Teaching and Learning the Trigonometric Functions through Their Origins: Episode 2 - Hipparchus’ Table of Chords

Daniel E. Otero (Xavier University)

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 Hipparchus’ Table of Chords is the second of a series of six units in the Convergence series Teaching and Learning the Trigonometric Functions through Their Origins that attempts to provide students with a rich motivation for the study of trigonometry by presenting a series of episodes from the long history of the subject. These episodes contextualize the main ideas of trigonometry in the questions that earlier mathematicians addressed in developing the subject over thousands of years.

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 Almagest (second century CE) that explains why the geometry of the circle was so important to astronomers. This is followed by an investigation of a (modern reconstruction of a) table of chords attributed to Hipparchus of Rhodes (second century, BCE), designed to show why and how degree measure works, as well as gently introduce students to the study of trigonometrical functions through an examination of a table of chords in a circle.

Figure 1. Hipparchus of Rhodes,
Convergence Portrait Gallery.

Figure 2. Modern Reconstruction of
Hipparchus' Table of Chords.

The unit Hipparchus’ Table of Chords (pdf) is ready for student use. It is meant to be completed in two 50-minute classroom periods, plus time in advance for students to do some initial reading and time afterwards for them to write up their solutions to the tasks. 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.

As mentioned above, the unit Hipparchus’ Table of Chords is the second 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 has been redesigned to 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.