History Lecture for Unit 7: Greek Mathematics

For Class

• Period: 600 bce to 100 bce

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• Geographic Location: Greece, Asia Minor, Egypt and Babylonia
• People to know: Pythagoras, Apollonius, Euclid
• Concepts to know: Proof, geometry, conic sections (parabola, ellipse, hyperbola, circle)
• See science topics: Mathematics

Outline/Summary

• Mathematics in Ancient Greece
  • Pythagoras
  • Euclid
  • Apollonius
• Study/Discussion Questions
• Further Study

Mathematics in Ancient Greece

This week, we depend heavily on the web for our history reading as we take a look at the beginning of Greek mathematics. The Greeks, through scholars like Pythagoras, were well aware of the work that the Egyptians and Babylonians had done in mathematics, but they were able to take it one step further and abstract the concept of pure number from the concept of counting. To understand the difference, think of how we normally teach a child to count. We hold up fingers, or spoons, or blocks, and we count things. Our first idea of number is always associated with the how-many-ness of real objects.

The Greek philosophers in Ionia on the coast of Asia Minor appear to be the earliest philosophers in the Western tradition to look at number as separate from the objects. Instead of adding 2 cookies + 2 cookies to get 4 cookies, they realized that 2 of anything plus 2 of anything made 4 of that thing, and then that 2 + 2 = 4, without regard to what things were being counted and summed. Once the Greeks had made this step, they could look for patterns and associations between numbers by themselves.

Pythagoras

The first Greek mathematician of note is Pythagoras. Rather than have you read through a brief biography here, I'm going to send you to another site, one which has been developing an extensive site on the history of mathematics for the past decade.

We have no direct writings by Pythagoras himself, but reports of his teachings were widespread Within two generations, Pythagorean ideas of the soul, life and death, mathematics, and the nature of the universe had became part of the intellectual inheritance of Plato and Aristotle.

Euclid

We jump forward several hundred years to Euclid, or rather to the book Euclid wrote about Geometry. We actually are not sure exactly when Euclid lived, or where, or even if the really was an historical Euclid. Some historians think the book of geometry that bears this name was really written by a team of mathematicians.

Euclid uses deduction to show that if certain ideas are true, then we must conclude mathematical figures have very specific relationships. To prove a proposition, Euclid starts with definitions and postulates.

A definition describes an object or explains the meaning of a term, but does not guarantee the existing of the thing it describes. For example, "A point is that which has no part" (i.e., no dimensions) is a definition. Euclid's geometry relies on twenty-three definitions that describe points, lines, straight lines, surfaces, plane surfaces, plane angles, right angles, obtuse and acute angles, circles, the center, radius, and diameter of a circle, three-sided and four-sided figures (triangles and rectangles), and parallel lines.

A postulate or axiom is a statement that must be accepted without proof. Three of Euclid's postulates are constructions, or instructions on how to draw straight lines and circles. The fourth postulate claims that all right angles are equal to one another, and the fifth that two non-parallel lines lying in the same plane will meet.

Euclid also relies on some "common sense" notions, for example, that two things that are equal to a third thing are equal to each other, and that in mathematics, the whole of something is greater than any fraction of it.

By combining definitions and postulates, Euclid shows a number of relationships between angles and lines lying in the same plane (plane geometry). Proposition I in Book I contains a proof that if you start with a short straight line, you can construct and equilateral triangle and know that all three sides are the same length.

Apollonius

Our third Greek mathematician is Apollonius of Perga.

Apollonius did a lot of work on what we now call conic sections -- slices and lines drawn on the surface of a cone. Using the same methods as Euclid, and starting with definitions and postulates, he proved that the different figures such as a circle, an ellipse, a parabola, and a hyperbola can be defined from specific intersections of a flat surface, or plane, and a cone. You can see how this works by looking at how mycroft's very short movie on Conics, which has four sections:

• A cone by itself, tilting so that you can see inside.
• The cone with a plane (flat surface) that intersects it parallel to the plane of the base. This kind of intersection creates a circle.
• The cone with the plane intersecting it at an angle greater than the angle of the side. This kind of intersection creates an ellipse if the plane cuts all the way across the cone.
• The cone with the plane intersecting it parallel to the axis of the cone. This kind of intersection creates a parabola.
• Not shown is the case where the plane intersects the cone in an angle equal to the side of the cone. This kind of intersection is a hyperbola.

Understanding these figures will be of great importance to Kepler and Newton in determining how the planets move around the sun. Later on, Einstein would realize that the "well" of force caused by gravitational mass maps to a funnel-like surface similar to a cone, which is why orbits of planets and comets take the shape of ellipses and parabolas.

Study/Discussion Questions:

• Is the abstraction of characteristics (like number, or patterns of behavior) useful in science? Why or why not?
• What is the purpose of having a method of proof? Can we use geometric methods of proof in science? What are the limitations of proof?

Further Study/On Your Own

Optional Websites to check

We actually know very little of Pythagoras directly, since he worked in community, and we have none of his own writings. We have to look at the collection of fragments in which he is mentioned to piece together his life. These fragments have been collected and translated as part of the Hanover Historical Texts Project.

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