It’s Pi Day again! I used to feel a profound disconnect with this holiday, but over the years since I started writing about math, I’ve grudgingly started to appreciate it as an opportunity to learn or write about math I wouldn’t have otherwise. I’m sure you can find a pie to eat or a digit reciting contest to partake in, but I think a better way to celebrate the circle constant is by learning something new.

In 1897, the Indiana legislature famously tried to decree that π was 3.2. In fact, this was part of a bill that claimed to square the circle; the revised value of π was only collateral damage. Squaring the circle is a millennia-old problem in mathematics that asks whether it’s possible to construct a square that has the same area as a given circle using only a compass and straightedge. It was proved impossible in 1882, but word apparently hadn’t reached the Indiana legislature yet.

But perhaps we’re being a little too hard on Indiana and Edward J. Goodwin, the hapless math enthusiast whose square-circling led to the widely-ridiculed bill. As Kelsey Houston-Edwards explains in this excellent video, π isn’t always 3.14159…. If you measure distance differently, your circles will look different, and you’ll get a different circle constant.

James Propp also writes about these and other alternative π’s in his most recent post. Maybe Goodwin just neglected to note that he was using the L^{p} metric for p=2.1 or something. (I haven’t crunched the numbers, so the precise value of *p* is left as an exercise for the reader.) Of course, that would require us to believe he was trying to square the circle using a pretty unconventional compass and ruler. If so, I’m even more impressed. But dude, you’ve got to show your work!

Looking at π's in different L^{p} metrics has a nice bonus: any number between 3 and 4 is π in some L^{p} space, so you can celebrate Pi Day on any day in March.

If L^{p} metrics aren’t your thing, I’ve written a few nice posts for π and Pi Day, if I do say so myself.

In 2012, I wondered exactly how many digits of π we need in order to build buildings or fly to outer space. The answer: a lot fewer than the winner of your local π digit recitation contest knows!

In 2013, feeling rather curmudgeonly, I wrote a link roundup of all the ways other people were celebrating the holiday I was attempting to ignore.

In 2014, I wrote about a different π, the prime counting function. Pi Day doesn’t just have to be about geometry! That function is lurking behind the scenes of an article I wrote last year about a weird anti-sameness bias in the last digits of prime numbers. That article led me down a rabbit hole to the Hardy-Littlewood conjectures, two reasonable-sounding statements in number theory that just can’t both be true.

In 2015, the “Pi Day of the century,” I gorged myself on continued fractions, an objectively better way of writing numbers than the decimal system, and wrote the Pi Day Link Roundup of the Century.

Finally, π can get spooky. One year for Halloween, I wrote about something that scares me: higher homotopy groups, denoted π_{n}.

Whatever you mathematical preferences, I hope you find something new to sink your teeth into this Pi Day and the rest of Pi Month.