Main Field Modeling
The history of modeling the geomagnetic field stretches back four centuries, to William Gilbert's De Magnete, the first book on geomagnetism. His assertion, that the Earth is a great magnet, can be regarded as the first model of the geomagnetic field. The development of geomagnetic field models is explained by the need to draw magnetic maps. The practical reasons were mainly connected with navigation: the use of the magnetic compass and portable sundials that included a compass needle, the problem of determining longitude at sea, and also the construction of road maps on land. These practical applications in turn provided the motivation to seek a more fundamental understanding of the geomagnetic field.
Before modeling the geomagnetic field, some fundamental properties of the field had to be discovered, like the existence of magnetic variation or declination, the spatial variation of declination, the existence of magnetic inclination or dip, and the secular variation. The history and...
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