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 name 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-days 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,  (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 this 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 stars. 

How did they estimate speed?  One way was to time how long it took the ship to pass an object into the water at the bow. A later, more precise method involved dropping over board a piece of wood attached to a rope knotted at specific regular intervals.  The floating wood drew the rope as the ship advanced.  After a prescribed time, the rope was hauled  in and knots drawn out were counted.  This indicated that 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 thing draw 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 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 it 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.

Next time: Navigating by Water, Sky, and Wind - Sailing by the Sky

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