# What Makes Pi Special

Roll a wheel, lay out its width, spin a wave and compare pizzas to discover why π is special

By cosmic\-fox\-0f21ea73

Topic: Maths

[Open the interactive Jig](https://ask.jig.com/j/M8SDr5LwlXi2)

## Transcript

### 1. A number hiding in every circle

Meet π, said "pie"\. Measure the distance around any circle, divide it by the distance across, and you always get the same number: a little more than 3\. Let's find out why that makes it special\.

The distance across a circle through its centre is called the diameter\.

### 2. Roll it out\!

Drag the wheel along the ruler, or press the button\. One full turn lays its rim out flat as a red line\. The ruler counts in wheel widths: where does the line stop?

### 3. Three widths and a bit

Lay the wheel's width end to end along the red line\. Three fit, plus one small piece\. That leftover is about 0\.14 of a width, so the trip around is about 3\.14 widths: that is π\.

π is not exactly 3\.14 or 22/7\. Those are handy shortcuts\.

### 4. Big or small, same π

Slide to shrink or grow the wheel, from a coin to a bike wheel\. The red line and the blue widths always grow together\. Every circle in the universe shares the same π\.

### 5. Digits that never end

π's digits go on forever, and they never settle into a repeating pattern\. Computers have worked out about 314 trillion of them, and there are always more\.

Pi Day is March 14, because the date 3/14 looks like π\.

### 6. Spin a circle, make a wave

Spin the wheel and follow the red dot\. Its height draws a wave, and one full turn makes one full wave\. Sound, light and ocean swells are waves too, so π helps describe them all\.

### 7. Pizza maths

A circle's area uses π too: it grows with the width times itself\. So a 16\-inch pizza holds as much as four 8\-inch pizzas\. Pick one and compare\.

### 8. Steering through space

Engineers use π for orbits, wheels, pipes and cans\. NASA steers spacecraft with only 15 decimal places of π\. About 40 places would measure the whole visible universe to the width of an atom\.

With 15 decimals, a circle 30 billion miles wide is off by less than half an inch\.