**Pi has been known for almost 4000 years—but even if we calculated the number of seconds in those 4000 years and calculated pi to that number of places, we would still only be approximating its actual value. Here’s a brief history of finding pi: **

The ancient Babylonians calculated the area of a circle by taking 3 times the square of its radius, which gave a value of pi = 3. One Babylonian tablet (ca. 1900–1680 BC) indicates a value of 3.125 for pi, which is a closer approximation.

In the Egyptian Rhind Papyrus (ca.1650 BC), there is evidence that the Egyptians calculated the area of a circle by a formula that gave the approximate value of 3.1605 for pi.

The ancient cultures mentioned above found their approximations by measurement. The first calculation of pi was done by Archimedes of Syracuse (287–212 BC), one of the greatest mathematicians of the ancient world. Archimedes approximated the area of a circle by using the Pythagorean Theorem to find the areas of two regular polygons: the polygon inscribed within the circle and the polygon within which the circle was circumscribed. Since the actual area of the circle lies between the areas of the inscribed and circumscribed polygons, the areas of the polygons gave upper and lower bounds for the area of the circle. Archimedes knew that he had not found the value of pi but only an approximation within those limits. In this way, Archimedes showed that pi is between 3 1/7 and 3 10/71.

A similar approach was used by Zu Chongzhi (429–501), a brilliant Chinese mathematician and astronomer. Zu Chongzhi would not have been familiar with Archimedes’ method—but because his book has been lost, little is known of his work. He calculated the value of the ratio of the circumference of a circle to its diameter to be 355/113. To compute this accuracy for pi, he must have started with an inscribed regular 24,576-gon and performed lengthy calculations involving hundreds of square roots carried out to 9 decimal places.

Mathematicians began using the Greek letter π in the 1700s. Introduced by William Jones in 1706, use of the symbol was popularized by Euler, who adopted it in 1737.

An 18th century French mathematician named Georges Buffon devised a way to calculate pi based on probability. You can try it yourself at the Exploratorium exhibit Throwing Pi.

## A Slice of Pi (π) Day History

*How a gathering at San Francisco’s Exploratorium became a national holiday?*

March 14 is Pi (π) Day, the annual celebration of a never-ending number—and Albert Einstein’s birthday. How did pi inspire a national holiday and an international celebration thousands of years after its discovery? It all started at the Exploratorium with former staff physicist, tinkerer, and media specialist Larry Shaw.

In 1988, three years after the death of Exploratorium Founder Frank Oppenheimer, staff gathered at a retreat in Monterey, California, to soul search and brainstorm. It was there that Shaw linked March 14 (3.14) with the digits of pi (3.14159…), seeing it as an extraordinary opportunity to bring Exploratorium staff together. And π Day was born.

On the first π Day, at 1:59—the π numbers that follow 3.14—Larry and his wife, Catherine, set up a table on the museum’s floor topped with fruit pies and a tea urn for the celebration.

A few years later, Larry’s daughter, Sara, discovered that π Day was also Einstein‘s birthday (b. 1879) so a celebration of his life was added to the π Day festivities.

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Larry created and installed the “Pi Shrine,” a circular brass plaque, in the center of a circular classroom constructed of circular cinderblocks. He led a winding parade around the museum with his boombox blaring the digits of π to the music of “Pomp and Circumstance.” The parade ending by circumnavigating the Pi Shrine 3.14 times while singing “Happy Birthday” to Albert Einstein.

π Day became an annual Exploratorium tradition for staff and the public, and the idea snowballed into something much bigger. Now it’s celebrated by math lovers and educators worldwide. In March 2009, π Day became an official U.S. national holiday.

Until his passing in 2017, Shaw, affectionately known as the Prince of π, led π Day parades yearly at the Exploratorium. Today, the Exploratorium continues the tradition by hosting an annual π Day, a public celebration that features a π-themed activities, antics, rituals, and—and, of course, plenty of pie.

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