Background on Earth and Magnetism


What are magnets and what is a magnetic field?

Ancient Greeks were the first to discover that certain stones obtained from the city of Magnesia attracted certain metals, an amusing phenomenon the origin of which was a mystery to the Greeks. These stones were magnets very similar to the ones we put on our refrigerators. Today we know that the magnets generate something called a magnetic field everywhere. Two magnets that do not even touch other can interact with each other via magnetic fields. We live in an environment not only filled with atoms and molecules but also filled with magnetic and other fields. Even though these fields don't interfere with our everyday life, they exist and we can see them using a compass.

Why do compasses point towards the Magnetic North Pole?

Earth has a magnetic field that is produced by its core. The core is believed to contain liquid iron, which is at high temperatures and high pressures. We have a limited understanding of earth's magnetism, as we cannot generate conditions similar to that of earth's core in a laboratory. However, the magnetic field generated by earth's core can easily be detected using a compass. A compass is a simple navigational tool made out of a needle-shaped magnet, which is freely suspended. The needles are the directed towards the north magnetic pole due to the earth's magnetic field. Note that the magnetic north pole is different than the true north pole, which is defined as the point where earth's rotational axis intersects earth's surface. The angle between magnetic north and the true north direction is called magnetic declination.

Was the Earth's magnetic field always the way it is now?

No. For reasons that we do not understand, the position of the magnetic north pole and therefore the magnetic declination angle changes over time. The Buddhist astronomer Yi-Xing first measured the magnetic declination in China about AD 720. The position of the North magnetic field has been closely monitored in the last hundred years and has been shown to be moving with an average speed of 10 km per hour. This is a very significant motion; for example, due to this motion the magnetic declination in Seattle changed by 1° in the last 10 years. We have also biological and geophysical evidence that the Earth's magnetic field has reversed many times.