Pi Day

Pi Pie at Delft University

Pies for a celebration at the Massachusetts Institute of Technology

Free pie being prepared at the University of Waterloo

Larry Shaw, the founder of Pi Day, at the Exploratorium

Pi Day and Pi Approximation Day are two holidays held to celebrate the mathematical constant $\pi$ (pi) (in the mm/dd date notation: 3/14); since 3, 1 and 4 are the first three digits of π. Pi Approximation Day is observed on July 22, because of Archimedes' popular approximation of π being 22/7. However, this may be considered misleading, as all cited dates are "approximation days" (as π is an irrational number) and 22/7 is actually a closer approximation of π than 3.14. Typically, March 14 is more popular for countries using the month/day format (22/7 being an impossible date in this format), and the 22nd of July is more popular for countries using the day/month format (since 3/14 and 31/4 are impossible dates in this format).

Sometimes Pi Minute is also celebrated; this occurs on March 14 at 1:59 p.m. If π is truncated to seven decimal places, it becomes 3.1415926; making March 14 at 1:59:26 p.m., Pi Second (or sometimes March 14, 1592 at 6:53:58 a.m.). An upcoming Pi Day in 2015 will have the date 3-14-15 and a Pi Second of 9:26:53.589... a.m.

There are a large variety of ways of celebrating Pi Day and most of them include eating pie and discussing the relevance of π. The first Pi Day celebration was held at the San Francisco Exploratorium in 1988, with staff and public marching around one of its circular spaces, then consuming fruit pies. The museum has since added pizza pies to its Pi Day menu. The founder of Pi Day was Larry Shaw, a now-retired physicist at the Exploratorium who still helps out with the celebrations.

The Massachusetts Institute of Technology often mails its acceptance letters to be delivered to prospective students on Pi Day.

Some also celebrate alternative Pi Days and/or Pi Approximation Days in addition to the two listed above; these can fall on any of several dates:

• March 4: When 14% of the 3rd month has elapsed.
• April 26: The Earth has traveled two radians of its orbit on this day (April 25 in leap years), reckoning from the start of the civil year on January 1. Thus the entire orbit divided by the distance traveled equals π; two radians equals 1⁄π of our orbit. This is celebrated exactly on the 41^st second of the 23^rd minute of the 4^th hour on April 26 or the 116^th day. (In leap years, it is celebrated exactly on the 3^rd second of the 2^nd minute of the 12^th hour on April 25 or the 116^th day.) This celebration is not a Pi Approximation Day; many, mathematicians included, celebrate April 26 as Pi Day and say that it should be the official Pi Day.
• November 10: The 314th day of the year (November 9 in leap years).
• December 21, 1:13 p.m.: The 355th day of the year (December 20 in leap years), celebrated at 1:13 for the Chinese approximation 355/113.

On Pi Day 2004, Daniel Tammet recited 22,514 decimal digits of π.

The U.S. House of Representatives passed a non-binding resolution (HRES 224), recognizing March 14, 2009 as National Pi Day.

See also

• Square Root Day
• Mole Day

External links

• Pi Day in Maine with Dr. Wilson's Memory Elixir

• The Skeleton Show celebrates Pi Day

• Celebrate Pi Day by memorizing the digits of pi