How Do We Know Where We Are?
I have often wondered how navigators on ships at sea tell where they are when it’s cloudy and they are out of sight of land. They have compasses, and have had for a long time, but how accurate are they? I think of the Titanic, but who could predict the position of an iceberg in those days.
I ran across a book, Longitude: the True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time by Dava Sobel. In it he describes a disaster when a whole fleet of ships was dashed to bits because the captain miscalculated his position. On the night of October 22, l707, Admiral Sir Clowdisley Shovell’s fleet was returning victorious from Gibraltar after skirmishes with the French Mediterranean forces, and wrecked against the Scilly Isles just twenty miles from the southwestern tip of the British Isles. He made a fatal miscalculation, and 2,000 sailors lost their lives.
Someone lived to tell the story because it was said that the day before the disaster, Sir Clowdisley was approached by a member of his flagship’s crew “who claimed to have kept his own reckoning of the fleet’s location during the whole cloudy voyage.” Such subversive navigation by an inferior was forbidden in the Royal Navy, as the unnamed sailor well knew. However the danger appeared so enormous, by his calculations, that he risked his neck to make his concerns known to the officers. Admiral Shovell had the man hanged for mutiny on the spot. His refusal to listen caused the death of 2,000 men, but it was the precipitating event that led to the Longitude Act of l7l4. In it, Parliament promised a prize of 20,000 pounds sterling (approximately $l2 million in today’s currency) for a solution to the longitude problem, which was simply put: How do you know where you know where you are once you lose sight of land? The problem was not solved until the middle of the l8th century, when John Harrison invented the chro nometer. This instrument is a large strongly built watch especially designed for precise time keeping on board ship. It differs from a watch in that it has a heavy balance wheel which is always kept vertical because it is mounted within two concentric rings so pivoted as to make sure that the chronometer always remains undisturbed despite tilting of the ship. This was the original design. Modern chronometers are the same except they have quartz digital chronometers. Chronometers always carry the exact time from Greenwich, England.
Our readers may ask as I did, “What does a clock which keeps exact time have to do with determining the ship’s location?” This has to do with the fact that at exactly 12 noon where the ship is in the ocean, the navigator can reference that time to the time on the chronometer and determine how far west or east he might be from Greenwich, England. Thus if it is exactly 12 noon as determined by the sun on board ship and the chronometer says 2:00 PM, then the ship is two clock hours away. Since there are 24 hours in a day and 12 hours from the time the sun passes the Greenwich meridian until it passes the international dateline in the Pacific, navigators can use the distance between meridians to determine their location east or west.
By the time the chronometer was invented, navigators were already using a sextant, invented by Thomas Godfrey, an American, and John Hadley, an Englishman independently in 1730. This instrument helps them determine the ship’s position by the sun in each season and the stars.
A typical day’s work for a marine navigator included plotting dead reckoning, observing celestial bodies at morning twilight, reporting of 0800 position to the commanding officer, checking the noon sun line, advancing the morning sun line for running fix and reporting of 1200 position to the commanding officer, checking the compass, determining of the time of sunset and preparing a list of bodies available for observation during evening twilight, using the sextant for evening twilight observation, and recording the time of moon rise and moon set.
Today these methods are backup to more sophisticated methods using satellites that circle the globe if they are used at all.. Now fishermen on Lake Erie or other bodies of water can use low cost navigation receivers to determine within 16 feet what their position is. There are even hand held receivers available for the hiker. These Global Positioning Systems are encrypted or “dithered” for civilian use, so that the military can be even more precise in their locations. Perhaps this explains the ability of NATO air forces to bomb targets at 15,000 feet.
-Mary