thumbnail What s going on in this sky?
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What s going on in this sky? © Ben Coffman/Tandem Stills + Motion
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.