Even more rare than the supermoon, a blue moon can be generalized as the second of two full moons in one calendar month. In addition, most seasons have just three full moons, but sometimes there are four — and the blue moon is the third of four full moons in that season. When people say "once in a blue moon," they're not joking about something happening rarely. In the next Metonic cycle (a period of 19 years), we have only eight monthly blue moons and eight seasonal blue moons. And just an FYI, the blue moon isn't blue at all.