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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| History of geomagnetism | 3/4 | https://en.wikipedia.org/wiki/History_of_geomagnetism | reference | science, encyclopedia | 2026-05-05T16:17:55.648373+00:00 | kb-cron |
In the late 1590s Henry Briggs, a professor of geometry at Gresham College in London, had published a table of magnetic inclination with latitude for the earth. It agreed well with the inclinations that Gilbert measured around the circumference of his terrella. Gilbert deduced that the Earth's magnetic field is equivalent to that of a uniformly magnetized sphere, magnetized parallel to the axis of rotation (in modern terms, a geocentric axial dipole). However, he was aware that declinations were not consistent with this model. Based on the declinations that were known at the time, he proposed that the continents, because of their raised topography, formed centers of attraction that made compass needles deviate. He even demonstrated this effect by gouging out some topography on his terella and measuring the effect on declinations. A Jesuit monk, Niccolò Cabeo, later took a leaf from Gilbert's book and showed that, if the topography was on the correct scale for the Earth, the differences between the highs and lows would only be about one tenth of a millimeter. Therefore, the continents could not noticeably affect the declination. The sixth book of de Magnete was devoted to cosmology. He dismissed the prevailing Ptolemaic model of the universe, in which the planets and stars are organized in a series of concentric shells rotating about the Earth, on the grounds that the speeds involved would be absurdly large ("there cannot be diurnal motion of infinity"). Instead, the Earth was rotating about its own axis. In place of the concentric shells, he proposed that the heavenly bodies interacted with each other and Earth through magnetic forces. Magnetism maintained the Earth's position and made it rotate, while the magnetic attraction of the Moon drove the tides. Some obscure reasoning led to the peculiar conclusion that a terella, if freely suspended, would orient itself in the same direction as the Earth and rotate daily. Both Kepler and Galileo would adopt Gilbert's idea of magnetic attraction between heavenly bodies, but Newton's law of universal gravitation would render it obsolete.
=== Guillaume le Nautonier ===
In about 1603, the Frenchman Guillaume le Nautonier (William the Navigator), Sieur de Castelfranc, published a rival theory of the Earth's field in his book Mecometrie de l'eymant (Measurement of longitude with a magnet). Le Nautonier was a mathematician, astronomer and Royal Geographer in the court of Henry IV. He disagreed with Gilbert's assumption that the Earth had to be magnetized parallel to the rotational axis, and instead produced a model in which the magnetic moment was tilted by 22.5° – in effect, the first tilted dipole model. The last 196 pages of his book were taken up with tables of latitudes and longitudes with declination and inclination for use by mariners. If his model had been accurate, it could have been used to determine both latitude and longitude using a combination of magnetic declination and astronomical observations. Le Nautonier tried to sell his model to Henry IV, and his son to the English leader Oliver Cromwell, both without success. It was widely criticized, with Didier Dounot concluding that the work was based on "unfounded assumptions, errors in calculation and data manipulation". However, the geophysicist Jean-Paul Poirier examined the works of both le Nautonier and Dounot, and found that the error was in Dounot's reasoning.
=== Temporal variation ===
One of Gilbert's conclusions was that the Earth's field could not vary in time. This was soon to be proved false by a series of measurements in London. In 1580, William Borough measured the declination and found it to be 111⁄4° NE. In 1622, Edmund Gunter found it to be 5° 56' NE. He noted the difference from Borough's result but concluded that Borough must have made a measurement error. In 1633, Henry Gellibrand measured the declination in the same location and found it to be 4° 05' NE. Because of the care with which Gunther had made his measurements, Gellibrand was confident that the changes were real. In 1635 he published A Discourse Mathematical on the Variation of the Magneticall Needle stating that the declination had changed by more than 7° in 54 years. The reality of geomagnetic secular variation was rapidly accepted in England, where Gellibrand had a high reputation, but in other countries it was met with skepticism until it was confirmed by further measurements. The observations of Gellibrand inspired extensive efforts to determine the nature of variation - global or local, predictable or erratic. It also inspired new models for the origin of the field. Henry Bond Senior gained notoriety by successfully predicting in 1639 that the declination would be zero in London in 1657. His model, which involved a precessing dipole, was strongly criticized by a royal commission, but it continued to be published in navigational instruction manuals for decades. Dynamic models involving multiple poles were also proposed by Peter Perkins (1680) and Edmond Halley (1683, 1692), among others. In Halley's model, the Earth consisted of concentric spheres. Two magnetic poles were on a fixed outer sphere and two more were on an inner sphere that rotated westwards, giving rise to a "westward drift". Halley was so proud of this theory that a portrait of him at the age of eighty included a diagram of it (above right).
== Magnetic navigation ==
Early mariners used portolan charts for navigation. These charts showed coastline with windrose lines connecting ports. A mariner could navigate by aligning the chart with a compass and following the compass heading. Early charts had distorted coastlines because the cartographers did not know about declination, but the charts still worked because mariners were sailing in straight lines. An accurate determination of longitude was necessary to determine the proper "magnetic declination", that is, the difference between indicated magnetic north and true north, which can differ by up to 10 degrees in the important trade latitudes of the Atlantic and Indian Oceans.