Earth's magnetic field

"... You can imagine the Earth as an enormous battery whose magnetic field is formed by the movement of molten metal (caused by the Earths rotation) around the solid metal inner core. At present the positive terminal is at the , but in times past the orientation has switched so that the positive terminal is at the . Over the past 76 million years the Earths magnetic field has reversed 171 times. No-one knows exactly why this happens, but at present the magnetic field is weakening a little each decade. Many researchers think that this presages another shift. Just how long before the reversal happens, however, is unknown."

"Since the development of simple compasses, the Earths magnetic field has been an important aid for navigation, enabling travelers to find their way in uncharted regions. It became an essential tool in the exploration of the planet and facilitated the great voyages of discovery in the late 15th century that established links between the continents. It is one of the central properties of the Earth and has been studied with purpose-built equipment on land, at sea, and in the past half-century from satellites. The magnetic field extends far into space around the planet, where it interacts with the energetic particles from the Sun and interstellar space that impact on the Earth. It produces sideways forces on the rapidly moving, electrically charged particles and deflects them around the planet. In this way, the magnetic field acts as a protective shield against harmful radiation and enables life to exist on the planet."