portraying what was known of it on his famous maps, describIng it in his books on geography and cosmography. Ptolemy’s world reached from the British Isles and the west coast of Africa as far east as China, and from about the Arctic Circle in the north to a line well below the equator running through Africa and the Indian Ocean. Most important for the development of navigational science, however, Ptolemy laid down lines on his maps. These were the first terrestrial coordinates, the parallels of latitude and the meridians of longitude—indispensable for locating a ship on the face of the globe. With this concept—the division of the earth’s spherical surface into geometrical coordinates—the science of navigation was provided with a foundation, a basic language: the degrees and minutes of the great circles of longitude that converge on the poles like lines separating the segments of an orange; the degrees and minutes of the circles of latitude that girdle the earth like belts, becoming smaller as they get farther from the equator. With longitude for determining locations east and west, and latitude for ascertaining distance north and south, it was now theoretically possible to locate one’s position at sea with scientific certitude and accuracy. If a mariner could correctly establish his east-west line of position and his north-south line of position, he knew his ship would be where the two lines intersected.
The Odyssey and The Stars For thousands of years before Ptolemy, mariners had steered by the stars. Book V of the Odyssey tells how Ullysses set sail from Kalypso’s isle, and set his course by the Pleiades, the Plowman, the Great Bear and Orion.
THE GEOGRAPHY OF THE WORLD was better known at the close of antiquity, about 150 A.D. (green area, top drawing), than it was some 900 years later in the Middle Ages (bottom). Ancient scholars diligently inquired into the size and shape of the earth and drew maps from reports by the armies of Alexander the Great and Roman traders with the East. Scientific inquiry languished during the Middle Ages and though Norse explorers got to Greenland and beyond, knowledge of parts of Africa and the East was forgotten.
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