Usually, when we talk about a Blue Moon, we’re referring to the relatively rare occurrence of a second full moon in a single month.
But just like months sometimes squeeze in an extra full moon, so do seasons. Instead of having just three full moons, seasons – from time to time – tack on a fourth.
So there’s an alternative definition of Blue Moon: the third full moon in a four moon season. As tonight’s full moon is the third of four this summer, it’s a Blue Moon by this standard.
“[T]his definition actually preceded the more modern definition of a Blue Moon as being the second of two full moons to occur in one calendar month,” writes Earth Sky.
A Blue Moon is equally rare whichever definition you prefer.
“[I]t’s inevitable that 7 out of 19 years will feature two full moons in one calendar month,” Earth Sky notes. “And it’s also inevitable that 7 out of 19 years will have four full moons in one season.”
“Once in a Blue Moon,” as the saying goes.
Of course, under either definition, the term “Blue Moon” is a misnomer, as the moon is not actually the color blue.
In Washington, tonight’s Blue Moon rises at 7:27 p.m. in the eastern sky and sets Wednesday morning at 7:05 a.m. Thus, from dusk to dawn, the Blue Moon will glow. It is officially 100 percent full at 9:45 p.m.
Tonight’s full moon is also known as the Sturgeon Moon. According to the Old Farmer’s Almanac, fishing tribes near the Great Lakes are credited with naming this moon – identifying the season when ample sturgeon were caught.