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.
put into a simple instrument all the tools that the navigator needed
to find and map variation at any point on the globe. This
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.
Michigan Technological University
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