Many Americans write dates in numerical form, with the first digits for the month, the second for the day of the month, and the third for the year. The notation 3/14/15, for example, represents March 14, 2015. When dates are written in this manner, the digits can be compared to those of the irrational number π (spelled in English "pi"), the ratio of the circumference to the diameter of a circle. This number is roughly 3, more precisely 3.14, and more precisely still 3.1415.
The digits of π were known to a few dozen places by the 1600s. Mathematical discoveries of that time made it possible to represent π by the sum of an infinite number of terms, each one smaller than the previous one. This led to more precise calculations. In the mid-20th century, calculating machines and then electronic computers, combined with new mathematical methods, prompted further computations. For example, in 1961 Daniel Shanks and John W. Wrench Jr. used an IBM 7090 computer to compute π to 100,000 places. A copy of their printout is in the Smithsonian collections.
Further calculations now extend known values of pi to trillions of digits. It also is possible to calculate single digits of pi arbitrarily far in its expansion, if one carries out the computations in base 16, rather than the more familiar base 10.
Scientists and engineers have also used pi in their calculations. Around 1900, slide rule owner Charles C. Brush of Philadelphia wrote in the number on an instrument he owned.
Slide rule manufacturers adopted Brush's example later in the century. In the 1970s, when the handheld electronic calculator displaced the slide rule, scientific versions of the machine generally had a key for π.
Since at least 1988, some mathematics educators in the United States have celebrated March 14 as Pi Day. Festivities were especially widespread in 2015, because 3.1415 includes the digits of that year as well as the day and month. At the hour of 9:26, one might even consider the approximation to include three more digits of π, being 3.1415926. The noisemaker shown in the image was distributed at a meeting on the history and pedagogy of mathematics held in Washington, D.C., on March 14, 2015. It was sounded at 9:26 a.m.
Pi Day is not an international holiday. Most of the rest of the world writes dates in the form day/month/year. Under this scheme, the first three digits of Pi Day would be 31/4, representing April 31; or 3/14, representing the third day of the fourteenth month. However, there is no April 31 or 14th month under the present calendar scheme. One also might have 3/1/4, representing January 3 in, say, 2004, or 3/1/41 for a date in say, 2041. Each of these dates occurs once a century—not too convenient for a celebration!
Peggy Aldrich Kidwell is curator of mathematics. She has also blogged about the math objects you may be able to spot in the movie Hidden Figures.
