Reply to Robin, Diminishing Circle Argument

Jesse L. Yoder

Subject: Reply to Robin, Diminishing Circle Argument

Author: Jesse Yoder < >

Organization: Epigone

Date: 30 Oct 1997 10:56:23 -0500

Robin - Thanks very much for your response to the note I posted on calculus and geometry. You

"I'm intrigued, but how does the diameter equal circumference divided by 2? It seems you have eliminated Pi from the equation. Perhaps this is why Pi figures in so importantly when computing curved areas within the Cartesian coordinate system.

Good luck with your work. Send me more info any time."

Diminishing Circle Argument

I concede that the idea that diameter of a circle = the circumference divided by 2 is
counterintuitive. This comes about from what I call the "diminishing circle argument." This argument goes as follows:

Take any circle E and draw two smaller circles inside the larger circle. The two smaller circles should be equal in area, with one edge intersecting the outside edge of the circle E (the point where the diameter intersects the circumference) and the other edge passing through the radius
of E (this is easier to draw than to describe). This gives you two smaller circles, A and B, each with a diameter that is half the diameter of the original circle E. This is because the area of E is (pi x r squared), and the area of circle B is (pi x (r/2) squared), (where r refers to the radius of A).
This equals pi x r squared/4, or 1/4 the area of the original circle E. Since A and B are equal in size, they have the same area, which is equal to 1/4 the area of E. Hence the areas of A and B together equal 1/2 the area of E, the large circle. This means that the remaining areas, C and D (which are also equal, but not circles) are together each equal in area to the areas of the smaller circles A and B.

The above shows that circle E is divided into four equal areas: the areas of A, B, C, and D. The next step shows that the circumference is B = the Circumference of E/2. The circumference of E = 2 x pi x r. Since the radius of B is 1/2 the radius of E, the radius of B is 2 x pix r/2 (where r refers to the radius of E). But 2 x pi x r/2 = pi x r, which is 1/2 the circumference of E. So the circumference
of B (one of the smaller circles) is half the circumference of E. This means that half way around the circumference of E, which is C/2, equals the entire circumference of circle B. This also means that halfway around the circumference of B plus halfway around the circumference of A equals halfway around the circumference of E (since A and B are equal, their circumferences are equal).
So going from one end of the diameter of E to the other via the circumference of E is the same distance as going from one end of the diameter of E to the other via the two circumferences of the smaller circles A and B.

This last result is the main result I need to get the argument that the diameter of a circle equals the circumference divided by 2. Now we have circle E divided into four equal areas, the areas of A, B, C, and D. Now imagine further dividing circles A and B each into four smaller areas in the
same way, by drawing two smaller but equal circles within A and B, each dividing the areas of A and B into four smaller areas. Again, going halfway around A is the same distance as traveling along circumferences of the two smaller circles within A. Continue this process indefinitely. At the end point, we have the diameter consisting of the smallest allowable points packed up against one another. But the distance around all these points still equals the distance around the
circumference of E, so the diameter of circle E will equal half the circumference. The same argument can be repeated for any circle.

At this last stage of the argument, I used to say "Now imagine this process completed to infinity. At infinity, the circles will be infinitely small, thereby equaling the points on a line, and the distance along the diameter will equal half the circumference." Sandy Norman of the University of Texas at San Antonio has convinced me of the folly of, on the one hand, rejecting the notion of infinity and then, on the other hand, relying on it when it is convenient to do so. But I don't believe that I need to rely on the notion of infinity to complete the diminishing circle argument.

In this geometry I am proposing, the points on a line are densely packed, each with a specifiable area. In other words, they are not "infinitely small" (whatever that is supposed to mean). How small the points will vary with what is being measured-but once the units are specified, they can only be made smaller by re-specifying the unit of measurement. (This is the only way to avoid
Zen's paradox.) This means that in this geometry, there is a finite number of points on the number line, not an infinite number, and that I reject the idea that between any two points, there always lies another.

Of course, I realize that the diminishing circle argument is counterintuitive, since it certainly seems shorter to walk along the diameter of a circle than to go along its circumference. The difference is in how you analyze motion along a diameter. If you analyze it as moving along the circumferences of a very large number of very small circles (or points), then the distance is in fact the same as the distance along the circumference of the circle whose diameter you are traversing. On the other hand, if you analyze the motion along the diameter as motion along the path of a moving point, giving you motion along a "straight line," then the distance is less than
traveling along the circumference of the circle whose diameter you are traversing. But since I reject straight lines, I can't accept this "straight line" analysis of motion along a diameter.

I hope that this answers your question (if you've gotten this far). Since you asked about my background, and what level I'm working at, I might as well answer that too. I have a PhD in philosophy (from U. of Mass. Amherst). While my main area of concentration is in philosophy of mind, I have also done some philosophy of mathematics. My main area of interest in this subject is to try to understand if the assumptions that underlie mathematics are valid, and to try to
suggest alternatives where I feel they are not. My view of philosophy is that it mainly consists in evaluating the assumptions the other fields of study take for granted. In the case of mathematics, I have always been bothered by the notion of infinity, and I object to the idea of analyzing what is continuous as being infinitely many discrete things. So I am trying to come up with a different way of analyzing continuous phenomena (such as the number "line") without relying on the notion of infinity.

I am working as a market analyst for a market research company (Flow Research), writing about industrial applications for flowmeters and pressure transmitters. I am trying to figure out whether my geometry can be used to improve or change the ways in which flow rate is calculated, since the fundamental flow equation is Volumetric Flow = Area x Velocity, and pipe areas typically are around, thereby invoking pi.

Thanks again for your question, Robin, and I welcome any comments you may have on my response.


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Jesse L. Yoder
Flow Research
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781-224-7552 (Fax)
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Flow Research
27 Water Street
Wakefield, MA 01880
781-224-7552 (fax)