What is pi?

One of the most popular mathematical constants, pi (π) is equal to approximately 3.14159. Ancient Greek mathematician Archimedes is commonly credited as the first to accurately calculate pi. The number is irrational and transcendental, which means it goes on, technically, to infinity — but its approximations, 3.14 and the fraction 22/7, are accepted as approximates.

The number is defined as the ratio of a circle’s circumference to its diameter, but it has other definitions in the larger scope of physics and mathematics.

Why do we celebrate Pi Day?

Pi Day falls on March 14 (3/14) and has become an international sensation wherein people celebrate partly by eating pies — from pizzas to fruit pies and savory meat pies.

But many don’t know of Pi Day’s roots in San Francisco.

It all began in 1988, from the brain of a physicist named Larry Shaw who was working at the Exploratorium. Shaw was at a staff retreat in Monterey when he realized the connection between the first few digits of the mathematical constant pi — 3.14 — and March 14.

At 1:59 p.m. — the digits that follow 3.14 in pi — Shaw and his wife, Catherine, set up a table with fruit pies and a tea urn on the museum’s floor for the staff to enjoy. Shaw then conceptualized a “Pi Shrine,” a circular brass plaque, in the center of a circular classroom constructed of circular cinder blocks. The celebration continued with a march around the museum to the music of “Pomp and Circumstance” and the group circling the shrine 3.14 times while singing “Happy Birthday” in honor of Albert Einstein, who coincidentally was born on March 14, 1879.

The tradition soon spread widely to math enthusiasts. Celebrators recite pi’s digits, discuss the idea of circumference and, of course, eat a lot of pie.

Shaw, a graduate of Pleasant Hill High School, had no idea his kooky brainstorm about pi would turn into a global party, or that he’d become known as the Prince of Pi. He had started it as a way to inspire and engage with students who never thought of math as being fun.

Until his death at age 78 in 2017, Shaw led parades every year, with his iconic red cap sporting the pi digits, at the Exploratorium, which still continues the tradition in his memory.

Drama in Pi-radise

The holiday only works for those who write dates in month-day format, which means it’s a holiday that primarily makes sense in the United States.

Some refuse to celebrate Pi Day on March 14, instead opting for July 22 as what’s been called Pi Approximation Day, with 22/7 a common approximation of pi.

Others choose to celebrate on what’s called Two Pi Day, or Tau Day, in observance of pi’s rival, tau (τ), a common multiple in mathematical formulae that others consider a more integral constant.

A tau fan named Michael Hartl is so committed to the cause he wrote a manifesto arguing that “Although π is of great historical importance, the mathematical significance of π is that it is one-half τ.”

Bay Area Events on Pi Day 2020

Editor’s note: Events originally listed here have been canceled due to city- and statewide restrictions on large gatherings in an effort to slow the spread of the COVID-19, the illness caused by the new coronavirus.

Pi Day webcast

Exploratorium educators Ron Hipschman, Lori Lambertson and Paul Dancstep, along with host by Sam Sharkland, discuss what’s so special about the famous mathematical constant π via a live stream on the museum’s website, and on its Facebook, YouTube and Twitter feeds starting at noon.

The webcast will also immediately archive to the Facebook and YouTube channels for later viewing.

The Exploratorium remains closed in accordance with city- and statewide bans on large gatherings of people. For updates, go to www.exploratorium.edu.