# Estimate How Far Away

Here is a clever method to estimate how far away something is:

- Hold your arm straight out, thumb up
- Close one eye, align your thumb with distant object
- Switch eyes (don't move your thumb!)
- Your thumb will seem to change position

Now ... estimate how far it moved sideways (you could imagine the length of a car or something).

**Multiply that by 10** and you have an estimate of how far away.

### Example

Here your thumb seems to jump about **half a car length**.

Half a car length is about **2.5 meters**.

Times 10: the car is **about 25 meters away**.

## How it Works

The distance from your **eyes to your thumb** is about 10 times the distance **between your eyes**

And so the distance **to the far object** is also about 10 times the **width your thumb seems to move** at the far object.

This works because the triangles are similar

and so the relative lengths are the same

## Learn the Size of Things

To be useful you need to know how long, wide or tall things are!

- Small cars are 4 m long
- Large cars are 5 m long
- Cars are about 1.8 m wide
- Adults are about 1.8 m tall
- A 5 year old is about 1 m tall
- A normal doorway is 2 m high and 0.8 m wide
- A truck and trailer is about 20 m long
- The width of a small house is about 8 m
- The width of a large house is about 12 m
- The height of a single-storey house is about 5 m
- The height of a two-storey house is about 8 m
- Tall buildings have about 3.5 m for every storey

(Note: to use this method for height, tilt your head and thumb 90° to the side.)

## Your Turn

Go outside and stand on a high spot where you can see lots of things (roads, building, etc).

Write down what you see and estimate how far away:

Object | "Thumb move" Distance |
×10 = How Far Away |
---|---|---|

Afterwards: get a map, mark where you stood and where each object is, and work out how far away they really are. How good were you?