Navigation and Cartography

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Navigation and Cartography

The Earth. Basic knowledge of the earth's geography survived the economic decline of the Western Roman Empire in the quadrivial subjects of astronomy and geometry, which delineated the climatic zones on the spherical earth. The classical Greek division of the habitable world into the three continents of Asia, Africa, and Europe, sur-rounded by ocean, is evident in the earliest medieval maps, and knowledge of the earth's sphericity was evident among the Vikings, who had been successfully navigating westward to Greenland and Nova Scotia by following fixed latitudes. However, efforts to map both land and sea

in a way that represented real distances and relations between places did not happen until the mathematical methods of the ancient scholar Ptolemy were rediscovered and used in conjunction with compass and triangulation during the Renaissance.

Portolan and Compass. Europeans became aware of the directional properties of the lodestone, a naturally occurring magnetic mineral, in the twelfth century and began to use magnetized iron needles for navigation in the thirteenth century. Previously, sailors piloted by land-marks and did not stray far from the coastline. On the rare occasions when they ventured out into the ocean, they followed fixed latitudes by measuring the height of the polestar with a simple cross-staff, or quadrant. Such simple methods permitted the Vikings to find and settle Iceland and Greenland, which were due west of Norwegian ports, and even to make their way to Vinland along the North American coast; but, generally speaking, sailing out of sight of land, or at least land birds, was fraught with danger and thus avoided. The introduction of the magnetic compass corrected this problem by permitting merchants to cross open sea on oblique directional headings, thus shortening travel times and enabling them to avoid pirate-infested coasts. This method was facilitated by the creation of a new kind of map, the portolan, which combined detailed knowledge of the coastal geography of Europe and North Africa with well-placed compass roses, radiating directional rhumb lines that intersected each other and ran to key ports. The fifteenth-century navigator could follow one magnetic heading to another and find his way with reasonable accuracy, but the maps were not suitable for long voyages at sea and did not give a realistic portrayal of distances. The limitations of such maps became increasingly evident as the voyages of discovery by Vasco da Gama and Christopher Columbus revealed new lands and aroused scientific curiosity about the surface of the globe and its inhabitants. Geographers did not doubt the sphericity of the earth but needed more data to determine its size and to map its lands.

Accurate Representation. The problem of accurate portrayal lay with the difficulty of representing a spherical section of the earth's surface on a flat map. Ptolemy had described methods for projecting a grid of latitudinal and longitudinal lines onto a conic surface, which could be unrolled flat, in his Geography, but this text was not rediscovered by Western scholars until the early fifteenth century. Printed in 1475, it quickly became a source for Peter Apian, whose Cosmographia (1524) was a basic textbook for sixteenth-century cartographers and mapmakers. One reader was Gerardus Mercator, who incorporated the latitudes of distant lands as they were reported by adventurers such as Columbus.

Mercator Projection. Mercator realized that Ptolemaic projections were of limited use to sailors, since

lines of constant magnetic heading, called loxodromes, were represented by arcs, so he invented a new technique by which the globe's surface is projected onto a cylinder that is tangent at the equator. The resulting “Mercator projection” represented loxodromes as straight lines and permitted navigators to lay out courses with a straight edge, making them more useful than the portolans. Maps made in this way remained the standard for navigation into the twentieth century.

Determining Longitude. Mercator's teacher in mathematics and cartography, Gemma Frisius, introduced two new ideas to geography and navigation: triangulation and the determining of longitudes by using a portable clock to measure azimuthal passages of known stars. In theory, if one could observe an azimuthal passage (when a star or the Sun was at its highest point in the sky, passing through the observer's meridian) and compare the time of the observation with respect to when the passage should have occurred at a standard place, such as Greenwich, England, one would know how many hours east or west of the standard meridian one had traveled, and thus could determine the longitude by adding fifteen degrees for every hour. Unfortunately, using clocks in this way was not possible in the sixteenth century, and the pressing problem of finding longitudes remained unsolved until the technology was refined.

Triangulation. In the 1533 edition of Peter Apian's Cosmographia, Frisius explained how land maps could be accurately established by using two known points, a graduated arc to measure angles, and standard linear measures to locate a third point, which could then serve as one of the base points needed to locate a fourth, and so on. This method, called triangulation, was first applied on a large scale map by Tycho Brahe, who printed an accurate map of the island on which his observatory was located and precisely oriented it with respect to landmarks on the coasts of the Danish sound. In the generations that followed, cartographers and mapmakers produced beautiful maps of even greater precision, as printing and surveying became more technically sophisticated. Europeans were now armed with the tools to explore, map, and colonize the globe.