Babylonian Trig

Evelyn Lamb in Scientific American:

Don’t Fall for Babylonian Trigonometry Hype

Separating fact from speculation in math history

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? In a 1980 paper titled “Sherlock Holmes in Babylon,” R. Creighton Buck implied that through mathematics and cunning observation, one could sleuth out the meaning of the tablet and offered an explanation he thought fit the data. But Eleanor Robson, in “Neither Sherlock Holmes nor Babylon,” writes, “Ancient mathematical texts and artefacts, if we are to understand them fully, must be viewed in the light of their mathematico-historical context, and not treated as artificial, self-contained creations in the style of detective stories.” It’s arrogant and will probably lead to incorrect conclusions to look at ancient artifacts primarily through the lens of our modern understanding of mathematics.

What did it do?

There are a few theories about how Plimpton 322 was created and used by the person or people who made it. Mansfield and Wildberger are not the first to believe it’s some sort of trig table. On the other hand, some believe it links the Pythagorean theorem (known by these ancient Mesopotamians and many other civilizations long before Pythagoras) with the method of completing the square to solve a quadratic equation, a common problem in mathematical texts from that time and place. Some believe the triples were generated using different numbers not included in the table in a “number theoretic” way. Some believe the numbers came from so-called reciprocal pairs that were used for multiplication. Some think the tablet was a pedagogical tool, perhaps a source of exercises for students. Some believe it was used in something more like original mathematical research. Academic but readable information about these interpretations can be found in articles by Buck in 1980, Robson in 2001 and 2002, and John P. Britton, Christine Proust, and Steve Shnider from 2011.

If it is a trigonometry table, is it better than modern trigonometry tables?

Mansfield and Wildberger’s contribution to scholarship on Plimpton 322 seems to be speculation that the artifact could be used to do trigonometry in a more exact way than we do now. In a publicity video by UNSW that must have accompanied the press releases sent to many math and science journalists (but not to me—what gives, UNSW?), Mansfield makes the claims that this table is “superior in some ways to modern trigonometry” and the “only completely accurate trigonometry table.”

It’s hard to know where to start with this part of their claims. For one, the tablet contains some well-known errors, so claims that it is the most accurate or exact trig table ever are just not true. But even a corrected version of Plimpton 322 would not be a revolutionary replacement for modern trig tables….

Is base 60 better than base 10?

Perhaps the utility of different types of trig tables is a matter of opinion, but the UNSW video also has some outright falsehoods about accuracy in base 60 versus the base 10 system we now use. Around the 1:10 mark, Mansfield says, “We count in base 10, which only has two exact fractions: 1/2, which is 0.5, and 1/5.” My first objection is that any fraction is exact. The number 1/3 is precisely 1/3. Mansfield makes it clear that what he means by 1/3 not being an exact fraction is that it has an infinite (0.333…) rather than a terminating decimal. But what about 1/4? That’s 0.25, which terminates, and yet Mansfield doesn’t consider it an exact fraction. And what about 1/10 or 2/5? Those can be written 0.1 and 0.4, which seem pretty exact.

Indefensibly, when he lauds the many “exact fractions” available in base 60, he doesn’t apply the same standards. In base 60, 1/8 would be written 7/60+30/3600 which is the same idea as writing 0.25, or 2/10+5/100, for 1/4 in base 10. Why is 1/8 exact in base 60 but 1/4 not exact in base 10? It’s hard to believe this is an honest mistake coming from a mathematician and instead makes me even more suspicious that his work is motivated by an agenda.

Plimpton 322 is a remarkable artifact, and we have much to learn from it. When I taught math history, I loved opening the semester by having my students read a few papers about it to show how much scholarship has gone into understanding such a small document and how accomplished scholars can disagree about what it means. It demonstrates differences in the way different cultures have done mathematics and outstanding computational facility. It has raised questions about how ancient Mesopotamians approached calculation and geometry. But using it to sell a questionable pet theory won’t get us any closer to the answers.

Astronomical Geometry

From the National Post:

Clay tablets reveal Babylonians invented astronomical geometry 1,400 years before Europeans

The medieval mathematicians of Oxford, toiling in torchlight in a land ravaged by plague, managed to invent a simple form of calculus that could be used to track the motion of heavenly bodies. But now a scholar studying ancient clay tablets suggests that the Babylonians got there first, and by at least 1,400 years.

The astronomers of Babylonia, scratching tiny marks in soft clay, used surprisingly sophisticated geometry to calculate the orbit of what they called the White Star — the planet Jupiter.

These tablets are quite incomprehensible to the untrained eye. Thousands of clay tablets — many unearthed in the 19th century by adventurers hoping to build museum collections in Europe, the United States and elsewhere — remain undeciphered.

But they are fertile ground for Mathieu Ossendrijver of Humboldt University in Berlin, whose remarkable findings were published Thursday in the journal Science.Ossendrijver is an astrophysicist who became an expert in the history of ancient science.

For a number of years he has puzzled over four particular Babylonian tablets housed in the British Museum in London.

“I couldn’t understand what they were about. I couldn’t understand anything about them, neither did anyone else. I could only see that they dealt with geometrical stuff,” he said this week in a phone interview from Germany.

Then one day in late 2014, a retired archaeologist gave him some black-and-white photographs of tablets stored at the museum. Ossendrijver took notice of one of them, just two inches across and two inches high. This rounded object, which he scrutinized in person in September 2015, proved to be a kind of Rosetta Stone.

Officially named BH40054 by the museum, and dubbed Text A by Ossendrijver, the little tablet had markings that served as a kind of abbreviation of a longer calculation that looked familiar to him. By comparing Text A to the four previously mysterious tablets, he was able to decode what was going on: This was all about Jupiter. The five tablets computed the predictable motion of Jupiter relative to the other planets and the distant stars.

“This tablet contains numbers and computations, additions, divisions, multiplications. It doesn’t actually mention Jupiter. It’s a highly abbreviated version of a more complete computation that I already knew from five, six, seven other tablets,” he said.

Most strikingly, the methodology for those computations used techniques that resembled the astronomical geometry developed in the 14th century at Oxford. The tablets have been authoritatively dated to a period from 350 B.C. to 50 B.C.

The people of Mesopotamia — what is now Iraq — developed mathematics about 5,000 years ago. Among them were the Babylonians who wrote in cunieform script and, over time, adopted a sexagesimal (base 60) numbering system. Early mathematics was essentially a form of counting, and the things being counted were mostly sheep and the like.

Mathematics progressed, as did the sharing of knowledge in the wake of Alexander the Great’s conquering journeys across Asia. The ancient Greek astronomer Aristarchus of Samos argued for a heliocentric universe — one in which the Earth orbited the sun, contrary to what seems to be the case when one looks at the sky. That view was shared by another astronomer, possibly Greek as well, who lived in Mesopotamia on the Tigris River and was known as Seleucus of Seleucia.

But Ossendrijver said nothing in the newly decoded computations suggests that the ancient scientist or scientists who etched the tablets understood that heliocentric model. The calculations merely describe Jupiter’s motion over time as it appears to speed up and slow down in its journey across the night sky. Those calculations are done in a surprisingly abstract way — the same way the Oxford mathematicians would do them a millennium and a half later.

“It’s geometry, which is itself old, but it’s applied in a completely new way, not to fields, or something that lives in real space, but to something that exists in completely abstract space,” Ossendrijver said. “Anybody who studies physics would be reminded of integral calculus.”

Which was invented in Europe in 1350, according to historians.

“In Babylonia, between 350 and 50 B.C., scholars, or maybe one very clever guy, came up with the idea of drawing graphs of the velocity of a planet against time, and computing the area of this graph — of doing a kind of computation that seems to be thoroughly modern, that is not found until 1350,” he said.

Alexander Jones, a professor at New York University’s Institute for the Study of the Ancient World, praised Ossendrijver’s research, which he said shows the “revolutionary brilliance of the unknown Mesopotamian scholars who constructed Babylonian mathematical astronomy during the second half of the first millennium BC.”