Moon Dog

Moondog

Mock Moon

Description

A member of the halo family, paraselene, also commonly known as a moon dog or mock moon, is an atmospheric optical phenomenon where bright spots or "mock moons" appear to the sides of the Moon, caused by light refracting through high-altitude hexagonal ice crystals in cirrus or cirrostratus clouds. They are analogous to Parhelia (sun dogs) and are most often seen during a full moon.

Paraselenae typically appear as a pair of faint patches of light, at around 22° to the left and right of the Moon. For paraselenae to be observed the moon must be bright, a quarter moon or more.

Formation and characteristics

Paraselenae are formed by the refraction of light through hexagonal, plate-shaped ice crystals that are typically suspended in thin, high altitude cirrus or cirrostratus clouds. The crystals act as tiny prisms, bending the moonlight by about 22°. The paraselenae are positioned at the same elevation above the horizon as the Moon itself and the vertical extent depends on the wobbling of the ice crystals; larger crystals create taller paraselenae.

When discernible, the paraselenae are red nearest the Moon with the outer parts fading through orange and blue but the colours are rarely vivid and often blend into the surrounding white halo or into the parhelic circle, if present, and appear as whitish patches rather than rainbow-coloured spots.

A rare example of 2 paraselenae either side of the Moon with clearly discernible rainbow colours. The Moon is shining through high level cirrus cloud which can be seen in the background [Source: wikicommons, Author: Annajohepworth, 25 October 2015, Didcot, England.]

Folklore

In folklore, the appearance of a moon dog is seen as a sign of approaching bad weather. This has some basis in science as the cirrus clouds often form a few days ahead of a storm system.