Moon dogs, also known as paraselenae or mock moons, represent a captivating atmospheric optical phenomenon. These events manifest as bright, luminous spots of light that appear alongside the Moon.
Understanding Moon Dogs
Moon dogs typically appear as one or two bright, sometimes faintly colored, patches of light positioned on either side of the Moon. These spots are always found at the same altitude as the Moon itself. They are part of a larger optical display known as a lunar halo, appearing as specific bright areas along the halo’s ring.
Moon dogs are more commonly observed when the Moon is bright, usually at or near its full phase, and positioned low in the sky. Because moonlight is dimmer than sunlight, moon dogs often appear white or pale to the unaided eye, as the light isn’t intense enough to fully activate color-sensing cone cells.
The Science Behind Their Formation
The formation of moon dogs relies on the presence of specific ice crystals within the Earth’s atmosphere. These crystals, typically hexagonal and plate-shaped, are found in high-altitude cirrus or cirrostratus clouds. Moonlight refracts (bends) as it passes through these tiny ice prisms.
Precise orientation of these plate-like ice crystals is important. As moonlight enters one face of a hexagonal crystal and exits another, it is bent at a specific angle. This refraction typically occurs at a 22-degree angle, determining the moon dogs’ apparent distance from the Moon. These ice crystals often form in very cold atmospheric conditions.
Moon Dogs Versus Other Sky Phenomena
Moon dogs are often compared to sun dogs (parhelia) because they are analogous phenomena. Both result from light refracting through hexagonal ice crystals in the atmosphere. The key distinction lies in the light source: moon dogs are created by moonlight, while sun dogs are formed by sunlight.
Sun dogs are generally seen more frequently and tend to display more vibrant colors than moon dogs, due to the greater intensity of sunlight. Moon dogs are also distinct from the more common 22-degree lunar halo. While moon dogs appear as bright spots on the halo, the halo itself is a continuous ring of light that encircles the Moon. Both moon dogs and the 22-degree lunar halo are products of light interacting with hexagonal ice crystals.