A rare super blue moon lit up the night sky across the world Wednesday. Blue moons occur when two full moons appear in a single calendar month, something that happens every decade on average. The term “supermoon” means the full moon is at the closest point of its elliptical orbit around the Earth. Wednesday's was this year’s closest full moon, at 357,344km or so away, and more than 160km closer than the August 1 supermoon.