Moon dog

This article is about the optical phenomenon. For other uses, see Moondog (disambiguation).

Pair of moon dogs

Partial Moon dog seen from Hillspeak, Shimla, Lower Himalayas.

A moon dog (or moondog) or mock moon, also called a paraselene^[1] (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 cirrus clouds 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 above the horizon as the Moon. They may also appear alongside 22° halos.^[2]

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 cells.^[3]

Formation and characteristics

Moon dogs are formed by the refraction of light through hexagonal, plate-shaped ice crystals that are typically suspended in thin, high cirrus or cirrostratus clouds. The crystals act as tiny prisms, bending the moonlight by about 22°, so the moon dogs appear at that angular distance from the Moon. Because the light of the Moon is fainter than that of the Sun, moon dogs are often pale or whitish, their colors, if visible, are muted, and when the Moon is especially bright—almost full moon—moon dogs are easier to see. Moon dogs are positioned at the same elevation above the horizon as the Moon, like sun dogs, their vertical extent depends on the wobbling of the ice crystals: larger crystals that tilt more create taller moon dogs.

The colors of moon dogs, when discernible, follow the same pattern as sun dogs: 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 crystals 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.

Terminology

A common misunderstanding is to call any halo seen around the Moon a “moon dog.” In reality, moon dogs are only a specific form of ice halo, created by refraction through plate-shaped ice crystals. The frequently observed 22° halo, which appears as a full ring around the Moon, is often mistaken for a pair of moon dogs and describing the phenomenon more broadly, the term ice crystal halos or simply halos is more accurate.

See also

• Halo (optical phenomenon)
• Circumhorizontal arc
• Circumzenithal arc
• Gegenschein
• Zodiacal light

References

1. "Definition of paraselene". Collins English Dictionary.
2. Lisle, Jason. The Stargazer's Guide to the Night Sky. p. 83.
3. "Moon Dogs, Paraselenae, Parselenae". atoptics.co.uk. Retrieved 2023-06-14.