Halley’s Magnetic Declination Map

Reprinted from Earth In Vision with permission by the author Fábio Rodrigues

Edmond Halley, 1701

Edmond Halley, 1701

Edmund Haley (8th November 1656 – 14th January 1742) was a British scientist who dedicated his life to Physics, Astronomy, Mathematics and Geosciences. His contribution to the scientific community was diverse, he had noticed and observed the Mercury’s Transit (from Earth, Mercury passes in front of the Sun) and deduced the same happens to Venus and not with other planets since Venus and Mercury are in between Earth and the Sun. He had published several papers about the monsoons, identifying the causes, also studied the atmospheric pressure differentials, being publish on charts. Also he made great advances on what is known today as the compass.

It was on this construction and conception process that this map was made, which shows the magnetic declination which was conceptualized by observations on two journeys on board of HMS Paramore (Murray 2012)

The methodology wasn’t publish, so little is known about the construction of the map just what could be read on Haley’s diaries and journals, but is known that it was made by observations. The main point of reference was London, and some researches have tried to reconstruct the methodology in other to understand how he made it.

Before going on, let’s just clarify what is magnetic declination. “The differences between the geographic north pole and the magnetic north pole” (compassdude.com) being the geographic pole the axis where the planet rotates and the magnetic poles the final of the magnetic pole.

The Earth acts like a great spherical magnet, in that it is surrounded by a magnetic field. This magnetic field changes both with time and with location on the Earth and resembles, in general, the field generated by a dipole magnet (i.e., a straight magnet with a north and south pole) located at the center of the Earth. The axis of the dipole is offset from the axis of the Earth’s rotation by approximately 11 degrees. This means that the north and south geographic poles and the north and south magnetic poles are not located in the same place. At any point and time, the Earth’s magnetic field is characterized by a direction and intensity which can be measured. Often the parameters measured are the magnetic declination, D, the horizontal intensity, H, and the vertical intensity, Z. From these elements, all other parameters of the magnetic field can be calculated.

“The compass points in the directions of the horizontal component of the magnetic field where the compass is located, and not to any single point. Knowing the magnetic declination (the angle between true north and the horizontal trace of the magnetic field) for your location allows you to correct your compass for the magnetic field in your area. A mile or two away the magnetic declination may be considerably different, requiring a different correction. NGDC has an on-line magnetic declination calculator where you can enter your location (or zip code for the USA) and get the Declination value.”


Haley realized, by observation, on his journeys, the changing on the position of the needle which made him portraying those changes on a map, as a beginning of Data Visualization, which each point had a value of latitude and longitude and a value of magnetic declination. Soon he realised that some places had the same value and he could draw a line over those points making lines with the same value of magnetic declination (isoline)

The data was collect at noon and the observation of the declination made while he observed the “sun’s magnetic amplitude, the angular distance then on the horizon at sunrise and sunset”.

“The magnetic declination was one-half the difference between the two amplitudes and applied to the geographical position at midnight” (Murray 2012)

The final map is a brilliant image of the Atlantic Ocean which was divided by Western Ocean and Southern Ocean due to the Historical and at the time social events of the European Society. The map is presented with a background colour of yellow, mainly the paper’s colour, the black for the outline and three colours, fuchsia, green and yellow making the coastal lines of the countries. The absence of boarders makes it easier to read, just because, as an isoline map, there are plenty of lines already.

On this, it is well marked the equator as a rose toned line; the cancer tropic as blacked tone line and a bold red vertical line dividing Africa and Europe to Americas. It is still seen a no variation lines, making a curve towards the North America, diving the west declination or negative to the east declination or positive.

There are still three images, one over the South America portraying two native over three palm trees – image that is easily associated with those people, on there it is written the title of the map saying right to the point what it is. Over the North America it is written the legend of the map on a beautiful flower outlined shield and over Africa, 3 women each of one having a set of instruments used by Haley for his study. It is noticeable a green dressed lady with 9 stars and the sun over the heart painted on the dress holding a telescope. A lady with a dark red dress is holding a ship and a sextant and having on her lap a solar clock. And, at least, a lady with a pink dress is holding a notebook. On this note Halley thanks the king of England William the Third.

Still, it is noticeable the presence the main coastal cities and the name of the islands and capes.

http://www.ngdc.noaa.gov/geomag/faqgeom.shtml  http://www.compassdude.com/compass-declination.shtml

Murray, L. (2012) –  The Construction of Edmond Halley’s 1701 Map of Magnetic Declination,  Master Dissertation Department of Statistical and Actuarial Sciences; The University of Western Ontario (p.72)

Rediscovering the Stories Lost in Maps

1672 Map by John Lederer of Carolina

As modern consumers, we like to think of maps simply as the representation of absolute truth.  We assume every map is created in a painstakingly unbiased manner and that the very act of viewing a map is to observe an accurate portrayal of some portion of the earth at a specific point in time.  We hold so strongly to this view that when we encounter recognizable errors in a historic map, say, showing California as an island or detail of a large lake that never existed, we chuckle at the obvious naivety of the cartographer or even sometimes suspect sinister motives.  It doesn’t even occur to us that the ignorance is primarily on our part for not understanding that maps must be interpreted within their own context rather than just being read in the absolute.  Scanning John Lederer’s map of his “Three Marches” published in 1672 without reading the text in which it was published would be like attempting to interpret the United States Constitution without considering the intent of the founding fathers or other legal precedents of the Supreme Court.

By their very nature, maps are created as a caricature, or cartoon version of reality, sometimes through a gross oversimplification of what exists in the real world.  The necessary interpretation of a map involves understanding not only the spatial language of representation but the competing demands of the craft as well as the economic and political influences on the mapmaker at the time of publication.  Consequently, every map is the product of its particular time and echos the beliefs and knowledge of its author or benefactor at that moment.  When studied in this light maps are more like eyewitnesses of a particular historic event – that is their creation – in addition to the geographic place that they describe.  The story they have to tell is less like that of a photograph and more like an essay from a personal journal.  Unfortunately, this sort of interpretation takes time and effort.  It is the scientific investigation of a mystery!

An excellent example of such detective work can be found in this article, A New and Correct Map of the Province of North Carolina: The Discovery of a 1737 North Carolina Manuscript Map, published in the Journal of Early Southern Decorative Arts which investigates not only the map, but its story.  As historians, it is up to us to honor the original intent of a map to help us both learn and share its story.

If you have such an investigation, we would love to publish it here as well.  It is our desire to make this blog a repository where stories of cartographic curiosities can be revealed and myths of map-making can be dispelled.

From Ptolemy to Pilgrimage

The following is abstracted from a lecture entitled From Ptolemy to Pilgrimage: Images of Late Antiquity in Geography, Travel and Cartography by Scott Johnson presented at the Library of Congress, on June 9, 2011 as published on the History of the Ancient World website.  The full video can be seen at the link to the article above.

In his presentation, the author argues that  the map known as Tabula Peutingeriana, or Peutinger Table named after its early modern owner Konrad Peutinger, which is the only world map to survive from the Greco-Roman world offers a unique opportunity to explore in detail the cultural history of the period in which it was originally made.  The period of the original map document, roughly 300 to 600 AD, is referred to in literature as “late antiquity”, basically from the time of Constantine to Mohammed.  This period that has come to be viewed as a crucial transitional period in Mediterranean and Middle Eastern history over the past 50 years.