Measurements of the magnetic field at the earth’s surface can tell only so much about the dynamo producing it in the planet’s core. Geophysicists have now used precise astronomical position data to calculate tidal damping in the core and determine for the first time the magnetic field in the center of the planet. The magnetic field strength is 25 Gauss, or 50 times stronger than the magnetic field at the surface that makes compass needles align north-south.