From “Do Penguins Have Knees?” by David Feldman

Why Was "pi" Chosen as the Greek Letter to Signify the Ratio of a Circle's Circumference to Its Diameter?

The history of "pi" is so complex and fascinating that whole books have been written about the subject. Still, if you want to make a long story short, it can be boiled down to the explanation provided by Roger Pinkham, a mathematician at the Stevens Institute of Technology in Hoboken, New Jersey:

There are two Greek words for perimeter, peri metros and periphireia. The circumference of a circle is its perimeter, and the first letter of the Greek prefix peri (meaning "round") used in those two words was chosen.

Mathematicians attempted to calculate the pi ratio before the birth of Christ. But it wasn't the ancient Egyptians or Greek mathematicians who first coined the term. San Antonio math teacher John Veltman sent us documentation indicating that although pi was earlier applied as an abbreviation of "periphery," the first time it was found in print to express the circle ratio was in 1706, when an English writer named William Jones, best known as a translator of Isaac Newton, published A New Introduction to Mathematics. "Pi" did not enjoy widespread acceptance until 1737, when Jones's term was popularized by the great Swiss mathematician Leonhard Euler.

So what did the ancient Greeks call the ratio? They probably did not have a handy abbreviation. Veltman explains:

It appears that even the Greek mathematicians themselves did not use their letter pi to represent the circle ratio. Several ancient cultures did their math in sentence form with little or no abbreviation or symbolism. It is amazing how much they achieved without such an aid. Archimedes, born about 287 B.C., is said to have determined that the value of pi was between 310/71 and 31/7 (or in our decimal notation, between 3.14084 and 3.142858).

In the centuries that followed, mathematicians in other countries produced even more precise calculations: Around A.D. 150, Ptolemy of Alexandria weighed in with his value of 3.1416. Around A.D. 480, a Chinese man, Tsu Ch'ung-chih, improved the figure to 3.1415929, correct to the first six numbers after the decimal point.

Why is our mathematical vocabulary a seemingly random hodgepodge of Greek and Latin terminology? Why do we say the Latin-derived "circumference" (originally meaning "to run or move around") rather than the Greek "periphery"? Diane McCulloch, a mathematician at the Mount de Chantal Academy in Wheeling, West Virginia, explains:

The Greeks were more avid mathematicians than the Romans, who preferred the practical uses and didn't have much time for the analytical aspect of mathematics. Thank goodness the writings of the Greeks were preserved by the Islamic scholars. We have access to the ancient Greek mathematical work because these Islamic scholars established libraries in Spain; when they were eventually driven out of Spain, their books were translated into Latin and then into other European languages, which themselves tended to be derived from Latin.

Therefore, we use the word "triangle" when the Greeks would probably have used the word "trigon," as in "polygon," "octagon," etc.

Submitted by Dennis Kingsley of Goodrich, Michigan.