National Geographic : 1951 Dec
The National Geographic Magazine in the librarian-philosopher's mathematical mind. At Alexandria he put up a pole, as perfectly erect as he could make it. Then at noon on June 21 he measured the angle of the pole's shadow. It was 71 degrees. Knowing geometry, Eratosthenes was aware that if the pole and the well were extended to the center of the earth they would meet at the same angle, 71 degrees (diagram, page 753). Since there are 360 degrees in a circle, this 7 -degree angle was 1 oth of the whole circle. So the distance between Syene and Alexandria must be /oth of the cir cumference of the earth. Thus, to find out how far it was around the earth, all Eratosthenes had to do was to multiply the Syene-Alexandria distance by 50. This he did in the unit of measurement called the stadium, probably equal to about ten feet. Syene was 5,000 stadia away, so the distance around the earth must be 250, 000 stadia. Even a coldly mathematical heart must have leaped with the joy of pure discovery. Standing on one little corner of the vast, mysterious, unexplored earth, one man-by the power of mind-had thrown a tape meas ure around the globe. Actually the measurement was not exact. If we substitute the now-known number of miles for Eratosthenes' 5,000 stadia, we get 520 miles times 50, or 26,000 miles. Today we know that the girth of the earth is 24, 901.96 miles. But Eratosthenes' result is remarkably close when considered in the light of the crudity of his instruments and data. Despite the much earlier work of Eratos thenes in the same city, Ptolemy of Alexan dria, in the 2nd century after Christ, conceived of the world as much smaller than it is. Be lieving in a Ptolemaic-sized world, Columbus thought he had reached India when he had sailed no farther than the West Indies. One Day Mysteriously Missing In those days the world, instead of getting smaller, was growing larger as mariners dis covered its extent. First to sail entirely around it was not Magellan-he was killed on the way-but members of his crew, includ ing the highly articulate Antonio Pigafetta. Nearing home with his gossipy round-the world diary, Pigafetta was puzzled to find it was Wednesday, July 9, 1522, aboard, but ashore in the Cape Verde Islands it was Thursday, July 10! "We could not persuade ourselves that we were mistaken," he wrote after the worm eaten, storm-battered Victoria reached Spain just 12 days short of three years after the start. "I was more surprised than the others, since I had every day, without intermis- sion, written down the day that was current. "But we were afterwards advised that there was no error on our part, since as we had always sailed toward the west, following the course of the sun, and had returned to the same place, we must have gained 24 hours!" * Sailing with the sun slows its apparent speed and thus lengthens each day. Pigafetta's calendar was a day behind because he failed to record these slight daily gains. New Knowledge of "Rivers in Sea" These pioneering circumnavigators would have made their voyage in less time had they possessed our modern knowledge of the world-wide, interlocking system of ocean cur rents, shown by brown arrows on this map. Prevailing winds, rotation of the earth, topography of the ocean floor, discharge from large rivers, melting of icebergs, heating and cooling of large bodies of water, evaporation, rain and snow, all play a part in maintaining the oceans' vast, ceaseless circulation system. Surface currents range in speed from hardly perceptible drifts to five miles per hour in the Gulf Stream, swift enough so that in 1513 Ponce de Le6n's ships could not stem it. Within the past year oil tankers surveying the Gulf Stream for the United States Navy in this same area found that ships can save 3 to 10 hours' steaming time between Cape Hatteras and Key West by staying close to the eastern edge of the Gulf Stream south bound, and that the strongest currents on the northbound route are well inshore. This American-born stream in the sea brings its warmth to Britain and Europe. It keeps Russia's Arctic port of Murmansk ice-free all year and makes far-north Spitsbergen a sum mer resort for Norwegians. The Gulf Stream's course is not always constant. Often it meanders like a river, turning up far from its supposed location, but whether these shifts affect weather in Europe has not been definitely proved. A two-way, "two-story" current through the Strait of Gibraltar completely changes the water in the Mediterranean Sea every 75 years. Salt tends to concentrate in the Medi terranean, since it loses more water by evapo ration than it gains from rainfall and rivers. The heavy salt water sinks and flows out below the surface, while fresher water from the Atlantic moves in above. All the great "rivers in the ocean" are mapped in the light of the latest knowledge on this new portrait of our world, 71 percent of which consists of the tidal, tossing salt water of Mother Sea. * See "Greatest Voyage in the Annals of the Sea." by J. R . Hildebrand, NATIONAL GEOGRAPHIC MAGA ZINE, December, 1932.