The saying “Once in a blue moon” doesn’t disappoint. Tonight’s “blue moon”—the second full moon within the month—is somewhat of a rare occurrence: The last one occurred in August 2012. Many have tried to understand the meaning for centuries: Shakespeare interpreted the phrase as meaning rare or absurd—something that may never happen—while the modern Farmer’s Almanac used ecclesiastical dates and the seasons to explain the phenomenon. The term settled on being the second full moon of the month in 1946. According to NASA, a truly blue moon typically occurs after a volcanic eruption. The ash from certain explosions emits particles that scatter red light and allow blue light to pass through, creating a sort of real-life Instagram filter for the night. There’s a chance it could become red if it sits low on the horizon, but most likely it will still be the same ivory white moon you see the rest of the year.