Calculating longitude Dead reckoning

Due to difficulties in calculating longitude, many of the great voyages of discovery owed as much to luck as to skill. It took a humble carpenter to provide the means for sailors to know exactly where they were.

The Scilly Isles disaster

When four ships and over 1,300 men were lost in 1707 due to a navigational error, the British government promised a reward to anyone who could solve the problem of measuring longitude

considering the many great voyages of discovery that were made in the 15th, 16th, and 17th centuries by the likes of Christopher Columbus, Ferdinand Magellan, and Vasco da Gama, it seems extraordinary that once they were out of sight of land, sailors were quite literally “all at sea.” Despite charts and compasses, their inability to determine longitude with any reliability left even the most experienced captains largely at the mercy of luck or the grace of God.

Dead reckoning

Determining position at sea requires knowing both latitude and longitude. Latitude gives the north-south position, and longitude the east-west. Any sailor could calculate the former by the height of the sun or known guide stars above the horizon. Christopher Columbus simply followed a straight line of latitude on his historic journey of 1492.

However, to gauge their distance east or west of a home port, sailors relied on a method called “dead reckoning,” by which a navigator threw a log overboard and observed how quickly it receded from the ship. This crude estimate of the ship’s speed, combined with the direction of travel (taken from the stars or a compass), and the length of time the ship had been on a particular course, plus or minus ocean currents and winds, gave a sailor a rough estimate of his longitude—of how far west or east he had traveled. It was a very imprecise method, and all too often fatally so.

On October 22, 1707, four homebound British warships miscalculated their position and ran aground on the Scilly Isles near the southwestern tip of England, and more than 1,300 men lost their lives.

The solution

The problem of longitude had taxed the wisest of minds for centuries. In theory, the best solution was for sailors to compare the time aboard ship with the time at the port they had left, since one hour’s difference in sunrise represents 15 degrees of longitude—a distance of some 1,035 miles (1,666km).

Marine Timekeeper H1

This is the first experimental marine timekeeper made by John Harrison between 1730 and 1735 as a first step toward solving the problem of calculating longitude

Prime meridian

To establish a ship’s longitude, and agreed zero point, or prime meridian, is required. In 1851, it was agreed this would be at Greenwich, also home of Britain’s great naval college, shown below

Okhotsk on the Pacific. Two small ships were built from materials transported for the purpose—ropes, sails, and iron parts, including the anchor (everything except wood). These ships carried the expedition to the remote Kamchatka Peninsula in northeastern Russia, where the men constructed another ship, the Archangel Gabriel, in which they took to the open sea. Bering kept so close to the Russian coast that he did not notice Alaska, which was a mere 68 miles (110km) away. Nevertheless,

Last word

This English map is based on a 1754 map published by the St Petersburg Academy of Science, showing the sea routes taken by Bering and Chirikov during the expedition.


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