Volpe Journal Summer 2000
Back to On Course with GPS
"I must go down to the sea again, to the lonely sea and the sky,
And all I ask is a tall ship and a star to steer her by..."
--Sea Feaver by John Masefield
The Search for Longitude
In 1492, Christopher Columbus sailed westward from Spain across the Atlantic Ocean in an attempt to reach Asia. Several months later, he landed in the Bahamas. His historic voyage marked the beginning of a new era where traveling by sea was one of the few ways to cover vast distances. However, pinpointing the exact location of a vessel on the sea was always a challenge.
For thousands of years, early navigators limited their sea voyages to coastal routes to avoid becoming lost. Dead reckoning was the only navigation method available. In dead reckoning, the navigator finds his position by measuring the course and distance that he has sailed from some known point. But at the end of the 15th Century, as trade between distant ports increased, new methods for determining position were needed. Early navigators turned to
the heavens for answers.
Using observations of the sun, stars, and planets to calculate position, celestial navigation or "shooting the stars" remains a basic skill even for the navigators of today. Until the early 18th Century, mariners used Polaris, or the North Star, in the northern hemisphere and the Southern Cross in the southern hemisphere to determine north-south latitude. However, a method for determining east-west longitude remained elusive.
The search for longitude spanned four centuries and was one of the most pressing scientific challenges of the time. Because longitude changes constantly with respect to the heavens as the earth rotates, the key element in determining longitude was a method for precise time keeping. However, in the era of pendulum clocks, precise time keeping at sea was impossible. Early astronomers such as Galileo Galilei and Sir Isaac Newton proposed a number of theories that looked to the natural world to provide a clock to be used by navigators. Galileo's theory, for example, involved calculating the position of the sun and the moon in relation to the position of the moons of Jupiter to determine longitude at sea. These theories, however, failed in practice when the navigators were unable to implement them reliably.
Periodically, the governments of the great maritime nations offered rewards for a workable method. In 1714, the British Parliament offered a reward of 20,000--an enormous sum of money at that time--for a practicable and useful means of determining longitude. An English clockmaker named John Harrison finally answered the challenge in the mid-1700s. Harrison successfully constructed a mechanical clock that could keep accurate time at sea. With this first marine chronometer, the modern era in navigation had begun.
Along the way, a number of navigation tools were created to aid mariners. Among these, the sextant has come to be widely recognized as the universal nautical symbol. In fact, the sextant, in conjunction with the magnetic compass, has been a basic navigational tool for more than two centuries. A sextant uses adjustable mirrors to measure the exact angle of the stars, moon, and sun above the horizon. In combination with a chronometer, a sextant is a simple, accurate, and low-cost way even today to calculate position.
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