Full moons, Friday the 13th and the paradox of exceedingly rare but totally mundane events

The big science-y story last week was the concurrence of a full moon and a Friday the 13th. The media correctly identified this combination of events as rare but forgot to mention that it’s altogether unremarkable. How can something be mundane, trivial, uninteresting and yet rare?

It’s not really paradoxical, but it lends itself to Daniel Kahneman’s notion of thinking fast and slow. In the thinking fast reaction, sometimes known as not thinking, a person hears that we won’t get another full moon on a Friday the 13th until 2049, and says, “oooh, cosmic, spooky”.

Here’s the Huffington Post with, Full moon on Friday the 13th won’t happen again until 2049

Here’s Vox: Today is Friday the 13th and a full moon. That won’t happen again until 2049.

Here’s the Weather Channel with:  Honey Moon On Friday the 13th brings rare celestial event. This story and several others did have a redeeming quality: They gave some background on the term honey moon, which apparently comes from the fact that full moons in June tend toward amber.

National Geographic has: Creepy Full Honey Moon Fills the Sky This Friday the 13th.

CNN has Solar Storms, Full Moon, Must be Friday the 13th, throwing in yet another unrelated and irrelevant fact: A couple of solar flares had led to a forecast for minor and unthreatening geomagnetic storms.

Most of the stories suggested people go out and look at the full moon because it’s such a rare event. One might think that Friday the 13th has some special power to repel full moons.

A moment of slow thought, however, would put it into perspective. Full moons are equally rare on Tuesday the 26th, or Saturday the 9th. They are even rarer on Friday the 31sts, since not all months have a 31st. If you combine any full moon with a day of the week and day of the month, you can rightly claim a rare event.

If you want to get an even more impressive level of rarity, just add some other independent variable – perhaps that the temperate in Philadelphia was ten degrees below normal. A full moon on Friday the 13th,  plus a weirdly cool day in Philadelphia and maybe factor in an underwear sale at Kohl’s and the combination probably won’t happen again for centuries. Better buy that underwear folks. It will be your last chance until 2356!

Here’s another way to consider the fact that so many unimpressive combinations are still rare. If I deal out a random poker hand, say 2 of clubs, 4 of diamonds, 6 of spades, jack of hearts, and queen of clubs, you might think it’s rather commonplace and therefore unlikely to win. You would be correct, but the exact combination of cards is exceedingly rare. I would bet a lot of money I’ll get a full house before you get that exact hand (including the suits) again. This crummy hand is rarer than four aces, because it represents just one of the thousands of combinations of five cards, while there 48 different five-card hands containing four aces.

That’s not to say that all combinations of common events are boring or trivial. Every night brings a different combination of the lunar cycle, planetary positions, atmospheric conditions, satellites and fireflies. So be sure to look up. It’s your last chance.

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