Pi, as everyone but samuraipandapoetry will recall, is the ratio between a circle’s circumference and its diameter. If the diameter of a circle is 1, than its circumference is π. Its value is traditionally represented as 3.14159… Some people celebrate the 22nd of July for its approximation of π (22/7). Others celebrate March 14 (3.14), although the deluded followers of the March date refer to it simply as Pi Day. But all values for π are approximations, because π is irrational. The ancients did not know this - and it wasn’t finally proven until the 18th century - but π cannot be expressed as any exact fraction. (This despite the Indiana House of Representatives passing a law stating that π = 3.2.) Pi extends forever and ever, and where π goes, its devotees follow.
In physics, π appears in the cosmological constant, Heisenberg’s uncertainty principle, and Einstein’s field equation for general relativity. Pi comes at us in unexpected ways, too, often having nothing to do with circles. It arises in the distribution of prime numbers, and in the probability that a pin dropped on a set of parallel lines will intersect one of them. The ratio of the actual length of a river and its “as the crow flies” length is, on average, π.