# What Makes Pi Special

How a rolling wheel, Archimedes' polygons and waves reveal why pi is the same for every circle, from bikes to spacecraft\.

By cal

Topic: Maths

[Open the interactive Jig](https://ask.jig.com/j/3UQc1COJw7tt)

## Transcript

### 1. A number in every circle

Measure around any circle, then straight across it\. Divide the first by the second and you always get the same number, a little more than 3\. It is called pi, written π\. Let's find it on this table\.

### 2. Roll it out

The red dot marks where the wheel touches the track\. Roll the wheel one full turn and its blue paint shows the distance all the way around it, called the circumference\.

Press Roll, or drag the wheel along the track\.

### 3. Three and a bit

Now lay the wheel's width, its diameter, along the paint\. Three diameters fit, with a small orange piece left over\. That piece is about 0\.14 of a diameter, so the trip around is 3\.14 diameters: that is π\.

### 4. Any size, same π

Make the wheel bigger or smaller\. The paint and the rods grow together, so it is always three diameters and the same little bit\. A bike computer uses this to turn wheel turns into distance\.

### 5. Archimedes squeezes π

Over 2,200 years ago, Archimedes drew shapes with straight sides inside and around a circle\. The more sides, the closer they hug it\. With 96 sides he proved π lies between 3\.1408 and 3\.1429\.

### 6. Pizza maths

A circle's area, the space inside it, also uses π\. Double a pizza's width and you get four times as much pizza, not two\. Bakers, pipe makers and tank builders all rely on this\.

### 7. Circles make waves

Spin the wheel and follow the red dot's height on the card\. Traced over time it draws a wave, and each turn draws exactly one\. Sound, light and the electricity in our homes travel in waves like this, so π is in their maths\.

### 8. π steers spacecraft

Space engineers use π to plan the paths of spacecraft\. NASA's navigators need only 15 decimal places\. That is enough to measure around a circle 64 billion km wide and be off by about 1\.5 cm\.

Source: NASA JPL\.

### 9. Digits that never end

No fraction equals π exactly, so its digits go on forever and never settle into a repeating pattern\. Computers have found 314 trillion of them, yet about 40 are enough to measure the whole visible universe to the width of an atom\.

Pi Day is 14 March: 3/14\.