A compass in New York points about 15 degrees west of true north; in California, it points about 15 degrees east of true north. Magnetic declination also varies with time. What are some possible explanations for magnetic declination?
closed as off-topic by probably_someone, StephenG, Martin Beckett, garyp, John Rennie Dec 11 '18 at 8:51
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The Earth's magnetic field is generated by a complicated process that involves convective motion of molten materials hundreds of miles below the surface, as well as by fields "frozen" into rocks that comprise tectonic plates. Convection is a turbulent process. Tectonic plates move more or less randomly around on the surface of the Earth at the rate of ~ 10 cm per year, carrying their "frozen" magnetic fields with them. As a result, the Earth's magnetic field is definitely not a simple dipole field. The dipole component is not very well aligned with the rotational axis of the Earth, and even wanders somewhat randomly. This Scientific American article gives a good presentation of the complexity of the processes that cause changes in the Earth's magnetic field.