Latitude was easy, longitude was not
By the 16th century, navigators could find their latitude fairly well by measuring the height of the sun at noon or of the Pole Star. Longitude, the distance east or west, was another matter. The Earth turns through 360 degrees in a day, so each hour of difference between local time and the time at a reference point corresponds to 15 degrees of longitude, and each degree to four minutes of time. A navigator who knew the time at his home port and could find local time from the sun could in principle work out how far east or west he had come.
The practical difficulty was knowing the home-port time. Pendulum clocks, accurate on land after the mid-17th century, were upset by the motion of a ship, and ordinary watches of the time drifted by minutes a day and were affected by heat, cold and damp. Without a reliable answer, ships relied on dead reckoning, and errors of longitude could reach many degrees on a long passage. At the latitude of southern England, a degree of longitude covers a little under 40 nautical miles, and at the equator about 60.
The cost of error
Errors of position were a source of loss. A well-known naval disaster occurred in 1707, when part of a British fleet returning from the Mediterranean under Sir Cloudesley Shovell was wrecked on the rocks of the Isles of Scilly, with the loss of well over a thousand lives. Poor knowledge of position was widely blamed. It stands among the famous shipwrecks that prompted official action, though it was one of several pressures.
Proposed solutions
Many ideas were put forward. Galileo suggested observing the moons of Jupiter, which are eclipsed at predictable times and so can serve as a clock in the sky; the method proved useful on land but was impractical on a moving ship. The lunar distance method measured the angle between the moon and a chosen star or the sun and compared it with predicted values in tables; this needed accurate tables, a good instrument and lengthy calculation. Other proposals, such as signal ships anchored at known positions, were less practical.
The Longitude Act
In 1714, the British Parliament passed an act that offered large rewards for a practical method of finding longitude at sea. The top prize of £20,000 was for a method accurate to within half a degree, which is two minutes of time, after a voyage to the West Indies. A body of naval officers, scientists and others, later known as the Board of Longitude, judged claims. Other nations, including Spain and the Netherlands, had made similar offers earlier.
John Harrison
John Harrison, a self-taught clockmaker from Yorkshire and later Lincolnshire, took up the challenge of building a clock that would keep time at sea. His first marine timekeeper, now called H1, was a large machine with balanced oscillating arms that were not upset by the motion of the ship, and it was tested on a voyage to Lisbon and back in 1736. The Board encouraged him to continue. He built H2 and H3, and after many years he produced a much smaller instrument, H4, which looked like a large pocket watch.
H4 was tested on a voyage to Jamaica in 1761 and 1762, with Harrison’s son William aboard, and it was reported to have lost only a few seconds on the passage, far inside the limits of the act. A second trial to Barbados in 1764 was also successful. Even so, the Board delayed payment and demanded further trials and the release of the design, and Harrison was not paid the full amount he thought he was owed. After he appealed to the king and to Parliament, he received a final payment in 1773. Historians continue to discuss how fair the Board’s treatment was, since there were scientific as well as personal motives in the dispute.
Lunar distances and the Nautical Almanac
While Harrison worked, the lunar method also advanced. Tobias Mayer’s lunar tables and the efforts of the Astronomer Royal, Nevil Maskelyne, led to the publication of the Nautical Almanac from 1767, which gave predicted lunar distances for use at sea. For some decades the two methods competed. A navigator needed practice and several minutes to work a lunar distance, whereas a chronometer gave the time at the reference meridian directly.
From one-off to everyday
H4 was costly and unique. Makers such as Larcum Kendall copied it, and one copy was carried by Captain Cook on his second Pacific voyage, in the 1770s, and performed well. In the following decades, John Arnold and Thomas Earnshaw developed simpler and more robust designs, including improvements in the detent escapement, which made chronometers less expensive to produce. By the early 19th century, the Royal Navy was supplying chronometers to its ships, and merchant ships with larger budgets were also fitted with them. The vessels of the East Indiaman fleets were among those that benefited.
Using a chronometer
A marine chronometer was kept in a padded box on gimbals, away from magnets and draughts, and was wound at the same time each day. Its error was checked against time signals or known longitudes at ports, and its rate of gain or loss was recorded. Ships often carried several, so that a failing one could be spotted by comparison with the others.
Longitude was then found by taking a sight of the sun, usually at a time other than noon, to determine local time, and comparing it with the chronometer’s reading. The calculations were eased by tables and later by the sextant and printed methods that made them routine. Today, satellite systems provide the time and position, but the chronometer’s principle, carrying the time of a reference place around the world, solved the longitude problem for the age of sail and steam.