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)