## Global distances

By David, 1 June 2016 Activity

Rule a line from Sydney to Reykjavik to Sydney.

With a globe and string, you’ll discover that finding the shortest path can be difficult on a round planet!

### You will need

- World map
- World globe
- Ruler
- Pencil
- Paper
- String
- Blu-Tack
Write down a few countries the line passes through.

### What to do

- We’re planning a holiday! Find Sydney on the map. Then search for Reykjavik, the capital of Iceland.
- Grab a ruler and find the straight line that connects the two. Write down a few countries that the line passes through at different places along the line.
Lay the string along the globe, passing through all the countries you wrote down.

- Take a second to think about this question – is this the shortest path from Sydney to Reykjavik?
- Cut a piece of string about 75 centimetres long.
- Get your globe and Blu-Tack one end of the string to Sydney.
- Carefully lay the string along the surface of the globe to Reykjavik, making sure to pass through the countries you wrote down earlier.
- Blu-tac the string to Reykjavik. You’ve just transferred your ‘straight line’ to the globe.
Can you find some slack in the string?

- Now see if you can make the journey shorter. Carefully, without detaching the Blu-Tack from either end, pull on the string and see if there’s any slack in it.
- Try repositioning the string so the journey is shorter. Hint – your string might have to go a bit further north.

### What’s happening?

Maps are really handy things – they show you where things are in relation to each other. If you look at a flat map of your town, 100 metres is the same length anywhere on the map. Football fields are the right shape and the shortest distance between two points is a straight line.

When you zoom out to the whole world, maps get a bit tricky. When you’re only looking at one town, you can ignore that Earth is round, and pretend it’s flat like a map. When you’re looking at the whole thing, Earth’s roundness is inescapable.

To get around this, map makers ‘stretch’ the way they show Earth. There are lots of different ways to stretch Earth, and each has advantages and disadvantages. Many online maps use the Mercator projection, which is good at preserving angles. However, the Mercator projection stretches areas near the poles, making them appear bigger than they really are.

The Gall-Peters projection is also very popular. It preserves areas, so you can make accurate comparisons between, say, Greenland and Africa. However, it changes the shapes, making Greenland shorter and wider.

To find the shortest distance between points, there are gnomonic projections. On these maps, the shortest path between two places is a straight line. However, Gnomonic projections stretch the world in very weird ways, making some countries almost unrecognisable!

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