The time variation (secular variation) of the magnetic field was first recognized in the late 17th century. In attempting to provide a theory to explain it, Halley (1693) (qv) noticed that a large part of the secular variation could be explained in terms of a “westward drift” of the field. If maps of declination are produced over the Earth's surface at different epochs, then there is a clear westward movement of field patterns, particularly lines of zero declination (so‐called agonic lines) (e.g., Langel, 1987). Using his map of Atlantic secular variation, Halley estimated that a full revolution of the field would take about 700 years, giving a rotation rate of just over 0.5° per year. To explain the drift, he posited a model of the interior of the Earth consisting of concentric shells of magnetic material rotating at varying rates with respect to the Earth's surface. In many ways, his theory has remarkable similarities to our current understanding. The Earth does indeed consist of...
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