Four Centuries of Geomagnetic Data from Historical Records

FOUR CENTURIES OF GEOMAGNETIC DATA FROM HISTORICAL RECORDS Art R. T. Jonkers,1 Andrew Jackson, and Anne Murray School of Earth Sciences, University of Leeds, Leeds, UK Received 14 March 2001; revised 12 February 2002; accepted 19 December 2002; published 21 June 2003. [1] We present a comprehensive review of historical dented number of early observations of the Earths geomagnetic data collected over the last 2 decades by magnetic field, in standardized format, of potential use many workers, culminating in the largest such compila- in many areas of geophysical research. INDEX TERMS: 1714 tion in the world. It spans over 4 centuries, from 1510 to History of Geophysics: Geomagnetism and paleomagnetism; 1560 1930 inclusive, and consists of 151,560 declinations, Geomagnetism and Paleomagnetism: Time variations-secular and long 19,525 inclinations, and 16,219 intensities. The greater term; 1532 Geomagnetism and Paleomagnetism: Reference fields (re- part of the new data set comprises unpublished obser- gional, global); 1507 Geomagnetism and Paleomagnetism: Core pro- vations recorded by mariners engaged in merchant and cesses (8115); KEYWORDS: geomagnetic data, secular variation, mag- naval shipping across the worlds oceans. Earlier pub- netic field, maritime history, navigation, historical geomagnetic lished compilations and printed accounts of voyages observations have also been included, in particular when their original Citation: Jonkers, A. R. T., A. Jackson, and A. Murray, Four centuries sources were inaccessible, missing, or no longer extant. of geomagnetic data from historical records, Rev. Geophys., 41(2), The current effort has brought together an unprece- 1006, doi:10.1029/2002RG000115, 2003. 1. INTRODUCTION [4] Furthermore, the local magnetic direction in the horizontal plane, sometimes called the magnetic merid- [2] The Earths magnetic field is commonly assumed ian, is denoted as H, defined by adding X and Y together. to originate in the planets fluid outer core. It has been The horizontal angle difference between geographic consciously observed at the surface for the past thousand north (X) and magnetic north (H) is usually written D, years or so, and geographically diverse observations are for the magnetic declination. A compass will try to align available for the last 500 years. It is likely that much can itself with H; navigators therefore have to apply a vari- be learned from an analysis of the field morphology and able correction to their compass courses to regain true evolution deduced from direct measurements. This was courses. Magnetic declination on board ship was often already realized early in the sixteenth century, when the referred to as magnetic variation; the needle was said to first global geomagnetic field models were postulated on be northwesting or northeasting a certain number of de- the basis of a handful of declination measurements, grees from true north. Last, the vertical angle between H compiled in Portuguese sailing directions. More sophis- and F is called the magnetic inclination or dip. It was ticated efforts by sundry individuals ensured in the fol- measured with an inclinometer (dip meter), which con- lowing centuries, largely based on original nautical data sisted of a magnetized needle able to rotate in the vertical [Jonkers, 2000]. Although many of these maritime plane. In the past, declination and inclination were the first sources have since been lost, a sufficiently large number measurements to be made, defining the direction of the have been preserved to warrant extracting a substantial local geomagnetic vector. Intensity (the magnitude of this sample for geomagnetic modeling purposes. vector) was first observed in a relative sense (from the [3] Geomagnetic data come in many shapes and sizes. 1790s) by comparing the “swing time” of a needle at the Nowadays, it is customary to define the field at any desired location with that obtained at a reference site. particular place and time in terms of the three compo- Absolute intensities were only recorded from the 1830s nents (X, Y, and Z) that together yield F, the full geo- onward. At present, directional data have been almost magnetic field vector (see Figure 1). completely replaced by three-component data, recorded at magnetic observatories, on surveys, and by satellites. [5] In 1839, Carl Friedrich Gauss published his classic method for the separation of magnetic fields into those 1Also at Department of Earth Sciences, University of Liv- due to internal and external sources by spherical har- erpool, Liverpool, UK. monic analysis [Gauss, 1839]. This technique was soon Copyright 2003 by the American Geophysical Union. Reviews of Geophysics, 41, 2 / 1006 2003 8755-1209/03/2002RG000115$15.00 doi:10.1029/2002RG000115 ● 2-1 ● 2-2 ● Jonkers et al.: GEOMAGNETIC DATA 41, 2 / REVIEWS OF GEOPHYSICS netic data (notably from ships logbooks) through a thorough analysis. Third, we desired to develop a core field model that was suitable for studies of the flow at the top of the core. [7] During the 1980s and 1990s several workers engaged in a geomagnetic data collection effort unprece- dented in numerical, temporal, and geographical scope. The result includes the available major compilations published in the past and the largest ever sample of original manuscript sources (mostly English, French, and Dutch ships logbooks). In particular, global measurements of declination (D) before 1850 have received detailed attention, as well as the gathering of early observations of inclination (I) and intensity (H and F, the magnitude of the horizontal and the full, three- dimensional magnetic vector, respectively). In addition, the several different types of relative intensity scales Figure 1. The three-dimensional geomagnetic vector F con- applied in the late eighteenth and early nineteenth cen- sisting of X, Y, and Z. Moreover, X and Y combine to form H, tury have been inventoried (see section 2.3). The com- the horizontal magnetic vector. The horizontal angle of H with plete data set (consisting of nine subsets described in X is defined as D, the magnetic declination, while the vertical section 5) comprises Ͼ187,000 geomagnetic observa- angle between H and F is called I, the magnetic inclination. tions, derived from Ͼ2000 ocean voyages. Each of these has been assigned a unique identification code (hereaf- ter referred to as ID code) and has been fully referenced applied to actual measurements of the Earths field, and in the table of sources (see Table 1). A histogram of data it was shown that most of the field was of internal origin. frequencies per decade over the complete interval 1510– In the ensuring years the field was reconstructed at 1930 is given in Figures 2a–2d. different epochs, the interest being primarily in the [8] Table 2 gives an indication of the data frequency evolution of global averages such as the dipole moment. spread for each measurement type per century. The Many such models have been collected by Barraclough unequal division over time is deliberate and will be [1978], who gives an extensive review. Only in the last maintained throughout this paper in Tables 3–7. The few decades has geomagnetic secular variation (the first era (up to 1589) contains the few geomagnetic change in the magnetic field of internal origin over readings taken before the advent of English and Dutch historical time) been modeled at its origin at the core- large-scale exploration of the East Indies. Moreover, the mantle boundary (CMB). Initial attempts mainly relied year 1590 constitutes the lower temporal limit of our upon published compilations of historical measure- time-dependent model gufm1 (1590–1990), and it there- ments, such as the work of Hansteen [1819], Sabine fore seemed useful to mark this boundary in Tables 2 [1872, 1875, 1877], and van Bemmelen [1899], which is and 7 as well. The next period spans the years 1590– treated in section 5. The earliest calculated main field 1699, a slightly extended seventeenth century, covering models at the CMB were epoch models, which reduced the early era of the East India Companies. The eigh- the data to a central year [Bloxham and Gubbins, 1985; teenth century (1700–1799) represents the time of these Shure et al., 1985; Bloxham, 1986; Bloxham et al., 1989]. companies greatest expansion, as well as vast increases A more sophisticated approach was soon thereafter ap- in Atlantic and Arctic oceanic traffic. The inclusion of plied to determine time-dependent models [Bloxham the first 3 decades of the twentieth century in the last and Jackson, 1989]. However, existing compilations have period (1800–1930), which contains extensive surveys as barely scratched the surface of the number of surviving well as maritime naval and merchant traffic, is due to the original sources, and printed versions were sometimes fact that practically all data points from that last 30 years incomplete, and values were truncated, rounded, or re- stem from the Veinberg subset, which itself covers the duced to grid. Furthermore, maritime historical data late nineteenth century as well. It therefore seemed suffer from peculiar errors (e.g., the ships navigational more appropriate to treat this time range as a single error), which have not been adequately accounted for in entity rather than to create an arbitrary division. previous work. [9] After archival processing and initial pruning of [6] Our goals that spurred the present data collection typographical error the large majority of original voy- exercise were threefold. First, we wanted to exploit the ages has been plotted and their estimated longitudes vast amounts of geomagnetic data that remain available brought into coincidence with the identified geographi- in archival and other repositories, going as far back in cal landmarks sighted, using a dedicated piece of soft- time as sources would permit. Second, our aim was to ware we call the voyage editor. The database therefore properly quantify the errors present in historical mag- contains two fields for longitude, reflecting the original 41, 2 / REVIEWS OF GEOPHYSICS Jonkers et al.: GEOMAGNETIC DATA ● 2-3 Table 1.

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