The magnetic field

A compass needle points north because the Earth is a magnet. But it is a strange one. It is not a lump of magnetised iron. It is made, continuously, by electrical currents in the slowly moving molten iron of the outer core, nearly 3,000 km below your feet. Because those currents shift, the field shifts too, year by year.

A pole on the move

There are two north poles. The geographic North Pole is where the Earth's spin axis comes out. The north magnetic pole, or dip pole, is where a compass needle allowed to tilt would point straight down. In 1900 the dip pole was in the islands of Arctic Canada, at about 70.5° north. For most of the 20th century it crept north. Then, from about 1990, it sped up, crossed the Arctic Ocean and headed towards Siberia. By 2020 it was within 4° of the geographic pole.

This matters in everyday life. Phones, aircraft and ships correct their compasses using a model of the field that is updated every five years. The difference between true north and magnetic north, called declination, changes from place to place and year to year.

When north becomes south

The field does more than wander. Every so often it reverses completely, so that a compass would point south. Rocks show it: when lava cools, or fine grains settle, tiny magnetic minerals line up with the field of the day and are frozen in place. On the sea floor this leaves stripes of rock magnetised one way and then the other, either side of the mid-ocean ridges, which became key evidence for plate tectonics.

Reversals come at irregular times. In the last 10 million years there have been about four or five every million years, on average. But for about 30 million years in the Cretaceous there were none at all. The last full reversal was nearly 800,000 years ago. Computer models suggest a reversal takes from about one to several thousand years. The field weakens but does not vanish while it happens.

A shield in space

The Sun blows out a steady stream of charged particles, the solar wind. The magnetic field steers most of them around the Earth. Some are funnelled down towards the poles, where they hit the upper air and make it glow: the northern and southern lights. When the Sun is very active they can sometimes be seen as far south as southern England.