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Welcome to the PI faq. This document was written after a
joke website of mine, 3point14159.info began getting a large number of visitors
(after being linked to from a popular link directory). After the initial link a
lot of blogs began posting the link as well. This faq is designed to
answer some common questions and resolve some misconceptions which I
saw about pi at those blogs.
What is pi and why is it significant?
PI is not a random number, and it is very important in maths, science and
engineering.
It is defined as the ratio of the diameter of a circle to its circumfrence.
In case you don't know (or need reminding), the diameter is the length down the
middle of a circle, and the circumfrence is the length around it.
Imagine that you can take the line down the centre (the diameter), bend it,
and fit it into the circumfrence. PI tells you the number of times the diameter
'fits' around the circle. If the diameter is 1 unit in length then it will fit
approximately 3.14 times. Knowing this, we can use PI to work out many things
about a circle. For example, that the size of the circumfrence is always PI
times the diameter, and the area is always PI times the radius squared (the
radius is half the diameter).
What is the exact value of PI?
There isn't one. This may seem confusing, but PI is termed in mathematics as
an irrational number. It never repeats itself and cannot be represented with a finite sequence of digits. It
is possible to calculate PI to a certain number of decimal places, but its exact
value can never be known.
Using some powerful computers, PI has been calculated to over a trillion
places, and people are always trying to find more digits. But for all practical
purposes five decimal places is enough (3.14159). Computer programs may use a
longer value such as 3.141592653589, but beyond that there are few situations in
which more accuracy makes any noticable difference.
Who discovered PI?
A greek mathematician, Archimedes is usually credited as having discovered
PI, however he was not the first. The first approximation of PI may go back to
the Babylonians, who gave a value of 3.125.
Why is it called 'pi'?
'PI' is the name of the Greek letter π, which is the first letter of the word
περιμετρος (perimetros), meaning 'measure around' in Greek.
Does the Bible say that PI = 3?
"And he made a molten sea, ten cubits from the one
brim to the other: it was round all about, and his height was five cubits: and
a line of thirty cubits did compass it round about." - 1 Kings 7:23; KJV
This passage is in reference to constructing a basin. It does seem to
say that the diameter of the basin was in a ratio of 1:3 (10:30) with the
circumfrence.
The first thing to note about this is the unit of measurement - the
cubit - was an unreliable measure defined as the distance from elbow to the tip
of the fingers. The second point is that the passage isn't saying anything about
the ratios in circles generally, but specifically about the measurements of one
particular circular object. This passage merely states some measurements which
were made by people with less accurate tools and knowledge than we have today.
There is little reason to doubt that the reported measurements are what
was actually measured, so in this sense the claims of this passage are probably true.
However this passage does not say "the ratio of the circumfrence of a circle to its diameter is 1:3",
and if it did say that then it would quite clearly be false.
This article discusses the topic in more
detail.
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Contents
PI - Frequently Asked Questions