North Pole, South Pole: The Epic Quest to Solve the Great Mystery of Earth's Magnetism

by Gillian Turner Phd

  Neckam’s reports and dates are backed up by a satirical poem, “Bible,” written by Guyot de Provins, a French poet and monk, around 1205. It includes the passage:

  … there is an art which the sailors have, which cannot deceive. They take an ugly brown stone, the magnet, to which iron willingly attaches itself, and touching a needle with it, they fix the needle in a straw, and float it on the surface of water, whereupon it turns infallibly to the Pole Star.

  Since Thales’ time it had been known that a magnet attracted a piece of iron, while two magnets would attract or repel one another. A compass needle was just a magnet so what attracted or repelled it, and what caused it to rotate into a north–south alignment?

  An interesting explanation emerged. Until the invention of the compass, the heavens had provided the chief means of navigation— the sun by day and the stars by night. It was commonly believed that the Earth lay at the center of Creation, with the moon, sun, stars and known planets—Mercury, Venus, Mars, Jupiter and Saturn— arranged on crystal spheres of increasing size, each of which revolved around the Earth daily. Beyond Creation lived God in his heaven, and beyond this lay infinite space. One star, however, seemed to remain fixed in place, because it lay on the axis about which the celestial sphere of the stars revolved. And, as de Provins pointed out, it was towards this star that the compass needle infallibly turned. Hence, the directivity of the compass came to be attributed to the Pole Star.

  An elaboration of the Pole Star theory appears in a poem by a thirteenth-century Italian, Guido Guinicelli:

  In what strange regions’neath the polar star

  May the great hills of massy lodestone rise,

  Virtue imparting to the ambient air

  To draw the stubborn iron; while afar

  From that same stone, the hidden virtue flies

  To turn quivering needle to the Bear

  In splendour blazing in the northern skies.

  This brief verse captured several important ideas. Although all eventually turned out to be wrong, they marked significant steps in scientific reasoning. The first was the notion that there were lodestone mountains at the Earth’s poles. Since lodestone was the only material Guinicelli knew that attracted a compass needle, he imagined there must be an enormous mass of it at the spot on Earth towards which all compasses were known to point—directly beneath the Pole Star. The notion of gigantic magnetic mountains at the poles spawned fantastic legends: apparently the mountains could even pull iron nails from passing ships.

  The medieval concept of a geocentric universe, as depicted by Peter Apian in Cosmographia, 1524. The moon, inner planets, sun and outer planets were all thought to revolve around the Earth. Beyond these bodies were the stars, the heavens and God himself.

  Also captured in Guinicelli’s verse was the idea that magnetic attraction, or “virtue,” was somehow transported through the air between the lodestone and the compass needle. Was this merely fanciful poetic language or an early glimmer of the concept of magnetic fields?

  Just a few years later the whole way in which men studied nature was to take a new turn, thanks to a little-known Frenchman and his investigation of magnets. Pierre Pèlerin de Maricourt is believed to have been a knight and a crusader. Commonly known as Petrus Peregrinus, or sometimes as Peter the Wanderer, he was also a military engineer, well educated and something of a scholar. In 1269, while serving in the army of Charles d’ Anjou at the siege of Lucera in southern Italy, Peregrinus had found time to reflect on and write about experiments he had earlier carried out. The result was Epistola de Magnete (Letter on the Magnet), dated August 8, 1269. Addressed to Sygerus de Foucaucourt, Peregrinus’s neighbor in Picardy, Epistola de Magnete has been lauded as Europe’s first work of true science.

  Peregrinus had introduced the one crucial element missing from previous scientific endeavors: the idea of learning from experimentation. A contemporary, Friar Roger Bacon, himself a progressive and vigorous proponent of experimental science—he devised methods of making gunpowder, spectacles, mechanical flying-machines, ships, carriages and much more—would describe him as a master of experiment and one of only two perfect mathematicians, the other being one of Bacon’s own students.

  Peregrinus’s most important discovery was magnetic polarity— the distinction between the north and south poles of a magnet. In his experiments he had explored the magnetic properties of a sphere of lodestone, and what happened when a needle or a short iron wire was placed at various points on its surface. He had found two unique locations at which a needle would align itself perpendicularly to the surface of the sphere. These were at opposite ends of a diameter, and the needle was attracted more strongly to these locations than to any others. Further, when the needle was positioned at other places on the sphere its orientations outlined a series of meridians—“just as all the meridian circles on the globe meet in the two opposite poles of the world,” Peregrinus wrote. For him, Earth’s poles represented the axis about which all the celestial spheres— indeed the whole of Creation—revolved. The “world” extended beyond the globe out into space: Earth was merely at its center.

  Peregrinus’s recognition of the similarity between the magnetism of the lodestone sphere and Earth’s magnetism led to the name “terrella,” or “little Earth,” being used to describe a lodestone sphere, and north (seeking) and south (seeking) labels being given to the poles of a magnet.

  Peregrinus was not finished yet. He now showed that whereas magnetic attraction occurred between the north pole of one magnet and the south pole of another, two north poles or two south poles repelled each other. This fundamental law of magnetism, which had eluded the ancient philosophers, had at last been discovered and enunciated: like poles repel, unlike poles attract.

  He then went on to debate why the compass needle should point to the north. First, he argued against the overriding influence of polar lodestone mountains. Even if such mountains existed— and in 1269, since no one had ventured anywhere near the north or south poles, there was no direct evidence either for or against— there were, he pointed out, known lodestone deposits in other places around the globe and these did not influence the compass needle, other than very locally.

  His observations of the symmetrical pattern of the needle’s orientations around his terrella led him to reason that the principal influence acting on the compass needle had to have the same symmetry, be of global proportions, and dominate any irregular influences due to local lodestone deposits or the possible effects of hypothetical lodestone mountains. In summary he wrote:

  … it is from the poles of the world [that is, the universe] that the poles of the magnet receive their virtue.

  The Petrus Peregrinus Medal of the European Geosciences Union, depicting Peregrinus’s sketch of a “continually moving wheel of wonderful ingenuity,” never realized in practice. An inventor of magnetic instruments, this thirteenth-century French knight and scholar is best known for discovering the difference between the north and south poles of a magnet, and that like poles repel one another, while unlike poles attract.

  Peregrinus had also noticed that the Pole Star, which could be seen from the northern hemisphere, was not absolutely fixed at the celestial pole. Instead, it moved very slightly around it—a movement the compass needle failed to follow. He reasoned, therefore, that the magnetic “virtue” of the compass could not be attributed directly to the Pole Star either.

  The final pages of Epistola de Magnete contain Peregrinus’s designs for three magnetic instruments. The first two are floating and pivoted compasses; in 1868 an Italian engineer named Bertelli would construct working instruments using these plans. However, neither he nor Peregrinus were able to make the third instrument work. Like all subsequent attempts to build a perpetual motion machine, Peregrinus’s “continually moving wheel of wonderful ingenuity” failed hopelessly to live up to his expectations. This was, he claimed, “through lack of skill, rather than a defect of nature.” In 2005 the European Geosciences Union honor
ed Peregrinus by creating the Petrus Peregrinus Medal, to be awarded annually for outstanding scientific contributions in magnetism and geomagnetism. In a display of eternal optimism, the medal depicts Peregrinus’s “continually moving wheel of wonderful ingenuity.”

  Peregrinus had certainly come a long way with his concept of Earth’s magnetism as perfectly symmetrical about an axis through the poles. That he meant the celestial poles rather than the poles of Earth does not matter—the effect was the same: wherever a compass needle was put on the surface of his terrella, or little Earth, it would point along the meridian or great circle from south to north. What he did not know was that the Chinese had already recorded discrepancies from this perfect picture.

  Voyages of Discovery

  Who would of his course be sure, when the clouds the sky obscure,

  He an iron needle must in the cork wood firmly thrust. Lest the iron virtue lack rub it with the lodestone black, In a cup with flowing brim let the cork on water swim. When at length the tremor ends, note the way the needle tends;

  Though its place no eye can see—there the polar star will be.

  —WILLIAM THE CLERK, c. 1230

  According to the Renaissance philosopher Francis Bacon, three things, all of them first invented in China, drew the Western world out of the cultural doldrums of the Dark Ages: the compass, gunpowder and the printing press. With the first of these, the compass, came unprecedented opportunities to explore and chart the world’s oceans, and to discover and eventually colonize new lands. Before its advent, mariners had often been forced to navigate along known coastlines.

  However, even with a compass to set a course, it was still difficult to determine a ship’s absolute position in the open ocean. You could find your latitude by measuring the altitude of the midday sun or the Pole Star with an astrolabe, an instrument dating back over two thousand years to the ancient Greek astronomers. However, longitude could be estimated only by dead reckoning— that is, by assuming your speed and direction from a previously known position. To use the stars to determine longitude you needed to know the time at the home port or some other known location, and sufficiently accurate timekeeping was not available. Even well into the seventeenth century it was difficult to avoid significant errors; for example, the positions logged by Abel Tasman when he sailed up the west coast of the North Island of New Zealand in 1642 erred by some two to four degrees of longitude, and put his ship in the center of the island.

  Within a hundred years of Neckam’s first reference to the compass, a series of nautical charts had begun to appear, based on the magnetic compass bearings between different ports. These portolan, or harbor-finding, charts showed only coastlines and coastal ports: details of the hinterland areas were not necessary for navigation at sea. From each port a series of straight “rhumb-lines” radiated out in the magnetic directions of other ports. A mariner setting a course from one port to another simply needed to set his chart to his compass and sail in the direction of the appropriate rhumb-line.

  Of the 130 or so portolan charts that survive today, most were produced in Italy, Catalonia and Portugal between the fourteenth and sixteenth centuries, and covered the Mediterranean Sea and the Atlantic region to Ireland, and the west coast of Africa. They were originally drawn by hand on sheepskin or vellum, but with the advent of line-engraving on paper in the sixteenth century they could be more easily reproduced. Beautifully engraved and colored compass roses became a characteristic feature.

  The shapes of coastlines on portolan charts often appear distorted when compared with modern maps. This is partly because modern map projections compensate for latitude, but also because of the deviation of compass needles from true north. This phenomenon, which came to be called “declination,” is the angular difference between magnetic north and true north—the same phenomenon unwittingly recorded by the Chinese in their street alignments in Shandan. Although declination was not widely recognized or understood by the makers of portolan charts, it did not affect the charts’ accuracy for navigation since both the chart-makers and the sailors worked from the same compass bearings.

  A medieval “portolan” chart of the eastern Mediterranean and Black Sea region, showing compass bearings, or rhumb-lines, between numerous harbors. Sailors simply needed to set their charts to the compass and sail in the appropriate direction. This chart has been attributed to Joan Rizo Oliva, a Catalan chartmaker who lived from 1580 to 1615.

  The realization that more often than not compass needles deviated from true north came gradually in Europe. Peregrinus’s contemporary Roger Bacon apparently referred to the fact that his compasses rarely pointed exactly along the meridian, and from the beginning of the fourteenth century this came to be noticed more and more frequently. Navigators and philosophers began to query whether the direction-seeking properties of lodestones might depend on their source and, if so, whether such differences, or some other systematic error, could be transferred to needles during the magnetization process. Or might the discrepancy simply be due to careless observation?

  Only when these possibilities were satisfactorily refuted was declination accepted as a real feature of Earth’s magnetism—a feature for which allowance had to be made whenever a compass was used for accurate direction-finding. German sundials made in the middle of the fifteenth century often incorporated compasses to help with alignment, and some of these had a mark that accurately indicated the adjustment necessary to allow for declination. Similarly, some road-maps produced in Germany showed an image of a compass in the margin, indicating a declination of 11°15′ E and instructions for setting the map accordingly, exactly as hikers, back packers and boy scouts do today.

  The discovery of declination meant that Peregrinus’s perfectly symmetrical picture of Earth’s magnetism was no longer strictly accurate. In the mid sixteenth century, when declination measurements were still relatively few and came mainly from Europe and the Atlantic region, it seemed the discrepancy was small and might be explained by a slight modification of Peregrinus’s model.

  Gerardus Mercator, a Flemish cartographer, had realized that differences in declination from place to place were also responsible for the curious distortion of coastlines on portolan charts. He figured the effect could be explained by supposing that the magnetic axis of the Earth was tilted with respect to the rotation axis—in other words, if the north and south magnetic poles were offset from the north and south geographic poles. If this were the case, the observed compass directions would still describe a series of “magnetic meridians,” but they would intersect at the magnetic poles, not the geographic poles. He estimated the north magnetic pole to lie at 85° N, at a longitude roughly 180 degrees from the Azore Islands (or 151 degrees east of Greenwich). At this time a compass in the Azore Islands pointed due north, meaning that the declination there was zero. Mercator and certain other map-makers of the time chose this as the prime meridian from which to measure longitude.

  Mercator was soon to discover, however, that his “magnetic meridians” did not intersect at single points at all—the declination was too variable and too complicated for that. In a map of the Arctic region that he drew about 1569, but which was not published until after his death in 1594, Mercator opted for not one but two magnetic north poles, one at the geographic pole and one at approximately 150° E and 75° N. Interestingly, he depicted both poles as magnetic islands. Was Mercator an adherent to the lodestone mountain theory or was he guessing? His mapping of the Arctic region was, after all, pure speculation since next to no polar exploration had actually taken place. California, which had recently been claimed by Spain, appeared north of the Arctic Circle.

  Map of the Septentrionalium Terrarum (Northern Lands or Lands of the Seven Stars) by Gerardus Mercator, published posthumously in 1595. It shows a Rupes Nigra, a black precipice of lodestone, at the north pole and a second magnetic island in the yet-to-be-discovered Bering Strait, as well as many other mythical features.

  By 1500 the Spanish and Portuguese vo
yages of discovery had begun in earnest, with ships sailing east around the Cape of Good Hope and west across the Atlantic Ocean, often making land more through luck than navigational skill. As John Cabot, his son Sebastian Cabot, Christopher Columbus, Vasco da Gama and Ferdinand Magellan sailed the high seas, the question of determining longitude at sea became a serious problem, and one that would persist for several centuries.

  Over this time many possible solutions were put forward, some ingenious, some almost practical, others quite incredible. Proposed solutions generally fell into one of two categories: the first were based on astronomical observation, the second mostly on magnetic declination. Astronomical observation was eventually to prove the more fruitful.

  In principle, longitude, like latitude, could be determined from the stars: you simply needed star charts that referred to a known place—say, your home port—and a clock set accurately to the time at that place. The difference in the positions of the stars between your location and your home port at a particular time would tell you how far around the globe you had traveled and, therefore, the difference in longitude between your location and your home port.

  However, although in theory the method was simple, in practice no one had come up with a clock robust enough to keep time to a sufficient degree of accuracy over long sea voyages. (It would not be until 1773 that a Yorkshire clockmaker, John Harrison, having constructed a rugged and reliable chronometer, would claim the £20,000 prize that had been offered in desperation by the British Board of Longitude some sixty years earlier.)