The Earth has a magnetic field. Not much surprise there, it has been known since William Gilbert did pioneering experiments in the 1580s and published his book De Magnete (1600). He saw Earth as a permanent bar magnet with positive and negative poles. The field surrounding such a magnet is thus termed a dipole field. Of course, Gilbert would not understand what we mean by this sentence because the conceptual model of a continuous field of force was developed only in the 1800s. Neither could Gilbert know the Earth's magnetic field is not that of a permanent magnet, but is continuously regenerated, slightly asymmetric, and ever changing.

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