Monday is Pi Day — in the U.S., at least. People in Europe might find it strange that March 14 would be the designated day to celebrate a ratio cherished by mathematicians and circle lovers around the world, but that’s because Americans also write the date as 3/14, similar to the first three digits of pi.

Pi Day celebrates the ratio of a circle’s circumference to its diameter — and the Greek symbol that represents it. |

“Pi (Greek letter ‘π’) is the symbol used in mathematics to represent a constant — the ratio of the circumference of a circle to its diameter — which is approximately 3.14159,” according to PiDay.org, which really is a thing. “Pi has been calculated to over one trillion digits beyond its decimal point … it will continue infinitely without repetition or pattern.”

Pi Day (#PiDay) got its start at the San Francisco Exploratorium in 1988. And the annual celebration enjoys a nerdy boost because Pi Day is also Albert Einstein’s birthday; the theoretical physicist was born on March 14, 1879.

**As American as Apple Pi**

“Pi is a great way to bring science awareness to kids and adults,” *San Jose Mercury News* stated. “It’s a number that almost everyone, regardless of educational level, has encountered and is used in fields from basic geometry to wave physics.”

Even the U.S. House of Representatives seems to love Pi Day.

“Pi is a recurring constant that has been studied throughout history and is central in mathematics as well as science and engineering,” according to the Congressional resolution that designated March 14 as Pi Day in 2009.

**Easy as Pi**

Pi is beautiful because it places infinity within humanity’s grasp, according to Cornell University mathematics professor Steven Strogatz’s article in *The New Yorker* last year:

“The digits of pi never end and never show a pattern,” Strogatz said. “They go on forever, seemingly at random — except that they can’t possibly be random, because they embody the order inherent in a perfect circle.”

Other ways Strogatz finds that “pi touches infinity” include:

- Formulas that add “an endless procession of smaller and smaller numbers” to equal pi
- Other irrational numbers, such as e and the square root of two, also consist of an apparently random sequence of digits — and they also serve as links to other mathematical fields
- Its inextricable link to cycles — which are related to circles — makes pi indispensable for people working with rhythms, from studying a planet’s orbit to earthquake-proofing a building to studying circadian rhythms

Theoretical physicist Albert Einstein was born on March 14, 1879 — 109 years before 3/14 became Pi Day. |

**Humble Pi**

Pi Day events abound, both online and on land. Look for clever corporate tweets, such as Major League Baseball’s spoof on the 2012 movie *Life of Pi* and Dunkin’ Donuts’ riff on the diameter equation (D = πr^{2}). And you might even find pie-eating contests, pizza cook-offs and other pi-related festivities near you.

But Princeton, N.J. probably takes the cake — or pie — with five days of activities planned (@PiDayPrinceton), starting with a lecture about Einstein on Thursday, and ending with a pub crawl on Monday. Einstein himself taught at Princeton University, living in Princeton for two decades until his death in 1955.

Last year’s Pi Day peaked at 9:26:53, the exact time — down to the month, day, year, hour, minute and second — that approximated pi to 10 places: 3.141592653. But don’t worry if you missed it; just catch it again in 2115.

*Follow Derek on **Twitter**: **@DKlobucher*

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