When the sky is clear, and the moon hangs low in the horizon, you can sometimes spot a halo around it, like the one captured in this image from Hug Point Falls on the Oregon coast. And occasionally within that halo, you may also see a bright spot that appears to be a second moon. No, it's not the moon's long-lost twin, but an optical phenomenon called a paraselene, more commonly referred to as a moon dog or mock moon. This 'false' moon can appear when the real moon is at least a quarter visible and is bright enough for its light to refract off hexagonal plate-shaped ice crystals floating in the atmosphere. Moon dogs are more commonly seen in winter months, when ice crystals are more prevalent in the clouds.
