plural paraselenae) in

meteorology Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...

, 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 is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

. They are exactly analogous to

sun dogs A sun dog (or sundog) or mock sun, also called a parhelion (plural parhelia) in meteorology, is an atmospheric optical phenomenon that consists of a bright spot to one or both sides of the Sun. Two sun dogs often flank the Sun within a 22° h ...

. Moon dogs are caused by the

refraction In physics, refraction is the redirection of a wave as it passes from one 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 commonly observed phenomen ...

of moonlight by hexagonal-plate-shaped

ice crystals Ice crystals are solid ice exhibiting atomic ordering on various length scales and include hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust. Formation The hugely symmetric shapes are due to depositional growth, n ...

in cirrus or cirrostratus clouds. 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 or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...

above the horizon as the Moon. They may also appear alongside

22° halo A 22° halo is an atmospheric optical phenomenon that consists of a halo with an apparent radius of approximately 22° around the Sun or Moon. When visible around the Moon, it is also known as a moon ring or winter halo. It forms as sunlight or ...

s. 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 retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cone ...

References

Atmospheric optical phenomena {{Climate-stub

See also

References