The Earth's Magnetic Field

An invisible shield around the planet

You cannot see it, feel it or hear it, yet Earth is wrapped in a vast magnetic field that stretches tens of thousands of kilometres into space. It guides every compass, paints the polar skies with auroras, and shields living things from a constant storm of particles streaming off the Sun. Without it, our planet would be a far harsher place. Understanding this field means looking deep into the Earth and far out into space at the same time.

Earth as a giant magnet

In many ways, Earth behaves like a colossal bar magnet tilted inside the planet. It has a magnetic north and south pole, and the field loops out from one and back into the other. A compass needle is simply a tiny magnet free to swing, so it lines up with this field and points roughly north.

Importantly, the magnetic poles are not in the same place as the geographic poles — the points the Earth spins around. The magnetic north pole currently sits in the Arctic and slowly drifts year by year, which is why precise navigators must correct for the difference.

The engine in the core: the geodynamo

So where does the magnetism come from? Not from a solid magnet — it is far too hot inside the Earth for that, as you can discover in inside the Earth. The answer lies in the outer core, a layer of liquid iron and nickel about 3,000 km down.

This molten metal is constantly moving, stirred by heat escaping the core and by Earth's rotation. Because moving metal carries electric charge, and moving charge creates magnetism, the swirling outer core acts like a natural electric generator. Scientists call this the geodynamo. The motion is self-sustaining, which is why the field has lasted for billions of years.

The magnetosphere: our space shield

The Sun constantly blasts out a stream of charged particles called the solar wind. If these particles reached the surface unchecked, they could strip away the atmosphere over time — as may have happened to Mars, which lost most of its magnetic field long ago.

Earth's field forms a protective bubble called the magnetosphere that deflects most of the solar wind around the planet, like water parting around the bow of a boat. Some particles are funnelled toward the poles, where they crash into atmospheric gases and make them glow. That glow is the aurora — the northern and southern lights. So the same field that protects us also creates one of nature's most beautiful displays. It works alongside weather and the atmosphere to keep the surface habitable.

When the poles flip

One of the strangest facts about the magnetic field is that it is not fixed. Over long timescales the poles wander, and every so often they completely reverse, so that magnetic north becomes magnetic south. This has happened hundreds of times in Earth's history, the last full reversal around 780,000 years ago.

We know this because lava that cools at mid-ocean ridges locks in the direction of the field at that moment. The seafloor preserves a striped pattern of normal and reversed magnetism — a record that, remarkably, became one of the key proofs of plate tectonics and moving continents. Reversals are slow, taking thousands of years, and life has always survived them.

Try it yourself: make a floating compass

1. Magnetise a steel sewing needle by stroking it about 30 times in the same direction with one end of a fridge magnet.
2. Push the needle through a small disc of cork or a flat piece of foam.
3. Float the cork gently in a bowl of water, well away from metal objects and magnets.
4. Let it settle. The needle should slowly swing and come to rest pointing roughly north–south.

You have just built a working compass and watched Earth's invisible field reach out and turn a needle. Sailors, explorers and migrating birds have relied on that same field for navigation for centuries.