Pi Day

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There are two days held in honor of the mathematical constant π (Pi): Pi Day and Pi Approximation Day.

Pi Day

March 14, written 3-14 in the USA date format, is an unofficial celebration for Pi Day derived from the common three-digit approximation for the number π: 3.14. It is usually celebrated at 1:59 PM (in recognition of the six-digit approximation: 3.14159). Some, using a twenty-four-hour clock rather than a twelve hour clock, say that 1:59 PM is actually 13:59 and celebrate it at 1:59 AM or 3:09 PM (15:09) instead. Parties have been held by the mathematics departments of various schools around the world.

This day has been celebrated in a variety of ways. Groups of people, typically pi clubs, give thought to the role that the number π has played in their lives and imagine the world without π. During such an event, pi celebrants may devise alternative values for π, eat pi (pie), play pi (piñata), drink pi (Piña Colada) or watch π (Pi (film)). Enthusiasts also note that the day happens to be Albert Einstein's birthday.

The "ultimate" pi day[1] occurred on March 14th, 1592, at 6:53 AM and 59 seconds. When written in American-style date format, this is 3/14/1592 6:53.59, which corresponds to the first twelve digits of pi: 3.14159265359 (rounded of course). However, considering this was well before any kind of standardized world time had been established, and the general public had no concept of π, the occurrence likely went unnoticed[2].

Pi Approximation Day

Pi Approximation Day is one of two days: either July 22 (written 22/7 — in some date formats — 22 divided by 7 is an approximation to π), or April 26 (April 25 on leap years), the day on which planet Earth completes two Astronomical units' worth of its annual orbit: on this day the total length of Earth's orbit, divided by the length already traveled, equals π (that is, the Earth has travelled two radians in its orbit).

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  1. ^  At least twice in the West, about a fortnight earlier for those countries (like Russia and the UK) that still used the Julian calendar in 1592.
  2. ^  Further, in order to "notice" such event on the day it occurs one needs (at the same time, at the same place): Gregorian (or comparable) calendar, American-style notation of date/time, possibility to calculate π with enough digits, and plain decimal calculus (that is, e.g. without the intermittent numbers of the Simon Stevin 1586 system) - well, that just never happened concurrently in real time.

be:Дзень Пі es:Día de π eo:Pi-tago fr:Journée de π ko:파이의 날 is:Π dagur it:Giorno di pi greco he:יום פאי nl:Pidag ja:円周率の日 uk:День пі zh:圓周率日