Atmospheric optics describes displays caused by light interacting with the Earth’s atmosphere. These phenomena occur when sunlight or moonlight is bent, reflected, or scattered by suspended particles. The resulting displays include rainbows, coronas, and various forms of halos. A moon dog represents a specific and captivating example of an atmospheric optical event that occurs at night.
Defining the Moon Dog Phenomenon
A moon dog is an atmospheric optical phenomenon that manifests as a luminous, spot-like patch of light appearing on one or both sides of the Moon. Scientifically, this event is known as a paraselene, which translates literally as “beside the moon”. These mock moons are part of the larger family of ice halos, which are all created by the same physical mechanism involving ice crystals in the atmosphere.
The visual characteristic that defines a moon dog is its fixed angular distance from the Moon. The bright spots always appear approximately 22 degrees to the left or right of the celestial body. A moon dog will always be positioned at the same elevation above the horizon as the actual Moon, giving the appearance of a false moon companion.
The phenomenon is only observable when the Moon is bright enough, typically during or near a full moon phase. When the conditions are right, the paraselene appears as an elongated, often faintly colored, spot of light. Though the moon dog is distinct, it may sometimes be seen connected to a larger, fainter ring known as the 22-degree lunar halo.
The Mechanics of Light Refraction
Moon dogs are created by the refraction of moonlight through tiny ice crystals suspended high in the atmosphere. These crystals are typically found in thin, high-altitude cirrus clouds composed entirely of ice. The precise shape of these particles governs the appearance of the moon dog.
The crystals responsible for this phenomenon are shaped like flat, hexagonal plates. As moonlight enters one face of the hexagonal prism and exits another, the light is bent, or refracted, similar to how light passes through a glass prism. For the moon dog to form, the light must pass through the crystal faces that are oriented at a 60-degree angle to one another. This specific geometry causes the light to be deviated by a minimum angle of 22 degrees.
The plate-shaped crystals must also be oriented horizontally as they flutter downward through the air. This horizontal alignment focuses the refracted light into a concentrated patch, which is then perceived as the bright moon dog. If the ice crystals were randomly oriented, the light would be scattered into a complete ring, forming a 22-degree circular halo instead.
Moon dogs often appear pale or whitish because the light source is the Moon, which is significantly dimmer than the Sun. The faint moonlight is not bright enough to stimulate the cone cells responsible for color perception. This leads to a display dominated by rod cell vision.
Comparing Moon Dogs to Related Optical Events
Moon dogs are often confused with other atmospheric phenomena. The moon dog (paraselene) is the nighttime equivalent of the more commonly observed sun dog (parhelion). The formation mechanism is identical for both, involving the refraction of light through horizontally aligned hexagonal plate ice crystals.
The primary difference between a moon dog and a sun dog is the light source, which affects visibility and coloration. Sun dogs, illuminated by the much brighter Sun, are typically more distinct and often display subtle rainbow colors. Moon dogs, conversely, are fainter and usually appear white or very pale due to the dimness of the moonlight.
Moon dogs are also distinct from the 22-degree lunar halo, even though they are often seen together. The lunar halo is a complete, continuous ring of light encircling the Moon at the same 22-degree radius. This full ring is created by ice crystals that are randomly oriented in the atmosphere, scattering the light in all directions around the central source. The moon dog, in contrast, is a concentrated, brighter spot caused specifically by the horizontal alignment of plate-shaped crystals.