Marine Chronometer

A marine chronometer is a precision timepiece carried on a ship and employed in the determination of the ship's position by celestial navigation. The instrument determines longitude by comparing Greenwich Mean Time with the time at the current location. The first true chronometer was the life work of John Harrison, spanning 31 years of persistent experimentation.

Longitude measures distance east or west of the prime meridian, and it is measured in degrees, minutes, and seconds. By 1700 skilled seamen could find their position north or south, their latitude, but still lacked accurate instruments or methods to calculate their east-west position, known as longitude. Navigators had measured latitude with basic tools for thousands of years, yet longitude demanded more advanced tools and calculations, and the best and simplest way to measure it from a ship was with an accurate clock.

Each hour of difference between local noon and the time in Greenwich equals 15 degrees of longitude, because the Earth rotates 360 degrees in 24 hours. For every four seconds that the time source is in error, the east-west position may be off by up to just over one nautical mile.

Aboard a swaying ship in varying temperatures and salty air even the most accurate clocks of the age kept time poorly, until marine chronometers were invented in the 18th century. For generations the longitude problem taxed the world's greatest scientific minds, including Galileo Galilei and Isaac Newton, and led to the foundation of the Royal Observatory. The Royal Society of London and the French Académie des sciences were founded in 1660 and 1666, while observatories were established in Paris in 1667 and at Greenwich in 1675 on government and royal initiative.

Spain established the first known incentive scheme for determining longitude in 1567; the States of Holland and other governments followed the Spanish example by offering rewards. The lunar distance method was initially proposed by Johannes Werner in 1514, while the Dutch scientist Gemma Frisius was the first to suggest in 1530 that a chronometer could determine longitude at sea.

Five of the Admiral's ships struck the rocks off the Isles of Scilly on 22 October 1707, and more than 1400 men perished, including the Admiral. There is no evidence of a public outcry urging government action after the Scilly disaster; instead the Longitude Act appears to have resulted from lobbying by mathematicians such as William Whiston.

The original Longitude Prize established in 1714 offered a £20,000 reward for a reliable method to determine a ship's longitude at sea. The British government created a Board of Longitude empowered to award £20,000 to the first man who developed a chronometer with which longitude could be calculated within half a degree at the end of a voyage to the West Indies. The 1714 act was first added to in 1741 and amended, repealed and replaced on numerous occasions.

John Harrison was born in March 1693 at Foulby in Yorkshire and died in London on 24 March 1776. John Harrison was a carpenter by trade who taught himself clock making, and his precision longcase clocks of the mid-1720s achieved an accuracy of one second in a month.

Harrison's first experimental marine timekeeper, H1, was made at Barrow-on-Humber between 1730 and 1735 and brought to London in 1735. It carried two interconnected swinging balances, an arrangement that kept it unaffected by the motion of a ship.

In the trial of 1736 Harrison and H1 were taken aboard HM ship Centurion for a voyage to Lisbon. On the return passage he correctly announced that a headland the officers took for the Start was in fact the Lizard, meaning the ship was 60 miles off course and in danger, and that H1 was working correctly.

Eight Commissioners of Longitude assembled on 30 June 1737 and agreed on a payment of £500 with half paid up front so that Harrison could build an improved clock within two years. Made between 1737 and 1739 H2 is a larger and more solidly built version of H1 with the additional refinement of a remontoire, a device to keep the drive to the two balances as uniform as possible. It was never tested at sea because Harrison found a fundamental flaw in it, and it was taken from him in 1766 by the Astronomer Royal under the conditions of the longitude prize.

H3 was begun in 1740 and took nearly 19 years to build and adjust, yet its two large, heavy, circular balances could not keep time well enough to win the prize. Two of its inventions remain relevant today: the bimetallic strip and the caged roller bearing.

Around 1751-52 Harrison commissioned John Jefferys to make a pocket watch with a radically new type of balance, completed in 1753, and it worked so well that he incorporated it into H4, which was completed in 1759. Its high-energy watch balance was less affected by the movement of a ship, and it ticks five times a second. The H4 watch has a dial diameter of 102 mm and an overall weight of 1.45 kg, and its materials include brass, steel, silver, diamond, ruby, enamel, copper and glass.

In 1761 the Commissioners gave permission for Harrison's son William to prepare a voyage to Jamaica to trial H4, and on the way out he used the watch to predict an earlier landfall at Madeira than the crew were expecting. Before departure the watch was tested by Robertson, Master of the Academy at Portsmouth, who reported that on 6 November 1761 at noon it was 3 seconds slow.

H4 kept time on board a rolling ship to about one-fifth of a second a day, ten times better than required to win the government prize. 4 timekeeper was in error by only five seconds after the voyage to Jamaica.

The new trial was to be held at Barbados with Nevil Maskelyne appointed as the astronomer in charge, determining the island's longitude by observations of Jupiter's satellites. Maskelyne departed England in September 1763 while Harrison sailed with H4 in March 1764 and arrived two months later.

When the Board met in February 1765 it confirmed that Harrison's timekeeper had kept time within the most stringent limits of the 1714 Act and recommended an award of £10,000 once he demonstrated the principles of his watch H4 to the Board. The Harrisons felt the full reward was already due, and the recommendations became law in the new Longitude Act of 10 May 1765 despite their objections.

On 5 May 1766 Astronomer Royal Nevil Maskelyne received Harrison's timekeeper from the Board of Longitude, locked up in a box sealed with three seals. Testing lasted 10 months and the dispute rekindled when Maskelyne published the results.

Although his chronometers all met the standards set up by the Board of Longitude, Harrison was not awarded any money until 1763 and was paid in full only in 1773 after further appeals. His rewards totalled £23,065 in all: £4,315 in increments from the Board of Longitude, £10,000 as an interim payment for H4 in 1765 and £8,750 from Parliament in 1773 which settled his claims.

Galileo Galilei discovered the moons of Jupiter in 1610 and they offered a celestial timekeeper visible at the same time from different points on Earth, though he failed to persuade the Spanish or the Dutch to invest in the method. Isaac Newton's theories in his book Principia failed to describe the Moon's motions well enough to find longitude accurately, and this flaw stimulated some of the finest mathematicians of the eighteenth century. The idea itself was not new: as long ago as 1514 it had been proposed that the Moon could be used to measure longitude at sea, and the Royal Observatory was founded to put this theory into practice.

Tobias Mayer, a brilliant astronomer in Hanover, believed part of the problem lay in the astronomical observations the theories were based on, and by the end of 1754 discussions of his ideas were taking place in England. James Bradley commissioned the London instrument maker John Bird to make a brass copy of Mayer's repeating circle, which after sea trials became the first marine sextant; Mayer's tables could then determine a ship's longitude to within 1°.

Nevil Maskelyne, a mathematics graduate from Cambridge who was ordained a minister in 1755 and admitted to the Royal Society in 1758, tested Mayer's ideas on the voyage to St Helena for the 1761 transit of Venus and found his longitude by lunars was only 1½° in error, against errors of up to 10° from dead reckoning; his interest in astronomy traced back to the eclipse of 25 July 1748, which he observed as a boy. Within months of his arrival at Greenwich in March 1765 Maskelyne had established the layout of the Nautical Almanac.

The work of successive Astronomers Royal culminated in 1766 when Nevil Maskelyne published the first Nautical Almanac, containing tables predicting the position of the Moon during 1767 relative to nearby bright stars. Maskelyne succeeded Nathaniel Bliss as astronomer royal in 1765 and continued the supervision of the almanac until his death.

In France in 1748 Pierre Le Roy invented the detent escapement characteristic of modern chronometers, and his 1766 chronometer with a temperature-compensated balance is considered by Rupert Gould to be the foundation of the modern chronometer. Thomas Earnshaw and John Arnold made the greatest strides toward practicality, developing and patenting simplified, detached spring detent escapements in 1780 while moving the temperature compensation to the balance.

The word chronometer was originally employed in 1779 by the English clock maker John Arnold. Ordinary clocks were useless at sea due to temperature changes and the ship's motion. Captain James Cook used K1, a copy of H4 made by Larcum Kendall, on his second and third voyages, having used the lunar distance method on his first. Chronometers were not regularly supplied to the Royal Navy until about 1825.

The marine chronometer quickly became a vital tool of maritime trade, revolutionizing sea travel and saving countless lives.

The Royal Greenwich Observatory Archives at Cambridge University Library include the complete run of the surviving papers of the Board of Longitude covering the eighteenth century until its abolition in 1828 by Act of Parliament. These papers throw a vivid light on the role of the British state in encouraging invention and discovery and on the culture of technical ingenuity in the long eighteenth century. The site transcribes 23 acts in total, beginning with the 1714 act for providing a public reward for such person or persons as shall discover the longitude at sea.

The bulk of the digitised collection was produced as part of a partnership between Cambridge University Library, the National Maritime Museum and the AHRC-funded Board of Longitude Project. Challenge Works has reimagined the original Longitude Prize as a series of multi-million-pound challenge prizes. In 1884 representatives at the International Meridian Conference in Washington met to define the meridian representing zero degrees longitude and chose a line through the telescope at the Royal Observatory in Greenwich.