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Geomagnetic Secular Variation: Direction and Intensity

What Constitutes Secular Variation?

The secular variation of the geomagnetic field denotes the gradual change with time of the Earth's magnetic field, measured on a timescale of months to thousands of years. Among the longest records we have of direct observations of the geomagnetic field are those from London and Paris dating from the mid sixteenth century. The record at Greenwich, London is illustrated in Fig. 1. It was fortunate that when the earliest measurements were being made in Europe, the amplitude of the secular variation was unusually large. If the study of terrestrial magnetism had developed in Australia, for example, where directional changes have been much smaller (Fig. 2), the secular variation would have passed unnoticed for much longer. An account of the discovery of secular variation is given in Geomagnetism: Historical Introduction.
FIGURE 1 Direction of the Earth's