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Digital Subscriptions > History Revealed > January 2016 > Longitude

Longitude

Knowing the exact location of a ship at sea had flummoxed the wisest thinkers since ancient times, until a humble clockmaker worked out what made longitude tick, writes Jonny Wilkes
DASHED ON THE ROCKS Without a reliable form of navigation, ship wrecks – as depicted in Johann Christoph Dietzsch’s 18th-century painting – were common tragedies

The Time to Find Longitude

DEAD RECKONING

Before the 18th century, the most common practice for establishing a ship’s longitude was ‘dead reckoning’, a crude calculation of the distance moved from a fixed point using a ship’s speed and the time taken. “Too often” historian Dava Sobel says, “the technique of dead reckoning marked him for a dead man.”

High winds and squalls had raged for almost the entire October 1707 voyage of Admiral Sir Cloudesley Shovell’s fleet, and morale was floundering. Following an unsuccessful campaign against French and Spanish forces at Toulon, the 21 ships had been ordered to withdraw and return to England, but rough conditions on the Atlantic – which Shovell described as “dirty weather” – severely hampered the crossing and blew them off course. It was with relief that they approached the end of their gruelling journey, with ship navigators, and Shovell himself, calculating the fleet’s position at somewhere near the coast of Brittany. But they were fatally wrong.

The ships were actually many miles to the west, and perilously close to the shallow waters of the Isles of Scilly, but by the time the error was realised, it was already too late. On the stormy night of 22 October, the flagship,HMS Association, struck the rocks first and sank in a matter of minutes, taking all of the 800 crew with her, before three more vessels were splintered and lost as crews couldn’t react quickly enough. In all, between 1,400 and 2,000 seamen perished in the disaster (one of the British navy’s worst) – all down to the age-old problem of navigating at sea. With the oceans busier than ever at the start of the 18th century, and wrecks all too common, it was finally time to solve the seemingly impossible problem of finding a ship’s longitude.

This c1600 engraving by Philip Galle shows how navigators tried, with little success, to find longitude using the movement of the Sun
BRIDGEMAN IMAGES X3, NATIONAL MARITIME MUSEUM, GREENWICH, LONDON X1
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The January 2016 issue of History Revealed
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