Once in a blue moon

If you ever played Trivial Pursuit back in the 80's then, like I did, you probably think you know the origin of the expression "once in a blue moon". It means something rare, because a blue moon occurs when there are two full moons in the same calendar month - a rare occurrence - right?

Wrong! It turns out that this is a misconception, which took hold in the popular imagination during the 1980's, promulgated by a (mistaken) American radio broadcast and the makers of Trivial Pursuit. The correct definition of a blue moon turns out to be a little more complicated, and refers to an event which is even rarer still.

What is the correct definition? Listen up, and I'll tell you. A blue moon is the third full moon in a season that has four full moons. How often does this happen? Let's take a look:



The "Maine Farmer's Almanac" Rule referred to in the diagram is the definition I just gave, 3rd full moon in a season with 4.

It's evident from the diagram that this happens somewhat less often than the occurrence of two full moons in the same calendar month. Furthermore, while the short duration of February means that the latter definition can never happen in February, the Farmer's Almanac definition implies that February is one of only four months in which a blue moon can occur.

The plain people of Ireland: Sure you could be making all this up. How do we know you're not just pulling our legs, in some kind of Saint Patrick's Day prank?
The management: Ye're a suspicious lot, aren't ye? But don't take my word for it. Check it out for yeerselves here. The good scientists at Sky and Telescope wouldn't be misleading the public.