*This article has been updated from the original post, which appeared on March 14, 2013.*

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March 14, when written as 3/14, represents the first three digits of pi (π), the ratio of the circumference of a circle to its diameter. To commemorate the world's most famous mathematical constant, enthusiasts around the world embrace their inner nerdiness by celebrating Pi Day. The date—which also happens to be Einstein's birthday—inspires a variety of events every year. This weekend will feature the ultimate Pi Day, as adding the year to our date notation, 3/14/15, encompasses even more digits in the sequence. We won't get this much pi again for 100 years.

Just why are people crazy about pi? The number—3 followed by a ceaseless string of random numbers after the decimal point—is irrational, meaning that it cannot be expressed through the division of two whole numbers. It is also a transcendental number, which means that it isn't the root of any algebraic number. This irrational and transcendental nature appeals to people, perhaps because pi's continuous flow of digits reflects the unending circle it helps to trace.

Pi has held an almost mystical quality to humans throughout time. Its unspoken presence can be felt in the circular ruins of Stonehenge, in the vaulted ceilings of domed Roman temples and in the celestial spheres of Plato and Ptolemy. It has inspired centuries of mathematical puzzles and some of humanity's most iconic artwork. People spend years of their lives attempting to memorize its digits, and hold contests to see who knows the most numbers after the decimal. Some write "piaku"—poems in which the number of letters in each word represents subsequent digits of pi. Still others create complex works of art inspired by the randomness of pi. The list goes on and on, like pi itself.

Here are some notable moments in the history of pi:

**1900-1650 B.C. **

Although the term pi wasn't yet in use, a Babylonian tablet gave a value for the ratio of the circumference of a circle to its diameter of 3.125, which isn't bad! In another document, the Rhind Papyrus, an Egyptian scribe writes: "Cut off 1/9 of a diameter and construct a square upon the remainder; this has the same area as the circle." This implies that pi is 3.16049, which is also fairly accurate, according to David Wilson of Rutgers University's math department.

**800-200 B.C. **

Passages in the Bible describe a ceremonial pool built in in the Temple of Solomon: "He made the Sea of cast metal, circular in shape, measuring ten cubits from rim to rim and five cubits high. It took a line of thirty cubits to measure around it." (I Kings 7:23-26.) This puts pi at a mere 3.

**250 B.C.**

Archimedes of Syracuse approximated the value of pi by finding the areas of two shapes—a 96-sided polygon inscribed within a circle and one drawn outside it. The areas of the polygons sandwiched the area of the circle, giving Archimedes upper and lower bounds for the coveted ratio. Though he knew he had not found the exact value of pi, he was able to set it between 3.1408 and 3.1429.

**Late 1300s **

Indian mathematician and astronomer Madhava of Sangamagrama was the first on record to posit that pi could be represented as the sum of terms in an infinite sequence—for example, 4 - 4/3 + 4/5 - 4/7 + 4/9 – 4/11 … ∞. His efforts yielded a value for pi that was correct to 13 decimal places, and he helped lay some of the groundwork for the development of calculus.

**1706 **

Welsh mathematician William Jones began to use π as the symbol for the ratio of the circumference of a circle to its diameter. Famed Swiss mathematician Leonhard Euler adopted this usage in 1737, helping to popularize it through his works.

**1873**

Amateur English mathematician William Shanks calculates pi to 707 digits. His number was written on the wall of a circular room—appropriately named the Pi Room—in the Palais de la Découverte, a French science museum. But his number was only correct to the 527th digit. The error was finally caught in 1946 and corrected on the wall in 1949.

**1897 **

Lawmakers in Indiana almost pass a bill that erroneously changes the value of pi to a solid 3.2. Cajoled by amateur mathematician Edwin Goodwin, the Indiana General Assembly introduced House Bill 246, which offered up "a new mathematical truth" for free use by the state. The purported truth was Goodwin's attempt to square the circle—a puzzle which requires that a circle and square of the same area be constructed using only a geometrical compass and a straightedge. The bill unanimously passed the House, but the Senate—and hence the state—was spared from embarrassment by C.A. Waldo, a Purdue mathematics professor who happened to be in the State House that day. "Shown the bill and offered an introduction to the genius whose theory it was, Waldo declined, saying he already knew enough crazy people," Tony Long of *Wired* wrote. Waldo gave the senators a math lesson, and the bill died.

**1988 **

Larry Shaw of San Francisco's Exploratorium introduces the first Pi Day celebration.

**2005 **

Chao Lu, then a graduate student in China, becomes the Guinness record holder for reciting pi—he recited the number to 67,980 digits in 24 hours and 4 minutes (contest rules required that no more than 15 seconds could pass between any two numbers).

**2009 **

Pi Day becomes a national event! Democratic Congressman Bart Gordon of Tennessee, along with 15 co-sponsors, introduced HR 224, which "supports the designation of a Pi Day and its celebration around the world; recognizes the continuing importance of National Science Foundation math and science education programs; and encourages schools and educators to observe the day with appropriate activities that teach students about Pi and engage them about the study of mathematics." The resolution was approved by the House of Representatives on March 12 of that year, proving that a love of pi is non-partisan.

How are you celebrating Pi Day?

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