The final supermoon of the year will illuminate the sky beginning tonight. According to the Farmer's Almanac, the "Harvest" supermoon will reach its peak just before 6 a.m. tomorrow on the east coast. A supermoon happens when the moon's orbit is closest to Earth at the same time the moon is full, according to NASA. A supermoon is typically about 14-percent bigger and 30-percent brighter than an average moon.