Wait... what? Butbutbut... I know this fact! And it's wrong? So I dived down the rabbithole and, sure enough, it is wrong. A Blue Moon is not the second full moon in a calendar month. As this...
Another definition of the blue moon, perhaps the more commonly used due to its simplicity, is actually a mistake [...] This definition describes the blue moon as the second full Moon in any calendar month with two full moons.
Wait... what?
Butbutbut... I know this fact! And it's wrong?
So I dived down the rabbithole and, sure enough, it is wrong. A Blue Moon is not the second full moon in a calendar month. As this article says:
Traditionally the definition of a blue moon is the third full Moon in an astronomical season containing four full moons.
Multiple sources confirm this.
However... I also found a few sources which accept that the newer meaning is now taking hold. For example, the Wikipedia article on blue moons talks about "seasonal blue moons" and "calendrical blue moons". The NASA page about different moon names doesn't even mention the seasonal blue moon; it just says the calendrical blue moon has been talked about since the 1940s.
It seems the misinterpretation came from a writer in the 1940s, which Wikipedia talks about:
The March 1946 Sky and Telescope article "Once in a Blue Moon" by James Hugh Pruett misinterpreted the 1937 Maine Farmers' Almanac. "Seven times in 19 years there were—and still are—13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon."
That phrase "so I interpret it" is the key. This writer didn't know the definition of a blue moon; he just guessed it based on the data in an almanac. And he guessed wrong.
However... despite my digging... noone knows why this "extra" full moon (whether in a season or a month) is called "blue". There are some guesses around, but nothing definitive. Someone even made a definitive statement about "blue" being derived from "belewe", meaning "betray" (this extra moon betrays the normal pattern of three full moons in a season), but he retracted that statement some years later. Noone knows why this moon is blue. Bugger.
Wait... what?
Butbutbut... I know this fact! And it's wrong?
So I dived down the rabbithole and, sure enough, it is wrong. A Blue Moon is not the second full moon in a calendar month. As this article says:
Multiple sources confirm this.
However... I also found a few sources which accept that the newer meaning is now taking hold. For example, the Wikipedia article on blue moons talks about "seasonal blue moons" and "calendrical blue moons". The NASA page about different moon names doesn't even mention the seasonal blue moon; it just says the calendrical blue moon has been talked about since the 1940s.
It seems the misinterpretation came from a writer in the 1940s, which Wikipedia talks about:
That phrase "so I interpret it" is the key. This writer didn't know the definition of a blue moon; he just guessed it based on the data in an almanac. And he guessed wrong.
However... despite my digging... noone knows why this "extra" full moon (whether in a season or a month) is called "blue". There are some guesses around, but nothing definitive. Someone even made a definitive statement about "blue" being derived from "belewe", meaning "betray" (this extra moon betrays the normal pattern of three full moons in a season), but he retracted that statement some years later. Noone knows why this moon is blue. Bugger.
And the next one is a Super Blue Moon, how cool!