A moon dog, scientifically known as a paraselene, is an atmospheric optical phenomenon that manifests as bright spots of light appearing on either side of the Moon. This spectacle is the lunar equivalent of a sun dog, and it is a member of the halo family of atmospheric events. The illusion is created when moonlight interacts with ice crystals suspended high within the Earth’s atmosphere. This display requires a specific set of celestial and meteorological conditions to become visible to an observer on the ground.
How Ice Crystals Create the Illusion
The formation of a moon dog depends on the presence of hexagonal, plate-shaped ice crystals suspended in the sky, typically found in high-altitude cirrus or cirrostratus clouds. These crystals must be aligned horizontally as they float or fall through the atmosphere. The moonlight enters one face of the six-sided crystal and is refracted as it passes through the crystal’s 60-degree prism angle. This process consistently deviates the light by a minimum of approximately 22 degrees from its original path. This uniform angle of deviation causes the light to appear to originate from a fixed point about 22 degrees away from the Moon itself. The overall 22-degree halo, which appears as a full ring around the Moon, is formed by light passing through randomly oriented ice crystals. In contrast, moon dogs are specifically caused by the horizontally-oriented plate crystals that concentrate the refracted light into two distinct, bright patches.
What Moon Dogs Look Like
Moon dogs appear as a pair of luminous patches, positioned horizontally to the left and right of the Moon, at the same elevation above the horizon. Their angular distance from the Moon is always 22 degrees. Though they are technically capable of displaying color due to the dispersion of light, the effect is often muted or entirely absent to the naked eye. Lunar light is significantly fainter than sunlight, meaning it is often not bright enough to fully stimulate the cone cells that perceive color. Consequently, most observers see moon dogs as bright, whitish patches, sometimes with a subtle reddish hue on the edge closest to the Moon. The size and vertical stretch of these patches can vary, determined by how much the flat ice crystals wobble as they descend through the air.
When and Where to Spot Them
To view a moon dog, the Moon must be at least a quarter phase, with a near-full or full moon providing the most favorable brightness. The moon should also be relatively low in the sky, as the phenomenon is most conspicuous when close to the horizon. The necessary ice crystals are present in high, thin cirrus clouds, which can form at any time of year and anywhere in the world. The conditions that favor their formation often coincide with cold weather, making winter nights a frequent time for sightings. Look for thin, wispy clouds passing in front of a bright moon, as this indicates the presence of the required ice crystals in the upper atmosphere.