A full moon that actually appears blue is very rare. It happens because of ash or dust in the air, sometimes from volcanic eruptions or major forest fires, which act like a color filter for your eyes making the moon appear blue. Because these types of blue moons only happen after a major event, they aren’t predictable. There are a few other reasons the moon could appear blue, according to Les Cowley, an atmospheric optics expert. “Our eyes have automatic ‘white balances‘ just like digital cameras. Go outdoors from a cozy cabin lit by an oil lamp (yellow light) and the moon will appear blue until your eyes adjust.”