How a magnetic compass works, and how to use bearings
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A compass needle is a thin magnet balanced on a pivot. Leave it alone and it swings round until one end points roughly north. No battery, no signal, no satellite: the Earth's own magnetic field does all the work.
Where the field comes from
The Earth's outer core, about 2,900 km down, is liquid iron and nickel. As it churns, it carries electric currents that produce a magnetic field around the whole planet, shaped rather like the field of a huge bar magnet. Our guide to the Earth's structure explains how scientists know the outer core is liquid.
Why the needle points north
The needle has a north-seeking pole and a south-seeking pole. Magnets obey one rule: opposite poles attract and like poles repel. So the north-seeking end of the needle is pulled towards the magnetic pole in the Arctic.
That leads to an odd but true conclusion: the magnetic pole in the Arctic behaves as the south pole of the Earth's magnet. It is called the north magnetic pole because of where it is.
Three different norths
- True north points to the geographic North Pole, where the Earth's axis meets the surface and all lines of longitude meet.
- Magnetic north is where a compass needle points. The north magnetic pole is in the Arctic, but hundreds of kilometres from the geographic pole, and it wanders. In recent decades it has moved from northern Canada towards Siberia.
- Grid north is the direction of the vertical grid lines on a map. On Ordnance Survey maps of Britain it is close to true north but not identical.
The angle between true north and magnetic north at a place is called magnetic declination, or variation. It differs from place to place and changes slowly, so walking maps print the current value in the margin.
Bearings
"Head north-east" is fine for a rough idea. For anything more precise, navigators use bearings: angles measured clockwise from north, always written with three figures.
Three figures make it obvious that a number is a bearing: east is 090°, not 90°.
Worked example: a back bearing
You walk on a bearing of 070° to a hilltop. What bearing brings you straight back?
- Turning round means a change of 180°.
- 070 is less than 180, so add: 070 + 180 = 250°.
If the outward bearing is more than 180, subtract instead. Outward 300°: 300 − 180 = 120°.
Following a bearing with a baseplate compass
Walkers and orienteers use flat, see-through baseplate compasses. To walk on a bearing:
- Turn the dial until the bearing you want lines up with the index mark at the base of the direction-of-travel arrow.
- Hold the compass flat in front of you, arrow pointing away from you.
- Turn your whole body until the red end of the needle sits inside the orienting arrow printed on the dial ("red in the shed").
- Look along the direction-of-travel arrow, pick a landmark, walk to it, then check again.
Keep the compass flat, and away from phones, keys and cars, which pull the needle off line.
A short history
Chinese writers described magnetised needles that settle north–south by the late 1000s, and Chinese sailors used them at sea in the early 1100s; in Europe the mariner's compass appears in writing by about 1190. By the Magellan–Elcano voyage of 1519, ships' compasses were marked with 32 points. A compass gave sailors their heading, but not their position: for that they needed latitude and longitude.
Close to a magnetic pole the field points steeply into the ground, so the needle swings about and becomes unreliable. Polar explorers fixed their position by measuring the height of the Sun instead, as both teams did in the race to the South Pole.
Activity: make a floating compass
You need: a steel sewing needle, a bar magnet, a slice of cork, a bowl of water, and an ordinary compass to check against.
Safety: do this with an adult. Needles are sharp, so hold them by the eye end. Keep magnets away from young children, who could swallow small ones, and away from phones and bank cards.
- Stroke the needle about 30 times with one end of the magnet, always in the same direction, lifting the magnet clear at the end of each stroke.
- Lay the needle across the cork.
- Float the cork in the middle of the bowl, away from metal objects.
- Let it settle, then compare its direction with the real compass. Mark the end that points north.
- Now bring the magnet slowly towards the bowl. How close does it need to be before your needle turns?
Sources and further reading
- An overview of the Earth's magnetic field – British Geological Survey – how the field is generated, how it changes and why it matters for navigation.
- World Magnetic Model – NOAA – the model used to work out magnetic declination, with maps and a calculator.
- Compass – Britannica – the history and types of navigational compass.
