For sure on the Circle of Fifths, and the whole "music theory is stacks of thirds, fourths, or fifths all the way down" thing is pretty neat.
But what blew my mind even more in music is that our 12 tone even-tempered scale is arbitrary and only firmed up a few hundred years ago. Everything before that (and for many years after, and in many non-western musical cultures) was written assuming just intonation. Modern electronic instruments have brought just intonation (and all sorts of weird stuff) back into reasonable usage.
But what blew my mind even more in music is that our 12 tone even-tempered scale is arbitrary and only firmed up a few hundred years ago. Everything before that (and for many years after, and in many non-western musical cultures) was written assuming just intonation. Modern electronic instruments have brought just intonation (and all sorts of weird stuff) back into reasonable usage.