March 14, 2018, is Pi Day — that's pi, not pie — and on Wednesday, Google marks the 30th anniversary of the math-inspired holiday with a special Doodle.

Pi, denoted by the Greek letter "π", has been part of human knowledge for millennia, but it wasn't until 1988 that physicist Larry Shaw organized what is now recognized as the first "Pi Day" celebration at the San Francisco Exploratorium science museum. Shaw chose March 14, or 3.14 — the first three digits of pi — as Pi Day. Shaw died last year, but Pi Day is still celebrated by lovers of mathematics around the world.

As we celebrate again in 2018, here are some Pi Day facts you should know.

## What Is Pi?

Pi represents the ratio of a circle's circumference to its diameter. It's an important part of the foundation of mathematics, most importantly geometry, where pi is key to equations calculating the area of a circle, A = π r^{2}, and the volume of a cylinder, V = π r^{2}h.

## The History of Pi

Various ancient civilizations calculated approximations of pi, including the Egyptians, Babylonians, and there's even a reference to the dimensions of a circle in the Bible, but the first calculation of pi as 3.14 is attributed to Greek mathematician Archimedes, who lived in the third century B.C.. It was also independently determined by Chinese mathematician Zu Chongzhi (429–501), who computed pi to six decimal places. Mathematicians adopted the symbol π for the expression in the 18th century: Welsh mathematics teacher William Jones is often credited with the first use of the symbol in 1706.

## How Many Numbers Are in Pi?

Pi is a mathematical constant, meaning it isn't changed by the size of the numbers it's used to equate, and it is irrational, meaning it has an infinite number of digits that never repeat. The rise of computing technology has led to an arms race of sorts to calculate ever more digits of pi: the current record was set last year by Christian physicist Peter Trueb, calculated pi to 22.4 trillion digits — 22,459,157,718,361, to be exact — outpacing the previous record set in 2013 by 9 million digits.

But adding new digits is little more than a pastime for mathematics fanatics: NASA's Jet Propulsion lab only uses 15 digits to calculate interplanetary travel, while mathematician James Grime argues that just 39 digits of pi is enough to calculate the circumference of the known universe.

## How Is Pie Day Celebrated?

Pi Day was officially recognized by Congress in 2009, and it's inspired quirky and pun-filled celebrations, including eating circular treats, from fruit pies to pizza, as well as dressing like Albert Einstein, whose birthday serendipitously falls on the math-imbued day. San Francisco's Exploratorium also hosts an annual day of pi-inspired activities. The Massachusetts Institute of Technology releases its undergraduate admissions decisions on Pi Day, and starting in 2012, it began sending the verdicts at 6:28 pm, or "Tau time," for the mathematical equation 2π. This year, NASA is inviting math whizzes to compete in its "Pi in the Sky" challenge to solve a series of interplanetary math problems.

Wednesday's Doodle is based on a pi-inspired dish, salted caramel apple pie, courtesy of Cronut creator and pastry pioneer Dominique Ansel.