Earth's magnetic field, though observed by humans for centuries, has been explained only recently by science. Most of the field is the result of electricity flowing through the molten metal outer core of the Earth as it rotates. The remainder is caused by the interaction of the main magnetic field and the solar wind and by electric currents in the ionosphere. As a result of the magnetic field, the Earth acts like a permanent bar magnet with its poles tilted eleven degrees away from its axis of rotation.

Although the Chinese did not understand the nature of the Earth 's magnetic field, they were the first to use it. They devised a compass with magnetized needle suspended by a thread. They did notice, however, that their compass needles did not exactly parallel a given geographic meridian and pointed slightly away from true north, a...