Why is pi = circumference / diameter ?

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Discussion Overview

The discussion revolves around the definition of pi as the ratio of the circumference to the diameter of a circle. Participants explore the reasons behind this convention, its historical context, and the implications of potentially defining pi differently.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Meta-discussion

Main Points Raised

  • Some participants question why pi is defined as the ratio of circumference to diameter rather than the reverse, suggesting it may be due to the circumference being larger.
  • Others point out that this is simply a definition and that alternative ratios could be considered, such as diameter over circumference.
  • A participant notes that pi is also related to the area of a circle, specifically that a unit circle has an area of pi.
  • There is a suggestion that the historical context of defining pi may stem from ancient geometric practices, where the perimeter was expressed in relation to a unit length.
  • Some participants emphasize that the choice of the symbol "pi" is a matter of convention, and alternative symbols could have been used.

Areas of Agreement / Disagreement

Participants express differing views on the significance of the definition of pi and whether it could have been defined differently. There is no consensus on the reasons behind the current definition or its implications.

Contextual Notes

Participants acknowledge that the definition of pi is rooted in historical conventions, but they do not resolve the implications of these conventions or the potential for alternative definitions.

Juwane
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Why do we take pi as the ratio of the circumference to the diameter, and not diameter to the circumference? Is it because circumference is always bigger than the diameter, so that it will be easy to work with the ratio? Or is it something fixed by those who discovered it and we can't change it?
 
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You also should realize that this is the ratio of the circle's area to the square of its radius. That is, a unit circle has an area of pi.
 
It's just a definition...
 
What if we turn it upside down; that is, diameter over circumference. This is still a ratio, isn't it?
 
Juwane said:
What if we turn it upside down; that is, diameter over circumference. This is still a ratio, isn't it?

That would be perfectly acceptable as well. pi is just a number someone defined (probably someone from ancient Greece), but you can define lots of other numbers related to it if you want. At some point we defined pi and since then we haven't found a major reason why we would rather have diamter/circumference or anything else related to it. In some areas of math we more often work with \sqrt{\pi}, 2\pi or 1/\pi than \pi, but we don't re-define pi or introduce a new symbol since it wouldn't be worth the trouble.
 
Why do we take pi as the ratio of the circumference to the diameter, and not diameter to the circumference? Is it because circumference is always bigger than the diameter, so that it will be easy to work with the ratio? Or is it something fixed by those who discovered it and we can't change it?

It's a 2500 year old convention.
 
You may as well ask why pi and not rho :smile:
 
Juwane said:
Why do we take pi as the ratio of the circumference to the diameter, and not diameter to the circumference? Is it because circumference is always bigger than the diameter, so that it will be easy to work with the ratio? Or is it something fixed by those who discovered it and we can't change it?

In ancient times geometers probably thought of the perimeter of a figure a derived or constructed from a unit length and so expressed the perimeter as a multiple of one. A circle is well approximated as a perimeter of a many sided regular polygon.

From this point of view the unit length would be the radius of the circle so I would guess that the original formula was circumference/2xradius.
 
Borek said:
You may as well ask why pi and not rho :smile:

Because it's the first letter in the word "perimeter".
 
  • #10
That still puts us in the world of conventions. What if pi was defined earlier by Egyptians? Or later by Muslims? It would be as obvious that we use some different symbol as it is that we use pi now.
 
  • #11
I don't know, why do we call a chair a "chair" and not a "kanildor?"
 
  • #12
Convention :smile:
 

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