Moon dog photographed at Hug Point Falls on the Oregon coast

When moonbeams meet ice crystals

When the sky is clear, and the Moon hangs low on the horizon, a halo sometimes appears around it, like the one captured in this image at Hug Point Falls, in Oregon, USA. Within that halo, you may sometimes see a bright spot that appears to be a second moon. This is an optical phenomenon called a paraselene, often referred to as a moon dog or mock moon. It can appear when at least a quarter of the Moon is visible and bright enough for its light to be refracted by hexagonal ice crystals floating in the atmosphere. Moon dogs are more commonly seen in the winter months when the ice crystals are more prevalent in the clouds.

The moon dog is the lunar equivalent of a sun dog, also known as a parhelion, and both appear as part of a related optical phenomenon called a 22-degree halo. Halos like these are also caused by crystal refraction in the atmosphere and are said to be visible up to 100 times a year - much more often than a rainbow.