Pi Day

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Pi Day and Pi Approximation Day are two unofficial holidays held in honor of the mathematical constant π (Pi). Pi Day is observed on March 14; Pi Approximation Day may be observed on any of several dates.

1 Pi Day
2 Pi Approximation Day
3 See also
4 External links
5 Notes

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Pi Day

March 14, written as 3-14 or 3/14 in the United States date format, represents the common three-digit approximation for the number π: 3.14. Pi Day is often celebrated at 1:59 p.m. in recognition of the six-digit approximation: 3.14159. Some, using a 24-hour clock, celebrate it at 1:59 a.m. or 3:09 p.m. (15:09) instead.

Pi Day is celebrated in a variety of ways. Parties or other observances may be held by mathematics departments in educational institutions. Math or science clubs might gather to consider the role that the number π has played in their lives and to imagine the world without π. During such an event, pi celebrants may devise alternative values for π, eat pie, play piñata, drink Piña Colada, eat pineapple, listen to the song "Pi" by Kate Bush, or watch Pi. The shape of the pie is sometimes square, due to the pronunciation of the equation for the surface area bounded by a circle = <math>\pi r^2</math>, i.e., "pie are squared."

Enthusiasts also note that the day happens to be Albert Einstein's birthday, in addition to other birthdays on this day. The renowned science and technology university Massachusetts Institute of Technology, known for its sometimes unconventional and quirky take on math, often mails out its acceptance letters to be delivered to prospective students on Pi Day.

The "ultimate" pi moment<ref>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.</ref> occurred on March 14, 1592, at 6:53 AM and 58 seconds. When written in American-style date format, this is 3/14/1592 6:53:58, which corresponds to the value of pi to twelve digits: 3.14159265358. 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 went unnoticed<ref>In order to have noticed this "ultimate" pi moment on the day it occurred one would have needed access to all of the following: the Gregorian (or comparable) calendar, the American-style notation of date/time, the ability to calculate π with enough digits, and modern decimal fraction notation (that is, e.g. without the intermittent numbers of the Simon Stevin system which was invented in 1586).</ref>.

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Pi Approximation Day

Pi Approximation Day may be marked on any of these days:

July 22, which is written as 22/7 in international (little-endian) date format; 22 divided by 7 is an approximation of π.
April 26 (April 25 in a leap year), 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).
July 1 (June 30 in a leap year), the 182nd day of the year, where the earth has completed exactly 1/2 of its orbital period, or π radians.
December 21 (December 20 in a leap year), the 355th day of the year, at 1:13 p.m., based on the approximation value of 355/113 derived by Zu Chongzhi.
November 10 (November 9 in a leap year), which is the 314th day of the year on the Gregorian calendar.