The Search for Longitude


Mankind has long known that they lived on a spherical world. Astronomers could see the circular shadow cast by the earth on the moon during an eclipse. Sailors noticed the suggestive way that a ship disappears over the horizon, the hull vanishing first, then the lower sails and finally the masthead. The ancient Greek mathematician Eratosthenes in the third century BC even calculated the circumference of the world, producing a result within 10% of the correct size.

At first navigating across the surface the world was reality straightforward, with most mariners content to follow a known coastline to a ship’s destination. But the growth in oceanic navigation that followed the discovery of the Americas and sea routes to the East produced a requirement for the world to be mapped accurately. This was achieved by laying a grid of imaginary lines over the earth’s surface. These were lines of latitude, which ran parallel with the equator, and lines of longitude, which ran through both poles. Knowing the coordinates of a given place would permit a cartographer to accurately plot its location on a chart, and studying that chart showed a sea captain how far they needed to travel, and in which direction, to arrive there.

However, even with an accurate map, there was still issues to be overcome. Once a ship left sight of land, how would it know where it was on the chart? The way used to calculate this was called dead reckoning. The sailing master would meticulously record each change in a ship’s course and speed. By plotting this detail on his chart, he could estimate the position of his ship, but as a method it was prone to inaccuracy. The sea surface over which a ship travelled might itself be on the move in a quite separate direction, thanks to ocean currents and the effects of tides. The intrinsic methods used to calculate ship speed and direction were not completely accurate. This introduced small errors that accumulated the longer the ship was away from land. Over a sustained period, the location of the last pencilled cross on the chart, and where a ship actually was, grew away from each other.

All was not hopeless, however, because a ship could check at least one element of its location. So long as the sun was visible at noon, the sailing master could use a back-staff or later a sextant to calculate his latitude from the angle of the sun above the horizon. This was useful information, but it only gave him one of his two coordinates. With no equivalent calculation available for longitude, there was a steady toll of disasters with ships unsure of their longitude being wrecked at night, or in poor weather.

This was the fate of the Royal Navy’s Mediterranean Fleet when it was returning home in 1707. Thanks to a longitudinal error, four ships were wrecked on the Scilly Isles and 1,550 seamen drowned, including the Commander-in-Chief, Sir Cloudesley Shovell. This calamity prompted the British Parliament to pass the Longitude Act, which provided a prize of twenty thousand pounds (a fortune at the time) for the person who produced a practical solution to calculate longitude at sea. Most expected that Sir Edmund Halley, the Astronomer Royal and discoverer of the comet that bore his name, would be the one to find the answer, but they were wrong. It was John Harrison, the cantankerous son of a Yorkshire carpenter who would solve the problem.

The solution to calculating longitude, argued Harrison, was time. The earth takes exactly 24 hours to rotate. Ships know the moment when it is noon in their location, because that is when the sun is due south of them* and it is already the moment when they calculate their latitude. If the ship had an accurate clock that told them the time at that precise moment in a place with a known longitude (say Greenwich) the difference between that time and 12 noon would tell them how far around the world they had travelled, and so their longitude. A difference of one hour, for example, would be a 24th of the way around the world, or 15 degrees of longitude.

Problem solved? Not quite. The early 18th century did have accurate clocks, but they had weights to power them and pendulums to regulate them. They could keep very good time when they were mounted on a drawing room wall, but would be useless on a moving ship. Harrison set about solving this engineering challenge, and would devote the next 43 years of his life to producing that clock. The challenge was formidable. His timepiece would have to work in all the conditions to be expected at sea. It might be on a whaler operating in fringes of the Arctic, or on an East Indiaman crossing the tropics. It had to pass through both calms and hurricanes, without deviating a second. Version followed version until in 1760 he produced his H5 marine chronometer, the device that would solve the longitude problem for the next two centuries and the ancestor of the wristwatch I am wearing as I write this now.

*or north in the southern hemisphere

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