First Mate

Hey there! Hope you’ve been enjoying your journey so far.. speaking of this journey I’m in charge of making sure we get to the right place at the right time. This means using maps and other equipment to make sure this big ship and its most precious cargo (the crew) get across these seas safely.

I’m having a spot of bother though… some of our fancy equipment isn’t working so well. Could you help me navigate our way across the seas?

How to Run

Everyone should look at a compass. A compass uses a small magnetic needle, floating in a protective case. It points north as it lines up with the magnetic fields of the earth. Everyone should look at a map and discuss what we mean by ‘north’. What is it and where is it? How is it shown on the map? The person leading the activity should explain that there are three different ‘norths’:

• True north is right at the top of the planet, at the geographic North Pole. The earth spins around this point so it never changes position. This north isn’t useful for navigating as we can’t find it with a compass.
• Magnetic north is the direction that a compass will point to. It’s slightly different from true north as the molten metals inside the Earth are constantly moving around, changing where magnetic north is.
• Grid north is the direction that the grid lines on a map point to. Magnetic north is used together with grid north for navigating.

Testing grid north

1. Everyone should take an orange and a marker pen.
2. Starting at the top of the orange, everyone should draw a straight line from the top of the orange to the bottom, as if drawing along the edge of an orange segment.
3. Moving one centimetre to the right of the first line, draw another line from top to bottom.
4. Continue working all the way around the orange, drawing straight lines from top to bottom until the orange is separated into roughly 10 segments.
5. Draw a small ring near the top of the orange, crossing through all your straight lines.
6. Move around half a centimetre further down from the top of the orange and draw another ring.
7. Continue drawing rings until you reach the bottom of the orange. The whole orange should now be split into lots of different sized squares.

This is exactly how the blue grid lines on an Ordnance Survey map work. This splits the entire planet into thousands and thousands of small squares. Different areas of the planet are then given different labels, and each square within these areas are numbered. This gives each grid square on Earth its own unique identity.

1. Peel the orange as carefully as possible, trying to keep it in one piece.
2. Now try to lay the orange peel flat. The orange peel will have to tear and deform to be able to lay flat.

Maps have the same problem. Trying to show the surface of a sphere on a flat map doesn’t work without changing the surface. Map makers get around this by slightly changing the shape of the Earth when they print it, to fit a flat map. This leaves the problem that grid north on a map is different to magnetic north on a compass. Helpfully, map makers tell us the difference between these two norths on our maps.

1. Everyone should look at a map to find where the adjustment for grid north and magnetic north are.

By adjusting our compass bearings by the amount of degrees shown on the map, we can convert from bearings on the map to bearing in real life, and vice versa.

What You Need

• Compass
• Permanent markers
• Oranges (or any fruit that’s easy to peel)