An introduction to Earth's magnetic field: what it is, where it comes from and what it's used for.
Why it matters
Earth's magnetic field is continuously changing.
Magnetic north wanders, and every few hundred thousand years the polarity gradually flips, so that a compass would point south instead of north.
To learn more about Earth's magnetic field, the three Swarm satellites were launched on 22 November.
Like 3D compasses, they are measuring the strength and direction of the magnetic field.