Plate 4 from Americae Retectio
by Johannes Stradanus

Instruments of Navigation


Deceptively like a common sundial, William Borough's instrument to determine magnetic variation pushed the envelope of scientific understanding in 1581. But more than that, it encapsulated a thorny scientific problem within a very simple instrument—so simple that a sailor could use it. Since the magnetic poles do not lie in the same place on the earth's globe as the true north and south poles, a compass needle will usually not point directly at the geographic pole. The deviation depends on where the compass is in relation to the magnetic pole as well as local geomagnetic conditions. Knowing this "magnetic variation" was of crucial importance to navigators trying to avoid being lost at sea. It was of particular concern to those plying the North Atlantic. Near the equator, variation might only be a degree or so, but in northern waters, it can vary up to 90°. Near Greenland, navigators could conceivably be sailing west when they thought they were sailing north.

Borough therefore put into a simple instrument all the tools that the navigator needed to find and map variation at any point on the globe. Click Here for a Larger ViewThis woodcut reduces a complex exercise in terrestrial geomagnetic and spherical geometry to a simple image of a simple instrument made up of simple and known commodities: compass and sundial. Designed for workaday purposes and not purporting to reveal any hidden truths of the earth or heavens, the instrument of variation allowed English sailors to map the North Atlantic. The timeliness of the book and the instrument it explained are underscored by recalling that the English only began seriously contemplating overseas exploration and colonies in the 1570s (Drake returned from his circumnavigation in 1578) and her first colony was (unsuccessfully) planted on Roanoke in 1584. Although described by Borough—the more able mathematician—the instrument was apparently invested with great meaning for Robert Norman (fl. 1590), who took the trouble to have his initials placed at the foot of the shadow line, identifying this invention and this embodied knowledge as his own.

The instrument worked as follows: Placing the instrument level and plumb on a stool and aligning the shadow from the cord on the N-S line of the compass dial, the navigator noted the position of the needle at two times in the day when the sun was at equal elevations above the horizon—that is, at equal times on either side of local noon. Half the difference in the absolute value of these two needle positions gave the local magnetic variation since magnetic north is not in quite the same direction as true north (the north magnetic pole lies at about 77° N lat., 45° W long.). So, for example, at 8 AM the needle might point 48°E and at 4pm point 45°W; the local magnetic variation would then be 1.5°E ([48°-45°]/2) at that point on the globe. The next time any ship came to the area, then, it would be able to compensate for that 1.5° and sail towards its destination reliably. One limitation of this instrument, however, was that to determine the most accurate declinations, the readings ought to have been taken as far apart in the day as possible, but that meant the mariner had to stay put for most or all of that day, not only a difficult task on the ocean, but also quite inefficient in terms of reaching one's destination. Ultimately, though, this instrument and many more like it defined the mathematical arts for late-sixteenth-century Englishmen and the world they sought to create.

Steven A. Walton
Michigan Technological University

Suggested Reading

Barlow, William. The navigators supply : conteining many things of principall importance belonging to navigation : with the description and use of diverse instruments framed chiefly for that purpose, but serving also for sundry other of cosmography in generall. New York: Da Capo Press, 1972.

Bourne, William. A regiment for the sea (1578) and other writings on navigation. Edited by E.G.R.Taylor. Cambridge: Cambridge University Press, 1963.

Davis, John. The seamans secrets. Delmar, N.Y.: Scholar's Facsimiles and Reprints, 1992.

Dee, John. The perfect arte of navigation. New York: Da Capo Press, 1968.

Markham, Albert Hastings. "The voyages and works of John Davis the navigator." Hakluyt Society 59 (1880): 1-392.

Smith, John. A sea grammar. New York: Da Capo Press, 1968.

Waters, David W. The Art of Navigation in England in Elizabethan and Early Stuart Times. London: Hollis and Carter, 1958.