Pi-Day was first celebrated in 1988 in San Francisco. In November 2019, Pi-Day (14 March of every year) was officially proclaimed International Day of Mathematics by UNESCO.
Why?
Writing this date in the format month.day it reads as 3.14 which is the two-digit approximation of the number Pi, usually written as π.
What is π?
For every circle, no matter how small or large it is, the ratio of the circumference and the diameter is always the same. Since the 18th century, the symbol π (which is a letter in the Greek alphabet) has been used as a name for this number. Contrary to the belief of some people, π is neither equal to 3.14 nor to the fraction 22⁄7. In fact, π is larger than 3.14 and smaller than 22⁄7. The first 13 decimal digits of π are 3.1415926535897…
A bit of History
Archimedes (more than 2,200 years ago) knew that π lies between the two fractions 223⁄71 and 22⁄7. Before that time, less accurate approximations were used, for example the Babylonians used 3 as an approximation for practical purposes but they knew about 3,800 years ago that 25⁄8 is closer than 3 to the true value of π. More on the history of Pi can be found on MacTutor, including a discussion of a verse from the Bible.
The number π cannot be presented as a finite decimal or as a fraction (rational number) – it is a non-recurring decimal. Finding as many as possible of its decimal digits became a challenge for mathematically interested people – well before the computer era. For example, in 1853, the amateur mathematician William Shanks correctly calculated the first 527 decimal digits of π. More on the history of the calculation of the digits of π, including the current world record, can be found in this wikipedia article.
π in Arts and Culture
The Poulnabrone Dolmen in the Burren, viewed from an appropriate position, resembles the shape of the symbol π.
There is a village named Pi in Bellver de Cerdanya, Catalonia (Spain), about 2 hours north of Barcelona by car. Coordinates: 42°21′6″N 1°45′43″E .
You may listen to the Kate Bush song π on YouTube from her 2005 album Aerial.
To memorise the digits of π, some people use Basic Pilish in which the number of letters in each word correspond to a digit of π. The following gives 3.14159265: How I wish I could recollect pi easily today!
π and your birthday
All tests undertaken so far have confirmed that it is likely that π has a very interesting property: all possible sequences of digits of any length appear eventually in π. For example, the sequence 141592 appears at decimal place 1 and 999999 appears at place 762. No proof is known to date whether π actually has this interesting property.
Patrick Sarsfield is said to have been fatally wounded at the battle of Landen on 29 July 1693. This date in the format 29071693 can be found among the decimal digits of π starting at decimal place 177,856,928. Mary Immaculate College was founded on 8 December 1898. The sequence 08121898 occurs at place 4,798,094 for the first time in π, and after that another 17 times among the first 2.14 billion digits.
By the way, this year's Pi-Day, 14 March 2025, can be found as 14032025 at position 72,734,798, as 3142025 at position 18,876,522, and as 20250314 at position 44,177,422.
You can try to locate your birthday or phone number in π and make a postcard. An 8-digit date is expected to be found about 21 times among the first 2.14 billion digits of π.
