Mapping Magnetism

The physical phenomenon known as magnetism occurs when the movement of an electric charge results in attractive and repulsive forces between objects.  In 77 CE, Roman author, naturalist, and military commander Gaius Plinius Secundus (Pliny the Elder) wrote in his encyclopedia Naturalis Historia (Natural History) of the origins of the discovery of natural magnetism attributed to earlier 2nd century BC Greek poet, physician, and grammarian Nicander of Colophon.  Nicander describes the legend of Magnes the shepherd who discovered magnetic rocks on Mount Ida, to which the nails of his shoes and the ferrule (metal cap) of his staff adhered.  In 1088 CE, Chinese polymath, scientist, and statesman Shen Kuo in his Dream Pool Essays was the first to describe the magnetic needle compass and magnetic declination, which is the angular difference between geographic or true north and magnetic north. The geographic north pole is a fixed position at the northern end of Earth’s axis of rotation, and the magnetic north pole is a slowly moving point (about a 30-mile shift per year).

The use of the compass for navigation spread from China to Europe by the end of the 12th century, with Christopher Columbus rediscovering magnetic declination while sailing to the new world in 1492.  Magnetic north had been attributed to different forces such as the north star or mountains. Flemish cartographer Gerhard Mercator depicted magnetic mountains on what was to be the first published map of the Arctic and North Pole, which was an inset on his world map published in 1569.  An enlarged second edition of his Arctic map titled Septentrionalium Terrarum (shown below) was posthumously published in Mercator’s Atlas of 1595.  It shows four islands divided by four rivers at the center of which was an island labeled “Rupus Nigra et Altissima” which translates to “very high black cliff.”

In a 1577 letter to English mathematician John Dee explaining his sources for the map, Mercator describes the fabled Rupus Nigra et Altissima as “right under the Pole there lies a bare rock in the midst of the Sea. Its circumference is almost 33 French miles, and it is all of magnetic stone.”  Mercator also depicts an additional magnetic pole/rock in Septentrionalium Terrarum north of the Straight of Anian (what became the Bering Strait after further exploration) along the 180th meridian, suggesting his knowledge of magnetic declination.

In his 1600 book De Magnete, English physicist, physician, and philosopher William Gilbert proposed the theory that the source of the magnetic field that pulled the compass needle north was the earth itself.  He argued that the earth’s core was iron, turning it into a giant magnet.  However, Gilbert erroneously concluded that magnetic variation (declination) does not vary over time, which was later proven by English mathematician Henry Gellibrand in 1633 after comparing his measurements of declination in London to earlier measurements that showed a shift of 7 degrees in 54 years (published in his work A Discourse Mathematical on the Variation of the Magneticall Needle). 

This theory of geomagnetic secular variation inspired further scientific investigations and models to explain it.  English mathematician, physicist, and astronomer Edmond Halley proposed an interesting concentric spheres theory/model in 1692 in which the earth was made of several independent rotating hollow spheres to explain its geomagnetic variations.  Halley also produced a 1701 map of magnetic variation (shown below) of the Atlantic Ocean after spending the previous two years aboard the ship Paramour on a scientific expedition sponsored by King William III.  Halley’s was the first published map to use isolines.  In this case, it showed lines of equal magnetic declination (isogonal lines or when the declination is zero, agonic lines), but other subsequently published maps used isolines to represent more familiar continuous phenomenon such as temperature (isotherms) and elevation (contour lines).

Further scientific expeditions were launched to not only to map magnetic declination around the world, but to map the location of the shifting magnetic poles themselves.  Between 1829 and 1833, John Ross commanded an Arctic expedition funded by wealthy British gin distiller Felix Booth in which Ross’s nephew, James Clark Ross led a party in 1831 that located the north magnetic pole.  The 1856 Chart of Magnetic Curves of Equal Variation by Peter Barlow below shows not only an expanded mapping of declination across the world using Halley’s isogonic lines, but the location of the 1831 discovery by James Ross labeled “Magnetic Pole” on the (now Canadian territory of Nunavut) peninsula labeled Boothia Felix (named after the patron who funded the trip).  As luck would have it, the magnetic north pole happened to be on land during this time period (it is now in the Arctic Ocean).

Although James Clark Ross completed another expedition to Antarctica between 1840 and 1843, the South Magnetic Pole was just inferred and charts were improved upon earlier expeditions from the American Charles Wilkes (1840) and French explorer Jules Dumont d’Urville (1838).  However, the South Magnetic Pole was not reached, or at least nearly reached until 1909 by a magnetic polar party of three men under an expedition by British explorer Ernest Shackelton (see map below).  Like the North Magnetic Pole, the South Magnetic Pole happened to be on land at this time (it is now in the Southern Ocean).

The magnetic compass and associated maps and charts, which had been very useful land and sea navigational aids for centuries, continued to find usefulness with the advent of aviation in the early twentieth century.  The magnetic declination world map below by the U.S. Hydrographic Office is annotated by the first aviator to make a nonstop solo transatlantic flight.  A note at the bottom right of the map reads: “Used in laying out route for flight from San Diego to St. Louis to New York to Paris, 1927” which is signed C.A. Lindbergh.  The delineated route is also shown on the map with segments from San Diego to St. Louis, to New York, and finally to Paris.  Knowledge of magnetic declination would have been useful to Lindberg to keep his plane flying in the right direction.

Magnetic declination on larger scale maps topographic maps used for land navigation typically aren’t represented by isogonal lines, but rather with a map surround graphic showing the declination angle and value for the map location and date it was published.  Below are two examples from the United States Geological Survey 1:24,000 scale map of Sherburne, New York for 1943 and 1994.  The 1943 map graphic shows (lower left corner) a magnetic declination of 11.5 degrees compared to 13 degrees for the 1994 map- a difference of 1.5 degrees change in magnetic declination over a 51-year period.

Declination and its shifting locations is the main and perhaps the most useful magnetic measurement on maps, but there are other magnetic measurements which are included on maps such as inclination/dip angle and intensity (see maps below).  The examples below come from a series of United States magnetic maps published mainly by the Coast and Geodetic Survey about every 5 years from 1874 to 1965 located in the Geography and Map Division’s title collection.

This sampling of the mapping of geomagnetism provides just a glimpse of the content that can be found in the Geography and Map Division and wider Library.  As the poles continue to shift, the need for updated scientific information about earth’s magnetic field will continue to be needed and the Library will continue to collect it.