![]() ![]() Given a measure of the field strength and many measurements of the field’s direction, the magnetic moment can be well defined. Instead, he found a century-old and somewhat obscure theory that links the intensity of a magnetic field to another property called its dipole moment, a quantity related to the strength of the poles and the distance between them. So, by itself, paleointensity data cannot answer the question.īut Gubbins wasn’t deterred. That’s more than enough to swamp any effect from a change in Earth’s field. ”The intensity measurements have typical errors of about 10 percent,” says Gubbins, a professor at the University of Leeds, in the north of England. The trouble is that measuring a trapped field is hugely difficult. These kinds of measurements have been made at 315 sites that date from the period between 15, creating a database of ”paleointensity” during that period. Measuring the trapped field gives you an indication of the field intensity when the rock solidified. The results have allowed Gubbins to build a remarkable picture of the behavior of Earth’s magnetic field in the centuries before detailed measurements were possible.īut how can the magnetic field be measured in retrospect? One important clue is that the field can become trapped when molten rock solidifies-in a volcanic flow, for example. Now British geophysicist David Gubbins and his colleagues have an answer from the most unlikely quarter: data hidden in the logbooks of ships that navigated the planet’s oceans in the 16th, 17th, and 18th centuries. Has Earth’s field been in a spiral of decay for longer than that? Or do we happen to live in a period when the decline is particularly striking? Since then, the field has decayed at the startling rate of about 5 percent per century. Earth’s magnetic field has been monitored carefully since the 1830s, when the German polymath Karl Friedrich Gauss invented a way to measure its intensity.
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