The largest, brightest full moon in last 60 years will be seen today. If skies are clear, the upcoming full moon will appear up to 14 percent bigger and 30 percent brighter than usual, making it what is called a supermoon, according to NASA. A supermoon occurs when the timing of a full moon overlaps with the point in the moon"s 28-day orbit that is closest to Earth. About every 14th full moon is a supermoon, said University of Wisconsin astronomer Jim Lattis. The next time a full moon comes as close to Earth will be in 2034.