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Lessons / Space around Earth

Space around Earth · 5 min · 4 steps

Your compass and the wandering pole

Plays here

Goal. Explain why a compass does not point to true north and where the Earth's magnetic field comes from.

Steps

  1. Predict

    Does a compass needle point to the North Pole?

    • Yes, exactly
    • No, towards the north magnetic pole, which is somewhere else
    • Only at night
    Show the answer

    No, towards the north magnetic pole, which is somewhere else. No. It lines up with the magnetic field, which points roughly towards the north magnetic pole, about 500 km from the geographic pole.

    The card shows your declination: the angle between true north and where your compass points. Each small needle on the globe shows the direction of the field.

  2. The line with years is the path of the north magnetic pole since 1590. In recent decades it has raced from northern Canada towards Siberia, at up to about 55 km a year.

  3. The needles are coloured by field strength. Over the South Atlantic the field is unusually weak: the South Atlantic Anomaly. Satellites passing through it, including Hubble and the ISS, meet more radiation there.

  4. The field is made by liquid iron moving in the outer core, which begins about 2,900 km below us. It turns aside most of the solar wind. When solar storms shake it, aurorae light up in rings around the magnetic poles.

The common mistake

Myth

A giant bar magnet in the core makes the Earth's magnetic field, and the poles stay put.

Fact. The core is far too hot for a permanent magnet. The field is generated by moving liquid iron in the outer core, a natural dynamo, which is why the poles wander and the field has reversed many times.

Check yourself

Question 1

Why does a compass not point exactly to true north?

  • It points along the magnetic field, towards the magnetic pole
  • It is faulty
  • The Moon pulls it
Show the answer

It points along the magnetic field, towards the magnetic pole. The magnetic pole is not at the geographic pole, and the field also bends locally, so the difference (declination) changes from place to place.

Question 2

What generates the Earth's magnetic field?

  • A permanent magnet
  • Moving liquid iron in the outer core
  • The Sun
Show the answer

Moving liquid iron in the outer core. Flowing molten iron in the outer core acts as a dynamo.

Question 3

Where is the field unusually weak?

  • Over Taiwan
  • Over the South Atlantic
  • At the North Pole
Show the answer

Over the South Atlantic. The South Atlantic Anomaly stretches from South America towards southern Africa.

Words in this lesson

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Curriculum

US Next Generation Science Standards
MS-PS2-5 · HS-ESS2-3
England national curriculum
KS3 Physics: magnetism (Earth's magnetism, compass and navigation)