History of geomagnetism - AbsoluteAstronomy.com

The history of geomagnetism is concerned with the history of the study of Earth's magnetic field

Earth's magnetic field

Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...

. It encompasses the history of navigation using compass

Compass

A compass is a navigational instrument that shows directions in a frame of reference that is stationary relative to the surface of the earth. The frame of reference defines the four cardinal directions – north, south, east, and west. Intermediate directions are also defined...

es, studies of the prehistoric magnetic field (archeomagnetism and paleomagnetism

Paleomagnetism

Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of...

), and applications to plate tectonics

Plate tectonics

Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...

.

Magnetism has been known since prehistory, but knowledge of the Earth's field developed slowly. The horizontal direction of the Earth's field was first measured in the fourth century BC but the vertical direction was not measured until 1544 AD and the intensity was first measured in 1791. At first compasses were thought to point towards locations in the heavens, then magnetic mountains. A modern experimental approach to understanding the Earth's field began with de Magnete

De Magnete

De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure is a scientific work published in 1600 by the English physician and scientist William Gilbert and his partner Aaron Dowling...

, a book published by William Gilbert in 1600. His experiments with a magnetic model of the Earth convinced him that the Earth itself is a large magnet.

Early ideas on magnetism

Knowledge of the existence of magnetism probably dates back to the prehistoric development of iron smelting

Smelting

Smelting is a form of extractive metallurgy; its main use is to produce a metal from its ore. This includes iron extraction from iron ore, and copper extraction and other base metals from their ores...

. Iron can be obtained on the Earth's surface from meteorite

Meteorite

A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...

s; the mineral lodestone

Lodestone

A lodestone or loadstone is a naturally magnetized piece of the mineral magnetite. They are naturally occurring magnets, that attract pieces of iron. Ancient people first discovered the property of magnetism in lodestone...

is rich in the magnetic mineral magnetite

Magnetite

Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...

and can be magnetized by a lightning strike. In his Natural History, Pliny the Elder

Pliny the Elder

Gaius Plinius Secundus , better known as Pliny the Elder, was a Roman author, naturalist, and natural philosopher, as well as naval and army commander of the early Roman Empire, and personal friend of the emperor Vespasian...

recounts a legend about a shepherd called Magnes on the island of Crete

Crete

Crete is the largest and most populous of the Greek islands, the fifth largest island in the Mediterranean Sea, and one of the thirteen administrative regions of Greece. It forms a significant part of the economy and cultural heritage of Greece while retaining its own local cultural traits...

whose iron-studded boots kept sticking to the path. The earliest ideas on the nature of magnetism are attributed to Thales

Thales

Thales of Miletus was a pre-Socratic Greek philosopher from Miletus in Asia Minor, and one of the Seven Sages of Greece. Many, most notably Aristotle, regard him as the first philosopher in the Greek tradition...

( BC – BC).

In classical antiquity

Classical antiquity

Classical antiquity is a broad term for a long period of cultural history centered on the Mediterranean Sea, comprising the interlocking civilizations of ancient Greece and ancient Rome, collectively known as the Greco-Roman world...

, little was known about the nature of magnetism. No sources mention the two poles of a magnet or its tendency to point northward. There were two main theories about the origins of magnetism. One, proposed by Empedocles of Acragas and taken up by Plato

Plato

Plato , was a Classical Greek philosopher, mathematician, student of Socrates, writer of philosophical dialogues, and founder of the Academy in Athens, the first institution of higher learning in the Western world. Along with his mentor, Socrates, and his student, Aristotle, Plato helped to lay the...

and Plutarch

Plutarch

Plutarch then named, on his becoming a Roman citizen, Lucius Mestrius Plutarchus , c. 46 – 120 AD, was a Greek historian, biographer, essayist, and Middle Platonist known primarily for his Parallel Lives and Moralia...

, invoked an invisible effluvium seeping through the pores of materials; Democritus of Abdera replaced this effluvium by atoms, but the mechanism was essentially the same. The other theory evoked the metaphysical

Metaphysics

Metaphysics is a branch of philosophy concerned with explaining the fundamental nature of being and the world, although the term is not easily defined. Traditionally, metaphysics attempts to answer two basic questions in the broadest possible terms:...

principle of sympathy between similar objects. This was mediated by a purposeful life force that strove toward perfection. This theory can be found in the writings of Pliny the Elder and Aristotle

Aristotle

Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

, who claimed that Thales

Thales

attributed a soul to the magnet. In China, a similar life force, or qi
Qi
In traditional Chinese culture, qì is an active principle forming part of any living thing. Qi is frequently translated as life energy, lifeforce, or energy flow. Qi is the central underlying principle in traditional Chinese medicine and martial arts...

, was believed to animate magnets, so the Chinese used early compasses for feng shui

Feng shui

Feng shui ' is a Chinese system of geomancy believed to use the laws of both Heaven and Earth to help one improve life by receiving positive qi. The original designation for the discipline is Kan Yu ....

.

Little changed in the view of magnetism during the Middle Ages

Middle Ages

The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

, and some classical ideas lingered until well after the first scientific experiments on magnetism. One belief, dating back to Pliny, was that fumes from eating garlic and onions could destroy the magnetism in a compass, rendering it useless. Even after William Gilbert disproved this in 1600, there were reports of helmsmen on British ships being flogged for eating garlic. However, this belief was far from universal. In 1558 Giambattista della Porta

Giambattista della Porta

Giambattista della Porta , also known as Giovanni Battista Della Porta and John Baptist Porta, was an Italian scholar, polymath and playwright who lived in Naples at the time of the Scientific Revolution and Reformation....

reported "When I enquired of mariners whether it were so that they were forbid to eat onyones and garlick for that reason, they said they were old wives fables and things ridiculous, and that sea-men would sooner lose their lives then abstain from eating onyons and garlick."

Measurement of the field

At a given location, a full representation of the Earth's magnetic field requires a vector with three coordinates. These can be Cartesian

Cartesian coordinate system

A Cartesian coordinate system specifies each point uniquely in a plane by a pair of numerical coordinates, which are the signed distances from the point to two fixed perpendicular directed lines, measured in the same unit of length...

(North, East and Down) or spherical

Spherical coordinate system

In mathematics, a spherical coordinate system is a coordinate system for three-dimensional space where the position of a point is specified by three numbers: the radial distance of that point from a fixed origin, its inclination angle measured from a fixed zenith direction, and the azimuth angle of...

with two angles (horizontal and vertical) plus the magnitude, or intensity. In China, the horizontal direction was measured as early as the fourth century BC, and its deviation from true north

True north

True north is the direction along the earth's surface towards the geographic North Pole.True geodetic north usually differs from magnetic north , and from grid north...

(or declination

Declination

In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...

) first recognized in 1088. In Europe, this was not widely accepted until the middle of the fifteen century AD. The vertical angle (magnetic dip

Magnetic dip

Magnetic dip or magnetic inclination is the angle made by a compass needle with the horizontal at any point on the Earth's surface. Positive values of inclination indicate that the field is pointing downward, into the Earth, at the point of measurement...

, or inclination) was first measured in 1544 AD. The intensity was not measured until 1791, after advances in the understanding of electromagnetism

Electromagnetism

Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...

Declination

The magnetic compass

Compass

existed in China back as far as the fourth century BC. It was used as much for feng shui

Feng shui

as for navigation

Navigation

Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...

on land. It was not until good steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

needles could be forged that compasses were used for navigation at sea; before that, they could not retain their magnetism

Magnetism

Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...

for long. The existence of magnetic declination, the difference between magnetic north and true north, was first recognized by Shen Kuo

Shen Kuo

Shen Kuo or Shen Gua , style name Cunzhong and pseudonym Mengqi Weng , was a polymathic Chinese scientist and statesman of the Song Dynasty...

in 1088.

The first mention of a compass in Europe was in 1190 AD by Alexander Neckham. He described it as a common navigational aid for sailors, so the compass must have been introduced to Europe some time earlier. Whether the knowledge came from China to Europe, or was invented separately, is not clear. If the knowledge was transmitted, the most likely intermediary was Arab merchants, but Arabic literature does not mention the compass until after Neckham. There is also a difference in convention: Chinese compasses point south while European compasses point north.

In 1269, Pierre de Maricourt (commonly referred to as Petrus Peregrinus) wrote a letter to a friend in which he described two kinds of compass, one in which an oval lodestone floated in a bowl of water, and the first dry compass with the needle mounted on a pivot. He also was the first to write about experiments with magnetism and describe the laws of attraction. An example is the experiment where a magnet is broken into two pieces and the two pieces can attract and repel each other (in modern terms, they both have north and south poles). This letter, generally referred to as Epistola de Magnete, was a landmark in the history of science.

Early mariners used portolan charts for navigation. These charts showed coastline with rhumb line

Rhumb line

In navigation, a rhumb line is a line crossing all meridians of longitude at the same angle, i.e. a path derived from a defined initial bearing...

s 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 know about declination, but the charts still worked because mariners were sailing in straight lines.

Petrus Peregrinus assumed that compasses point towards true north. While his contemporary Roger Bacon

Roger Bacon

Roger Bacon, O.F.M. , also known as Doctor Mirabilis , was an English philosopher and Franciscan friar who placed considerable emphasis on the study of nature through empirical methods...

is reputed to observe that compasses deviated from true north, the idea of magnetic declination was only gradually accepted. At first it was thought that the declination must be the result of systematic error. However, by the middle of the fifteenth century, sundials in Germany were oriented using corrections for declination.

Inclination

A compass must be balanced to counter the tendency of the needle to dip in the direction of the Earth's field. Otherwise, it will not spin freely. Compasses that are balanced for one latitude often do not work as well when they are taken to a different latitude. This problem was first reported by Georg Hartmann

Georg Hartmann

Georg Hartmann was a German engineer, instrument maker, author, printer, humanist, churchman, and astronomer....

, a vicar in Nuremberg, in 1544. Robert Norman

Robert Norman

Robert Norman was a 16th century-English mariner, compass builder, and hydrographer who discovered magnetic inclination, the deviation of the Earth's magnetic field from the vertical.- Work :...

was the first to recognize that this occurs because the Earth's field itself is tilted from the vertical. In his book The Newe Attractive, Norman called inclination "a newe discouered secret and subtil propertie concernyng the Declinyng of the Needle." He created a compass in which the needle was floated in a goblet of water, attached to a cork to make it neutrally buoyant. The needle could orient itself in any direction, so it dipped to align itself with the Earth's field. Norman also created a dip circle

Dip circle

Dip circles are used to measure the angle between the horizon and the Earth's magnetic field . They were used in surveying, mining and prospecting as well as for the demonstration and study of magnetism....

, a compass needle pivoted about a horizontal axis, to measure the effect.

Early ideas about the source

In early attempts to understand the Earth's magnetic field, measuring it was only part of the challenge. Understanding the measurements was also difficult because the mathematical and physical concepts had not yet been developed – in particular, the concept of a vector field

Vector field

In vector calculus, a vector field is an assignmentof a vector to each point in a subset of Euclidean space. A vector field in the plane for instance can be visualized as an arrow, with a given magnitude and direction, attached to each point in the plane...

that associates a vector with each point in space. The Earth's field is generally represented by field line

Field line

A field line is a locus that is defined by a vector field and a starting location within the field. Field lines are useful for visualizing vector fields, which are otherwise hard to depict...

s that run from pole to pole; the field at any point is parallel to a field line but does not have to point at either pole. As late as the eighteenth century, however, a natural philosopher would believe that a magnet had to be pointing directly at something. Thus, the Earth's magnetic field had to be explained by localized sources, and as more was learned about the Earth's field, these sources became increasingly complex.

At first, in both China and Europe, the source was assumed to be in the heavens – either the celestial pole

Celestial pole

The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the imaginary rotating sphere of stars called the celestial sphere...

s or the Pole star

Pole star

The term "Pole Star" usually refers to Polaris, which is the current northern pole star, also known as the North Star.In general, however, a pole star is a visible star, especially a prominent one, that is approximately aligned with the Earth's axis of rotation; that is, a star whose apparent...

. These theories required that magnets point at (or very close to) true north

True north

True north is the direction along the earth's surface towards the geographic North Pole.True geodetic north usually differs from magnetic north , and from grid north...

, so they ran into difficulty when the existence of declination

Declination

was accepted. Then natural philosphers began to propose earthly sources such as a rock or mountain.

Legends about magnetic mountains go back to the classical era. Ptolemy

Ptolemy

Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

recounted a legend about magnetic islands (now thought to be near Borneo

Borneo

Borneo is the third largest island in the world and is located north of Java Island, Indonesia, at the geographic centre of Maritime Southeast Asia....

) that exerted such a strong attraction on ships with nails that the ships were held in place and could not move. Even more dramatic was the Arab legend (recounted in One Thousand and One Nights) that a magnetic mountain could pull all the nails out of a ship, causing the ship to fall apart and founder. The story passed to Europe and became part of several epic tales.

Europeans started to place magnetic mountains on their maps in the sixteenth century. A notable example is Gerardus Mercator

Gerardus Mercator

thumb|right|200px|Gerardus MercatorGerardus Mercator was a cartographer, born in Rupelmonde in the Hapsburg County of Flanders, part of the Holy Roman Empire. He is remembered for the Mercator projection world map, which is named after him...

, whose famous maps included a magnetic mountain or two near the North Pole. At first, he just placed a mountain in an arbitrary location; but later he attempted to measure its location based on declinations from different locations in Europe. When subsequent measurements resulted in two contradictory estimates for the mountain, he simply placed two mountains on the map.

Beginnings of modern science

William Gilbert

1600 was a notable year for William Gilbert. He became president of the Royal College of Physicians of London, was appointed personal physician for Queen Elizabeth I

Elizabeth I of England

Elizabeth I was queen regnant of England and Ireland from 17 November 1558 until her death. Sometimes called The Virgin Queen, Gloriana, or Good Queen Bess, Elizabeth was the fifth and last monarch of the Tudor dynasty...

, and wrote De Magnete
De Magnete
De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure is a scientific work published in 1600 by the English physician and scientist William Gilbert and his partner Aaron Dowling...

, one of the books that mark the beginning of modern science. De Magnete is most famous for introducing (or at least popularizing) an experimental approach to science and deducing that the Earth is a great magnet.

Gilbert's book is divided into six chapters. The first is an introduction in which he discusses the importance of experiment and various facts about the Earth, including the insignificance of surface topography compared to the radius of the Earth

Earth radius

Because the Earth is not perfectly spherical, no single value serves as its natural radius. Distances from points on the surface to the center range from 6,353 km to 6,384 km...

. He also announces his deduction that the Earth is a great magnet. In book 2, Gilbert deals with "coition", or the laws of attraction. Gilbert distinguishes between magnetism and static electricity

Electrostatics

Electrostatics is the branch of physics that deals with the phenomena and properties of stationary or slow-moving electric charges....

(the latter being induced by rubbing amber

Amber

Amber is fossilized tree resin , which has been appreciated for its color and natural beauty since Neolithic times. Amber is used as an ingredient in perfumes, as a healing agent in folk medicine, and as jewelry. There are five classes of amber, defined on the basis of their chemical constituents...

) and reports many experiments with both (some dating back to Peregrinus). One involves breaking a magnet in two and showing that both parts have a north and south pole. He also dismisses the idea of perpetual motion

Perpetual motion

Perpetual motion describes hypothetical machines that operate or produce useful work indefinitely and, more generally, hypothetical machines that produce more work or energy than they consume, whether they might operate indefinitely or not....

. The third book has a general description of magnetic directions along with details on how to magnetize a needle. He also introduces his terella, or "little Earth". This is a magnetized sphere that he uses to model the magnetic properties of the Earth. In chapters 4 and 5 he goes into more detail about the two components of the direction, declination and inclination.

In the late 1590's Henry Briggs

Henry Briggs (mathematician)

Henry Briggs was an English mathematician notable for changing the original logarithms invented by John Napier into common logarithms, which are sometimes known as Briggsian logarithms in his honour....

, a professor of geometry at Gresham College

Gresham College

Gresham College is an institution of higher learning located at Barnard's Inn Hall off Holborn in central London, England. It was founded in 1597 under the will of Sir Thomas Gresham and today it hosts over 140 free public lectures every year within the City of London.-History:Sir Thomas Gresham,...

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 terella. 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

Topography

Topography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...

, 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

Niccolo Cabeo

Niccolò Cabeo was an Italian Jesuit philosopher, theologian, engineer and mathematician.-Biography:He was born in Ferrara in 1586, and was educated at the Jesuit college in Parma beginning in 1602...

, 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

Cosmology

Cosmology is the discipline that deals with the nature of the Universe as a whole. Cosmologists seek to understand the origin, evolution, structure, and ultimate fate of the Universe at large, as well as the natural laws that keep it in order...

. 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

Diurnal cycle

A diurnal cycle is any pattern that recurs every 24 hours as a result of one full rotation of the Earth.In climatology, the diurnal cycle is one of the most basic forms of climate patterns. The most familiar such pattern is the diurnal temperature variation...

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 Earths position and made it rotate, while the magnetic attraction of the Moon

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

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

Johannes Kepler

Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...

and Galileo

Galileo Galilei

Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

would adopt Gilbert's idea of magnetic attraction between heavenly bodies, but Newton's law of universal gravitation

Newton's law of universal gravitation

Newton's law of universal gravitation states that every point mass in the universe attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them...

would render it obsolete.

Guillaume le Nautonier

In about 1603, the Frenchman Guillaume le Nautonier (William the Navigator), Sieur de Castelfranc

Castelfranc

Castelfranc is a commune in the Lot department in south-western France....

, 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

Henry IV of France

Henry IV , Henri-Quatre, was King of France from 1589 to 1610 and King of Navarre from 1572 to 1610. He was the first monarch of the Bourbon branch of the Capetian dynasty in France....

. 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 – 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

Oliver Cromwell

Oliver Cromwell was an English military and political leader who overthrew the English monarchy and temporarily turned England into a republican Commonwealth, and served as Lord Protector of England, Scotland, and Ireland....

, 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

William Borough

William Borough was a British naval officer and the younger brother of Stephen Borough. He participated in the British attack on Cádiz in 1587...

measured the declination and found it to be 11° NE. In 1622, Edmund Gunter

Edmund Gunter

Edmund Gunter , English mathematician, of Welsh descent, was born in Hertfordshire in 1581.He was educated at Westminster School, and in 1599 was elected a student of Christ Church, Oxford. He took orders, became a preacher in 1614, and in 1615 proceeded to the degree of bachelor in divinity...

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

Henry Gellibrand

Henry Gellibrand was an English mathematician. He is known for his work on the Earth's magnetic field. He discovered that magnetic declination – the angle of dip of a compass needle – is not constant but changes over time...

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.

Age of sail

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