The Earth's magnetic field

Around 4000 years ago, a Greek shepherd called Magnes was looking after his sheep. The story of Magnes says that iron nails in his shoes stuck to one particular type of rock. The rock was called lodestone and contained a substance that was later named magnetite, which is a naturally occurring magnet. Naturally occurring means it is not made by people.

Both the Greeks and the Chinese started to investigate magnetic properties.

The Chinese discovered that a small needle of lodestone, split off the rock, could be made to float on water. When allowed to float, the needle of lodestone always turned to point in the same direction.

One end of the needle pointed toward the north and the other end pointed toward the south. This was the invention of the magnetic compass.

It was soon discovered that the compass needle pointed to a position close to the Earth’s north pole, but not exactly to the geographic north pole. This point is now called magnetic north. Magnetic north moves very slowly, and is currently in the Arctic Ocean, north of Alaska.

The invention of the compass was very important because it allowed people to navigate in places such as oceans and deserts, with less chance of getting lost. With a compass, you will always know what direction you are facing.

Even today with satellite navigation (satnav), ships and aeroplanes still use magnetic compasses.

Satnav systems do not use the Earth’s magnetic field.

Some animals use the Earth’s magnetic field to navigate over long distances.

The diagram shows what the Earth’s magnetic field lines look like compared with a bar magnet.

Notice, in the diagram, that the magnetic field lines around the Earth point towards the Earth’s north pole. You will remember from Topic 9.1 that magnetic field lines point from north to south.

This means that the north pole of Earth is actually a magnetic south pole.

The term magnetic north, when used in context of the Earth and navigation, means the magnetic pole that is close to the geographic north pole.

In the same way, magnetic south is the magnetic pole that is close to the geographic south pole.

The geographic north and south poles are the parts of the Earth through which the spin axis passes. The axis is the imaginary line around which the Earth spins.

The Earth’s magnetic field causes the natural appearance of lights visible in the night sky close to the north and south poles. These are caused by particles coming from the Sun arriving into the stronger parts of the Earth’s magnetic field.

Origin of the Earth’s magnetic field

People once thought that the Earth was made almost entirely from magnetic rocks. However, it is now known that the high temperatures deep inside the Earth would cause rocks to lose any magnetism that they had.

Scientists also know that the Earth’s magnetic field has reversed in the past. The last change was around 500 000 years ago, when north really was north!

It is now known that the Earth’s core is the origin of the magnetic field, but scientists have still to discover the exact reason for this. They think the heat generated in the core, which is mostly made from iron, causes it to continually create a magnetic field. The core also contains some nickel, which is another magnetic metal. The movement in the liquid outer core would explain why the magnetic poles move slowly, and have occasionally reversed. Magnetic north moves at a speed of about 60 km per year.