In this series, we present a collection of curricular units based on primary historical sources and designed to serve students as an introduction to the study of trigonometry. Each unit may be incorporated, either individually or in various combinations, into a standard course in College Algebra with Trigonometry, a stand-alone Trigonometry course, or a Precalculus course. These lessons have also been used in courses on the history of mathematics and as part of a capstone experience for pre-service secondary mathematics teachers.

Trigonometry is concerned with measurements of angles about a central point (or of arcs of circles centered at that point) and quantities, geometrical and otherwise, that depend on the sizes of such angles (or the lengths of the corresponding arcs). It is one of those subjects that has become a standard part of the toolbox of every scientist and applied mathematician. Why is it so valuable?

There is a key geometrical feature of the measurement of angles, or arcs that are traced out, about a point in the plane: as we might expect, the sizes of such angles (or the lengths of such arcs) grow as one end of the arc moves counterclockwise around its circle (while the other remains fixed); but when the moving point returns to the place of the fixed point after making a full turn around the circle, it continues to retrace the same path through another turn, and another and another as the angle/arc grows ever-larger. In other words, *periodic behavior* is at the core of the relationships between angles and arcs and the measurements we associate with them.

**Figure 1. **Diagram from 14th-century manuscript copy of Ptolemy's *Almagest*, folio 22 recto.

Manuscript owned and digitized by gallica.bnf.fr / Bibliothèque nationale de France.

As a consequence, wherever the mathematical description of cyclical phenomena is needed, trigonometric functions make appearances in pure mathematics and in applications of mathematics to the sciences. An introduction to trigonometry is a staple of the mathematics curriculum in high schools and colleges, many of whose students later study calculus and other forms of mathematical analysis in which periodic phenomena are explored.

It is the goal of the curricular units presented in this series to impart to students some of the story of where and how the central ideas of this subject first emerged, in an attempt to provide context for their study of this mathematical theory. Students who work through the entire collection of units will encounter six milestones in the history of the development of trigonometry. Our journey will span a vast interval of time, from some unidentifiable moment dating as far back as around 1000 BCE, to roughly the year 1500 CE. We will consider developments of the subject that took place in many different parts of the world: in ancient Mesopotamia; in Hellenistic Greece and Roman-era Egypt; in medieval India; in central Asia during the height of Islamic science; and in Renaissance Europe. The term “milestones'' is quite appropriate here, since we are only touching on a few moments in a long and complex history, one that brings us just to the edge of the modern scientific era in which we now live.

More specifically, each unit looks at one of the following episodes in the development of the mathematical science of trigonometry:

- the emergence of sexagesimal numeration in ancient Babylonian culture, developed in the service of a nascent science of astronomy;
- a modern reconstruction (as laid out in [Van Brummelen, 2009]) of a lost table of chords known to have been compiled by the Greek mathematician-astronomer Hipparchus of Rhodes (second century, BCE);
- a brief selection from Claudius Ptolemy's
*Almagest*(second century, CE) [Toomer, 1998] in which the author (Ptolemy) shows how a table of chords can be used to monitor the motion of the Sun in the daytime sky for the purpose of telling the time of day; - a few lines of Vedic verse by the Hindu scholar Varāhamihira (sixth century, CE) [Neugebauer and Pingree, 1970/1972] containing the "recipe" for a table of sines as well as some of the methods used for its construction;
- passages from
*The Exhaustive Treatise on Shadows*[Kennedy, 1976], written in Arabic in the year 1021 by Abū Rayḥān Muḥammad ibn Aḥmad al-Bīrūnī, which include precursors to the modern trigonometric tangent, cotangent, secant and cosecant; - excerpts from Regiomontanus'
*On Triangles*(1464) [Hughes, 1967], the first systematic work on trigonometry published in the West.

**Figure 2. **Woodcut showing Regiomontanus* *with an astrolabe, from the Nuremberg Chronicle by Hartmann Schedel (1493), folio 255 recto. Wikimedia image from copy owned by University of São Paulo, believed to be in the public domain.

Readers who want to learn more about the history of trigonometry are recommended to consult Glen van Brummelen's masterful *The Mathematics of the Heavens and the Earth: The early history of trigonometry* [Van Brummelen, 2009] from which much of this work took inspiration.

The onscreen introduction for each individual unit to appear in this series will provide additional historical detail about the episode in question, a brief instructor guide specific to the goals and implementation of that particular unit, and a downloadable pdf of the unit itself that can be shared with students (after removing the Notes to Instructors section). Readers who want to see the entire Primary Source Project (PSP) from which these units are drawn can obtain that PSP, *A Genetic Context for Understanding the Trigonometric Functions*, without waiting for future installments to appear, from the website of the NSF-funded project TRansforming Instruction in Undergraduate Mathematics via Primary Historical Sources (TRIUMPHS). LaTeX source code of the entire PSP and of each individual unit is also available directly from the author by request. The concluding chapter of this series will describe the various paths that instructors have followed in their implementation of these materials in the range of courses described in the opening paragraph of this introduction, and it will provide comments from instructors and students about their experiences on those journeys.

Our series of curricular units based on primary sources begins near the start of recorded history, the early centuries of the second millennium BCE in Old Babylonia. (A link to a pdf copy of the curricular unit itself appears toward the bottom of this page.) The term ‘Babylonia’ is used by historians and archaeologists to refer to that region of the world which is now part of present-day Iraq and whose most prominent cultural center was the ancient city of Babylon. The map pictured below shows modern cities and country boundaries superimposed on locations of the most important ancient Mesopotamian sites, including those of the Old Babylonian period. These are concentrated in Lower Mesopotamia, where the Tigris and Euphrates Rivers empty into the northern end of the Persian Gulf. (The map shows modern place names in roman type with cities marked as white dots, whereas ancient place names—all just ruins today—are in italic type with sites marked as a triangle of dots.)

**Figure 1.** Key sites of ancient Mesopotamia superimposed on a modern map.

Map created by Wikigraphist Goran tek-en, 28 January 2014, and licensed under the Creative Commons Attribution-Share Alike 4.0 International license.

In the Old Babylonian era, the earliest known systematic astronomical records were made. Some of these are recorded in a collection of clay tablets known today as the *Enuma Anu Enlil*. One such tablet fragment is pictured here:

**Figure 2.**** **Cuneiform tablet containing commentary on *Enuma Anu Enlil*.

Metropolitan Museum of Art, donated to Wikimedia Commons and made available under the Creative Commons CC0 1.0 Universal Public Domain Dedication license.

Exercising the natural tendencies of human beings to notice and make use of patterns they observe in the world around them, early astronomers recognized the circular motions of the stars and planets in the sky and began to use numbers to describe these motions. The curricular unit *Babylonian Astronomy and Sexagesimal Numeration*, the first episode in our walk through the history of trigonometry, describes how Old Babylonian astronomers gave us the sexagesimal system of measurement of angles and circular arcs now known as degree measure. Students are introduced to sexagesimal numeration through its similarities with the decimal numeration with which they are already familiar. They also learn how this vestige of ancient history continues to ring through modern life via its presence in angle measure, in the geometry of the circle, and in the measure of time.

The unit *Babylonian Astronomy and Sexagesimal Numeration *(pdf)* *is ready for student use. It is meant to be completed in one 75-minute classroom period, plus time in advance for students to do some initial reading and time afterwards for them to write up their solutions to the tasks. This particular unit is unusual in that it does not include any primary source text. Instructors who wish to expose students to a primary source connected with the sexagesimal numeration system are encouraged to consider using the Primary Source Project *Babylonian Numeration *by Dominic Klyve, also from the TRIUMPHS collection. A brief set of instructor notes offering additional background and practical advice for the use of these materials in the classroom is appended at the end of the student version of these projects.

This unit is the first in the *Convergence* series *Teaching and Learning the Trigonometric Functions through Their Origins*. Although these classroom projects are posted here as parts of a series, each episode can also stand alone. Readers who want to see the entire Primary Source Project (PSP) from which these units are drawn can obtain that PSP, *A Genetic Context for Understanding the Trigonometric Functions*, without waiting for future installments to appear, from the website of the NSF-funded project TRansforming Instruction in Undergraduate Mathematics via Primary Historical Sources (TRIUMPHS). The LaTeX source code of all TRIUMPHS projects, including the units to appear in this series, are available from the project authors by request.

Millions of students worldwide learn trigonometry every year as a rite of passage in science, technology, engineering, and mathematics (STEM) education. They pass through a precalculus course, or even a course specifically focused on trigonometry, that is typically wedged between their introductions to the more fundamental skills in algebra and the more advanced training they will receive in differential and integral calculus. These experiences in the study of mathematics are generally directed to provide students with a working knowledge of the algebraic, geometric and analytic fundamentals that have become associated with the six trigonometric functions. Given the ever-escalating pressures to accelerate student training in these subjects, motivation for the study of trigonometry often receives short shrift. And when students present the natural question “why?”, the responses are meager and unprepared: “you’ll see when you study X later.”

So, who decided that degrees would be divided into sixties rather than tenths, as is everything else in scientific measurement? Is trig about triangles, or circles? What’s the deal with periodicity? Why is the sine function so important?

The classroom unit In this unit, students are introduced to the basic elements of the geometry of the circle and the measure of its arcs, central angles and chords, whose interrelationships formed the foundation for trigonometry as a tool for Greek astronomy. Students read a brief excerpt from Claudius Ptolemy's |
Figure 1. Hipparchus of Rhodes,Convergence Portrait Gallery. |

Figure 2. Modern Reconstruction ofHipparchus' Table of Chords. |
The unit As mentioned above, the unit |

The development of the student projects presented in this series has been partially supported by the TRansforming Instruction in Undergraduate Mathematics via Primary Historical Sources (TRIUMPHS) project with funding from the National Science Foundation’s Improving Undergraduate STEM Education Program under Grants No. 1523494, 1523561, 1523747, 1523753, 1523898, 1524065, and 1524098. Any opinions, findings, and conclusions or recommendations expressed in this project are those of the author and do not necessarily reflect the views of the National Science Foundation.

Daniel E. Otero (Danny, to mostly everyone) is Associate Professor of Mathematics at Xavier University in Cincinnati, Ohio, where he has been on the faculty since 1989. A member of the MAA for over 40 years, he is currently Past Chair of the SIGMAA for History of Mathematics (HOM SIGMAA) and Section Representative to the MAA Congress for the Ohio Section. Trained as an algebraist and number theorist, Danny has been a student of the history of mathematics and an unabashed humanist since his school days and has devoted much of his scholarship to the use of history in the teaching of college level mathematics. He and Daniel J. Curtin (Northern Kentucky University) have co-organized the ORESME (Ohio River Early Sources in Mathematical Exposition) Reading Group since 1998; the group meets twice a year in the Cincinnati area to read influential texts drawn from the mathematics literature of long ago. Since 2015, he has been one of the seven Principal Investigators for the TRIUMPHS collaborative.

Berggren, J. L. *Episodes in the Mathematics of Medieval Islam*. Springer, New York, NY, 2003.

Chereau, Fabien, and Guillaume Chereau. *Stellarium Web.* Stellarium Web Engine Project, 2020.

Hughes, Barnabas, O.F.M. *Regiomontanus: On Triangles*. University of Wisconsin Press, Madison, WI, 1967.

Katz, Victor. *A History of Mathematics: an introduction*. Addison Wesley, Reading, MA, second edition, 1998.

Katz, Victor, Annette Imhausen, Eleanor Robson, Joseph W. Dauben, Kim Plofker, and J. Lennart Berggren. *The Mathematics of Egypt, Mesopotamia, China, India, and Islam: a Sourcebook*. Princeton University Press, Princeton, NJ, 2007.

Kennedy, E. S. *The Exhaustive Treatise on Shadows, by Abū Rayḥān Muḥammad ibn Aḥmad al-Bīrūnī*. Institute for the History of Arabic Science, Aleppo, Syria, 1976.

Neugebauer, Otto, and David Pingree. *The Pañcasiddhāntikā of Varāhamihira*. Munksgaard, Copenhagen, Denmark, 1970/1972.

North, John. *The Fontana History of Astronomy and Cosmology*. Fontana Press, London, 1994.

Pedersen, Olaf. *A Survey of the Almagest, with annotation and new commentary by Alexander Jones*. Springer, New York, NY, 2011.

Plofker, Kim. *Mathematics in India*. Princeton University Press, Princeton, NJ, and Oxford, 2008.

Richards, E. G. *Mapping Time: the calendar and its history*. Oxford University Press, Oxford, 1998.

Suzuki, Jeff. *Mathematics in Historical Context*. Mathematical Association of America, Washington, DC, 2009.

Toomer, G. J. *Ptolemy’s Almagest*. Translated and annotated by G. J. Toomer; with a foreword by Owen Gingerich. Princeton University Press, Princeton, NJ, 1998.

Van Brummelen, Glen. *The Mathematics of the Heavens and the Earth: the early history of trigonometry*. Princeton University Press, Princeton, NJ, and Oxford, 2009.