# What Makes π Special

Why π is the same for every circle, shown with a rolling wheel, tape measures, pizzas and a rolling\-dot wave\. Follow how one number ties circle size, area and waves together\.

By vivid\-moon\-4e230935

Topic: Maths

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

## Transcript

### 1. Meet π

Pi, written π, is a number that hides inside every circle\. Measure the distance around a circle, then the distance straight across it\. Divide the first by the second and you always get the same answer: a little more than 3\.

The distance straight across, through the middle, is called the diameter\. The blue stripe on the wheel is one diameter\.

### 2. Roll it out

Let the wheel roll exactly one turn and its edge leaves a yellow tape behind\. Lay three blue rods, each one diameter long, beside it\. The tape is three diameters plus a little extra\.

That extra bit is why π is about 3\.14\. Bike computers use this idea: every turn of a wheel moves you π times its diameter\.

### 3. Big or small, still π

Make the wheel bigger or smaller\. The tape and the rods stretch together, so the tape always ends at the same flag: 3\.14 diameters\. A coin, a tyre or a planet all share this one number\.

This is what makes π special: it never depends on the size of the circle\.

### 4. Digits that never end

π starts 3\.14159 and keeps going forever\. Its digits never stop and never fall into a repeating pattern, so no fraction can match it exactly\. Computers have now worked out more than 300 trillion of them\.

In 1949 the ENIAC computer took about 70 hours to find 2,037 digits\.

### 5. The pizza puzzle

Here is one wide pizza and four pizzas half as wide\. They hold the same amount of pizza\! A circle's area grows with the square of its size, and π is the number that turns that square into a circle\.

So doubling a pizza's width gives you four times as much to share\.

### 6. Circles make waves

Watch the yellow dot on the wheel's edge as it rolls\. Its height rises and falls, drawing a smooth wave, and one full turn makes exactly one wave, π diameters long\. Sound, light and the electricity in your walls all travel in waves like this\.

That is why π turns up in the maths of music, radio and power stations\.

### 7. Steering by π

Spacecraft, satellites and GPS all move in circles and curves, so their maths is full of π\. Try a rough π and the orbit path does not close; a better π closes the gap\. NASA uses 3\.141592653589793 to steer its spacecraft\.

With those 15 decimals, a circle as wide as Voyager 1's distance from Earth would be off by only about 1 cm\.

### 8. π is everywhere

Wheels, pizzas, waves and orbits all share one number\. People celebrate it on Pi Day, March 14, written 3/14\. Next time you spot a circle, remember that π is hiding inside it\.

Pi Day began at the Exploratorium science museum in San Francisco in 1988\.