Navigating by Water, Sky, and Wind
ARE you afraid of falling off the edge of the earth? Probably not. In times past, however, some sailors apparently had that very fear. Many sailed keeping land in sight. But other brave mariners cast off their fears and took to the open sea.
About 3,000 years ago, Phoenician seamen set out from their home ports on the eastern shore of the Mediterranean to carry on trade in Europe and North Africa. In the fourth century B.C.E., a Greek explorer named Pytheas sailed all around Britain and may have reached as far as Iceland. And long before European ships ever entered the Indian Ocean, Arab and Chinese sailors from the East had already been traversing it. In fact, the first European to sail to India, Vasco da Gama, arrived there safely with the help of an Arab pilot, Ibn Majid, who guided Da Gama’s ships through the 23-day crossing of the Indian Ocean. How did such ancient navigators find their way at sea?
Dead Reckoning Kept Them Alive
Early mariners had to depend on dead reckoning. It requires that the navigator know three things, as shown in the illustration below: (1) his ship’s starting point, (2) its speed, and (3) its heading (direction of travel). Knowing the starting point was easy. But how could the heading be determined?
To find his heading in 1492, Christopher Columbus used a compass. But compasses had been available in Europe only since the 12th century C.E. Without compasses, pilots navigated by the sun and the stars. When clouds obscured their view, sailors oriented themselves by the long, regular ocean swells produced by steady winds. They kept track of how these swells were aligned with the rising and setting of the sun and the stars.
How did they estimate speed? One way was to time how long it took the ship to pass an object cast into the water at the bow. A later, more precise method involved dropping overboard a piece of wood attached to a rope knotted at specific regular intervals. The floating wood drew the rope out as the ship advanced. After a prescribed time, the rope was hauled in and the knots drawn out were counted. This indicated the ship’s speed in knots—nautical miles per hour—a measuring unit still used today. Knowing his speed, the navigator could calculate the distance his ship had traveled in a day. On a chart, a map of the sea, he then drew a line to show his progress along his chosen heading.
Of course, ocean currents and side winds could push the ship off course. So the navigator periodically calculated and recorded the steering adjustments that were needed to keep the ship on track. Each day, he continued from where he had left off—measuring, calculating, drawing. When the ship finally dropped anchor, these daily notations on his charts formed a permanent record of how the ship had arrived at its destination. By means of dead reckoning, Columbus made his way from Spain to North America and back over 500 years ago. His carefully drawn charts allow modern seamen to retrace his remarkable voyage.
Sailing by the Sky
How did old-time navigators use the heavenly bodies to guide their vessels? The rising and setting sun indicated east and west. At dawn, sailors could note how much the sun had shifted by comparing its position with the fading stars. At night, they could get their bearings from Polaris—the North Star—which appears to be almost directly above the North Pole after dusk. Farther south, a bright constellation known as the Southern Cross helped them to locate the South Pole. So on a clear night, sailors on all seas could check their heading by means of at least one celestial reference.
But these were not the only astral guideposts. The Polynesians and other seafarers of the Pacific, for instance, could read the night sky like a road map. One of their techniques involved setting a course toward a star that they knew rose or set on the horizon in the direction of their goal. Throughout the night, these navigators also checked other star alignments to make sure that they were on course. If their heading was wrong, the heavens showed them how to correct it.
How reliable was this system? At a time when European sailors often clung to shore in fear of plunging off the edge of a flat earth, Pacific mariners were apparently making long mid-ocean crossings between relatively tiny islets. For example, more than 1,500 years ago, Polynesians left the Marquesas Islands and headed north across the vast Pacific Ocean. By the time they went ashore in Hawaii, they had journeyed 2,300 miles [3,700 km]! Island folklore tells of ancient Polynesian voyages back and forth between Hawaii and Tahiti. Some historians say that these accounts are mere legends. Nevertheless, modern-day seafarers have repeated that voyage, navigating by the stars, ocean swells, and other natural phenomena—without instruments.
Riding the Wind
Sailing ships were at the mercy of the winds. A breeze from behind moved a vessel along quite nicely, but a head wind slowed the boat considerably. No wind, as was often experienced in the doldrums—the region around the equator—meant no progress. In time, sailors discovered prevailing ocean winds that helped establish highways for sailing ships on the high seas. Navigators made good use of these winds.
Of course, if the winds were contrary, they could also bring misery and death. For example, when Da Gama set sail from Portugal to India’s fabled Malabar Coast in 1497, prevailing winds took him out into the South Atlantic and then carried him back southeastward and around Africa’s Cape of Good Hope. But in the Indian Ocean, he met the monsoons—winds that reverse direction seasonally. Early each year the summer monsoon rears up in the southwest part of the Indian Ocean, and for months it blows all that floats toward Asia. In late fall the winter monsoon takes over. Roaring in from the northeast, it blows back toward Africa. But Da Gama left India in August and soon faced unfavorable winds. Instead of the 23 days required for his eastward crossing, his return trip took nearly three months. Because of this delay, fresh food ran short, and he lost many of his men to scurvy.
Shrewd navigators on the Indian Ocean learned to check the calendar as well as the compass. Eastbound ships passing the Cape of Good Hope had to set out for India by early summer or risk waiting months for favorable winds. On the other hand, ships’ captains departed India for Europe in late autumn to avoid battling the summer monsoon. Thus, the Indian Ocean route was like an alternating one-way street—marine traffic between Europe and India’s Malabar Coast often moved in just one direction at a time.
Navigation Sails Onward
Time passed, and the art of navigation eventually set a new course. Mechanical instruments began to reduce dependence upon the naked eye and guesswork. The astrolabe and later the more accurate sextant—devices that determine the elevation of the sun or a star above the horizon—allowed mariners to find their latitude north or south of the equator. The marine chronometer—a reliable, seaworthy clock—gave them the ability to determine longitude, their east or west position. These instruments were far more precise than dead reckoning.
Today, gyrocompasses indicate north without a magnetic needle. The Global Positioning System can indicate one’s exact location at the push of a few buttons. Electronic displays often replace paper charts. Yes, navigation has become an exact science. But all this advancement only increases our respect for the courage and skill of the ancient seafarers who guided their crafts through vast and empty seas with only their knowledge of water, sky, and wind.
[Diagram/Pictures on page 12, 13]
(For fully formatted text, see publication)
Dead reckoning was carefully documented for future navigation
1 Starting point
2 Speed Determined by using a piece of wood, a rope
knotted at regular intervals, and a timer
3 Heading Determined by observing currents, stars,
the sun, and the wind
[Pictures on page 14]
State-of-the-art instruments make today’s navigation an exact science