Transcript: The ancient Greeks knew about loadstones. These were curtain rocks which, when suspended in a fluid, would appear to line themselves in response to a mysterious force. That mysterious force was magnetism, first understood through the experimentation of the physicist Michael Faraday. Now we understand magnetism very well. We understand that the Earth has a magnetic field. The north pole of the Earth's relatively strong magnetic field is not perfectly aligned with the geographic North Pole. Magnetic field is tethered in the iron and nickel solid core, and there's good evidence that occasionally throughout the Earth's history, not on a fixed periodic timescale but every 10 thousand to 100 thousand years, the Earth's core melts and the polarity of the Earth's magnetic field reverses. The projection of the Earth's magnetic field up above the atmosphere into space also causes effects familiar to many people. The interaction with the Earth's magnetic field and solar radiation and particles give rise to the aurora or the northern lights.