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| Photo: Evelyn J Lamb |
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| Plimpton 322, an ancient Mesopotamian math tablet, is a fascinating document, but it's not going to revolutionize trigonometry. Credit: Public Domain |
You may have seen headlines about an ancient Mesopotamian tablet. “Mathematical secrets of ancient tablet unlocked after nearly a century of study,” said the Guardian. “This mysterious ancient tablet could teach us a thing or two about math,” said Popular Science, adding, “Some researchers say the Babylonians invented trigonometry—and did it better.” National Geographic was a bit more circumspect: “A new study claims the tablet could be one of the oldest contributions to the study of trigonometry, but some remain skeptical.” Daniel Mansfield and Norman Wildberger certainly did a good job selling their new paper in the generally more staid journal Historia Mathematica. I’d like to help separate fact from speculation and outright nonsense when it comes to this new paper.
What is Plimpton 322?
Plimpton 322, the tablet in question, is certainly an alluring artifact. It’s a broken piece of clay roughly the size of a postcard. It was filled with four columns of cuneiform numbers around 1800 BCE, probably in the ancient city of Larsa (now in Iraq) and was removed in the 1920s. George Plimpton bought it in 1922 and bequeathed it to Columbia University, which has owned it since 1936. Since then, many scholars have studied Plimpton 322, so any picture you might have of Mansfield and Wildberger on their hands and knees in a hot, dusty archaeological site, or even rummaging through musty, neglected archives and unearthing this treasure is inaccurate. We’ve known about the artifact and what was on it for decades. The researchers claim to have a new interpretation of how the artifact was used, but I am skeptical.
Scholars have known since the 1940s that Plimpton 322 contains numbers involved in Pythagorean triples, that is, integer solutions to the equation a2+b2=c2. For example, 3-4-5 is a Pythagorean triple because 32+42=9+16=25=52. August 15 of this year was celebrated by some as “Pythagorean Triple Day” because 8-15-17 is another, slightly sexier, such triple.
The far right column consists of the numbers 1 through 15, so it’s just an enumeration. The two middle columns of Plimpton 322 contain one side and the hypotenuse of a Pythagorean triangle, or a and c in the equation a2+b2=c2. (Note that a and b are interchangeable.) But these are a little brawnier than the Pythagorean triples you learn in school. The first entries are 119 and 169, corresponding to the Pythagorean triple 1192+1202=1692. The far left column is a ratio of squares of the sides of the triangles. Exactly which sides depends slightly on what is contained in the missing shard from the left side of the artifact, but it doesn’t make a huge difference. It’s either the square of the hypotenuse divided by the square of the remaining leg or the square of one leg divided by the square of the other leg. In modern mathematical jargon, these are squares of either the tangent or the secant of an angle in the triangle.
We can interpret one of the columns as containing trigonometric functions, so in some sense, it is a trig table. But despite what the headlines would have you believe, people have known that for decades. The mystery is what purpose the tablet served in its time. Why was it created? Why were those particular triangles included in the table? How were the columns computed?
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Additional resources
Hints of Trigonometry on a 3,700-Year-Old Babylonian Tablet | New York Times - Science
Source: Scientific American (blog)
