## Pi Day: Celebrating the 14th Day of the Surd Month is Anything But Irrational

### Today, America celebrates a number without which maths, physics, engineering and art would never have got very far

Today we celebrate the transcendental surd pi, because today is 14 March, which in America is spelled out 3.14, which happen to be the first three digits of pi, aka π, which is that Greek letter you had to know in high school.

Pi is important past high school: it’s the ratio of a circle’s circumference to its diameter, and – before your eyes glaze over – humanity wouldn’t have come very far without its help in everything from engineering to physics to art.

(I gave up on math way back in high school, so I apologize to fanatics for stinting on detail. Alex Bellos likes numbers much more than I do; he can help you. I’m here for the lay folk.)

Anyway, besides its uses, what’s interesting about pi is that it’s an irrational number, aka a “surd”, which is more fun to say. Irrationals are real numbers that can’t be expressed as a ratio. In practical terms, it’s a decimal that’s neither finite (like 2.0 or 1.24) nor repeating (like 1.3333) nor periodic (like 9.1818). It just keeps going past 3.14159 for as long as you’d like to take it, and because humanity has had loads of spare time to kill, people have taken it out to 2,700 billion decimal places (Count von Count of Sesame Street is also on the case).

In other words, pi is “infinite” … in a sense … without getting into the various types of “infinite” and “infinity” that are out there (to which whole books are devoted).

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