Introduction of the Mercator World Map Revolutionizes Nautical Navigation

Overview

In 1569, Flemish cartographer Gerardus Mercator (1512-1594) broke away from the teachings of Greek mathematician and astronomer Ptolemy (90-168) and published a world map, which introduced a new system of projection for marine charts featuring true bearings, or rhumblines, between any two points. His system presented a revolutionary cylindrical projection where a straight line between any two points forms the same angle with all the meridians, and became the basis for modern day navigational charts.

Background

The practice of cartography, or map-making, can be traced back to early examples from Babylon, Egypt, and China, where the first maps were printed. European map-making can be traced to early Greek culture and the most significant contributions to the study of geography were made by Claudius Ptolemaeus known as Ptolemy, a Greek astronomer and mathematician. Ptolemy created an eight-volume cartographical work entitled Geographike Hhegesis or Guide to Geography , which featured his own research as well as that of his predecessors. These included Eratosthenes of Cyrene (c. 276-c. 194 b.c.), who developed an accurate measure of the circumference of the globe; Crates of Mallus (fl. early second century b.c.), who formalized the concept of a globe; Hipparchus (fl. late second century b.c.), who worked out a grid of Earth; and Poseidonius (c. 135-c. 51 b.c.), who developed a "corrected" figure of Earth's circumference, which was smaller and less accurate than that of Eratosthenes and, unfortunately, was the figure chosen by Ptolemy to represent the circumference of the globe.

The cartographic principles established by Ptolemy became the fundamental elements of geography for centuries. His Geography was translated to Latin in the early fifteenth century and maps of his world were constructed and became the model for Renaissance exploration. Before that, however, two developments of the twelfth and thirteenth century had an impact on navigation and cartography: the magnetic compass, in use in Western Europe by the late 1100s, and Portolan charts. Based on the portolano, a harbor-finding manual of the Middle Ages, Portolan charts, printed on goat and sheepskin and very rare and expensive, were the first charts developed exclusively for nautical use. Common in the Mediterranean, the Portolan chart eventually covered the upper coasts of Africa and those of northern Europe.

About the same time as Portolan charts were being enhanced to include Africa, Prince Henry of Portugal (1394-1460), known as Prince Henry the Navigator, established his "navigational" school at Sagres near Cabo de São, Portugal. The school employed cartographers, including Portolan chart-makers, to develop new maps based on data collected during expeditions sponsored by Prince Henry. By the time of his death, expeditions under his sponsorship had rediscovered Madeira and the Azores in the Atlantic and had explored southward along the coast of Africa as far as Gambia. As information from these discoveries were added to the cartographic record, more European countries became interested in maritime exploration and Prince Henry's lasting contribution (influenced by his accomplishments and the tales of the thirteenth century explorer Marco Polo (c.1254-1324)) was to instigate a tradition of European maritime discovery.

Men like Bartolomeu Dias (c.1450-1500), who discovered the Cape of Good Hope; Vasco da Gama (c.1460-1524), who rounded Africa and reached India in 1498; Christopher Columbus (1451-1506), who discovered North America but was convinced he had reached Asia; and Amerigo Vespucci (1454-1512), who rediscovered North America on his return from Brazil, all contributed to events which would radically alter the world map of the time. The 1519-1522 circumnavigation of the globe by Ferdinand Magellan brought back better understanding of the expanse of the Americas and the Pacific Ocean. These momentous discoveries by European explorers were matched by the invention of printing in Europe and the first European printed map. By 1507, much of the geographic knowledge and recent discoveries appeared on a world map, one of the first ever printed, published by Martin Waldseemüller (c.1475-1522) of Germany. His map depicted the new continent discovered across the Atlantic as a separate entity and named America in honor of Vespucci.

As men explored the oceans and coastlines of the world, they found that the Portolan charts were inadequate for navigation over the expanses of oceans. The need for a chart of latitudes and longitudes instead of directions and distances prompted Renaissance mathematicians to experiment with various map projections to accommodate both the new geographical data and the problem with navigation. This new scientific approach to cartography stimulated one Flemish cartographer to abandon the teachings of Ptolemy and develop a new system for navigation charts.

Impact

The growing volume of new geographical data persuaded Gerardus Mercator (1512-1594) to abandon the time-honored theories of Ptolemy and other cartographers and to construct his maps and globes to reflect the latest observations and geographic knowledge. Using his background in mathematics, Mercator sought a practical solution to represent the curved meridians and parallels of the globe on the flat surface of a chart. This problem was especially significant to mariners who were beginning to explore the vast expanse of the world's oceans after centuries of sticking to the coast lines. Compiling data from Spanish and Portuguese charts with his mathematical system of projection, Mercator published his world map for navigation at sea in 1569. The cylindrical projection enabled navigators to compute a compass bearing in a straight line, enabling the sailing of the shortest distance between two points. Because the compass bearings could be plotted as straight segments on Mercator's map, a ship's course could be found by laying a straight rule across it. The map, of which only four copies survived, was made up of 24 sheets with dimensions being 131 × 208 cm. Intended for navigators, it also represented the land surfaces of the globe as accurately as possible based on current geographical data gathered on expeditions around the world. Mercator regarded his world map as one part of a coordinated scheme of cartographical research that culminated in the posthumous publication of his greatest work, his Atlas (1595), a collection of maps which included 27 originally prepared by Ptolemy (with corrections and commentary by Mercator), and his own maps of the countries of Europe and the world. Mercator is credited with coining the phrase atlas for a collection of maps.

Although sailors were slow to adopt the new Mercator mapping device, the projection was eventually used by some of the most important explorers of the time. The concept of Mercator's projection was embraced by scientists who were determined to discover the mathematical reasoning behind it. Because Mercator did not explain his map—he merely included a graphical device showing how it could be used to solve the nautical triangle—it was not practical for chart-makers and sailors. His solution was not perfect; it approximated longitude because neither the knowledge of the true size and shape (many still, despite vast evidence to the contrary, believed the world to be flat) of the globe nor the mathematical resources of his time were adequate to permit great accuracy.

The problem of determining longitude delayed the use of Mercator's charts until two English mathematicians developed a solution using trigonometric tables. Thomas Harriot (1560-1621), working privately for Sir Walter Raleigh (c. 1554-1618), and Edward Wright (1558-1615), constructed tables of meridional parts, by which lines of latitude on a Mercator chart could be spaced. The compass bearing, or plot, on a Mercator chart is thus determined using the difference in meridional parts and longitude. (Wright published his table in 1599 and is often given sole credit for the solution.)

Despite the table of meridional parts, determining longitude was still problematic for sailors and would require the invention of an accurate chronometer, which was accomplished in 1759 by English inventor John Harrison (1693-1776). Harrison's seagoing chronometer was employed by James Cook (1728-1779) during his circumnavigation of the globe. The charts Cook compiled during his voyage were so accurate and detailed that they changed the nature of navigation and cartography forever. In 1884, the countries of the world agreed to adopt the meridian of Greenwich, England, as the Prime Meridian (0°), making longitude constant on all future navigational charts around the globe.

Today, nearly all navigational charts are constructed using the Mercator projection with the exception of maps of large areas, such as the entire Pacific Ocean, or charts of the Polar regions. However, modern cartography owes much to the inventions of other scientists, such as photography, the airplane, the computer, and space technology. The advent of aerial photography, first from balloon, then airplane, and now satellite, has had a tremendous impact on the development of more accurate and infinitely detailed maps of the globe. The use of modern technologies in the maritime realm, such as global positioning systems and highly sophisticated radar, has significantly changed the work of a navigator on the seas and oceans of the world. The Mercator map is still used, but it has been overshadowed by these commonplace modern inventions.

ANN T. MARSDEN