Historical Methods and Tools of Navigation

 

 

            From the time of Thales (circa 640-546 B.C.) who first postulated that the earth was a sphere until 1760 when John Harrison developed his first workable chronometers, the commerce of navigation left much to chance.  Ancient navigation relied mostly upon staying close enough to shore and familiar landmarks to get from one port to another.  Dead (from deduced) reckoning was a widely used navigation technique.  In dead reckoning, ships would start off on course and continue on it until they arrived at their destination.  The speed of a ship was calculated by throwing knotted lines overboard and counting how many knots passed through a sailor’s hands in a minute.  This technique worked well for voyages across the Mediterranean but voyages across oceans were often disastrous. 

            Celestial navigation relies upon using the positions of stars (especially the North Star, Polaris), the sun, and the moon to calculate one’s position around the world.  In fact, early navigation efforts using celestial navigation were limited to obtaining latitude only.  A variety of instruments such as the astrolabe, with early Persian models dating as far back as the 11th century, were used to determine latitude by sighting in on Polaris or Stella Maris (Star of the Sea).  Instruments related to the astrolabe, such as the alidade or sighting rule, the quadrant, the cross-staff which had developed from the 10th century Arab kamal were also used for determining latitude.  Pictures and descriptions of these instruments as well as a number of activities related to their use may be found at the following web sites: http://beatl.barnard.columbia.edu/students/his3487/cantrell/presentation1.html as well as at http://www.saundersandcooke.com/history.html .

            Eventually the sextant which allowed mariners to calculate the angular distance between two objects such as the sun and the horizon was developed.  With the compilation of detailed “declination tables” or “astronomical charts” which the captain kept under lock and key, it became easier to establish a ship’s latitude. 

            Of course, a navigation instrument that did not rely on the sky was the magnetic compass.  It appears that the compass was developed, probably independently, by the Chinese in the 11th century and the Europeans in the 12th.  The advantage of the compass was that it worked in both foul and fair weather and also during both the day and night.  The one great disadvantage, which became known by later in the 15th century was that the compass gave a reading for magnetic north which was usually not the same as true north as was indicated closely by the north star.  The magnitude and direction of this “magnetic declination” or error changed as one’s position changed throughout the world.  The size of the declination was particularly huge in the Southern Atlantic Ocean and it caused significant navigation errors.

            After a particularly tragic wreck on the Scilly Isles off the southwest coast of England on October 22, 1707 in which over two thousand English sailors were killed, the British Parliament relented to public pressure and passed the Longitude Act on July 8, 1714.  It offered three cash prizes for development of a method of determining longitude around the world.  It offered a prize of “20,000 pounds (the equivalent of millions of dollars today) for a method to determine longitude to an accuracy of half a degree of a great circle; 15,000 pounds for a method accurate to within two-thirds of a degree; and 10,000 for a method accurate to within one degree.”

            In principle, calculating one’s longitude was a simple problem.  All one had to do was to determine noon for the current position of a ship.  Then one had only to know the exact time back at the ship’s homeport and it was a simple matter of a few calculations to calculate how far around the globe the ship was (the longitude) from its homeport.  The problem of course was that there was no such thing as an accurate timekeeper on board a ship.  The sandglasses or hourglasses that were in use lost a great deal of time depending on the moisture and temperature conditions and had to be reset on the sun’s noon position every day.  What was needed was an accurate way of keeping shipboard time and it was into that void that John Harrison stepped.  His lifelong battle to win the longitude prize and to save thousands of sailors’ lives is the subject of a best-selling book Longitude by Dava Sobel as well as a Nova video “Lost at Sea – The Search for Longitude”.