According to Weather.com, when tonight’s full moon rises to its peak at 9:45 p.m. EST, it’ll be a “seasonal blue moon” — the third of four full moons in a season. A seasonal blue moon hasn’t happened in nearly three years. And owning to the rareness of the whole thing, won’t happen again until 2016.
Full moons occur every 29.5 days on average, when the moon is directly opposite the sun from the perspective of Earth. This causes its whole disk to be fully illuminated as a large, bright circle. Usually, when the moon is full, it passes either above or below Earth's shadow, but sometimes, when it is perfectly aligned, it travels right through the shadow, causing a lunar eclipse, when its disk is dark.
“There was a time, not long ago, when people saw blue moons almost every night. Full moons, half moons, crescent moons — they were all blue, except some nights when they were green,” Phillips wrote for NASA’s Solar System Exploration. “The year was 1883, the year an Indonesian volcano named Krakatoa exploded.”
The ash, he continued, caused the unlikely shades, with particles large enough to “strongly scatter red light, while allowing other colors to pass,” Phillips wrote. “White moonbeams shining through the clouds emerged blue and sometimes green.” Krakatoa’s not the only blue-moon causer. It happened after Mount St. Helens erupted in 1980, and again after Mount Pinatubo’s event in the Philippines 11 years later.
Tonight's Blue Moon has several other names including: Full Sturgeon Moon, the Full Red Moon, the Green Corn Moon and the Grain Moon.