John Harrison

British clockmaker and joiner John Harrison (1693–1776) revolutionized navigation at sea with the invention of the marine chronometer, a watch-like device that made the accurate measurement of longitude possible for the first time. Before the chronometer, inaccurate reckoning of east-west positions caused shipwrecks. Harrison spent decades perfecting his device, proving scientists wrong and saving countless lives.

Longitude was a problem that had vexed kings and governments for centuries. After a 1707 naval disaster resulting from navigational error destroyed four battleships and caused the death of over 1,500 crewmembers, the British government formed the Board of Longitude, which offered a reward of £20,000 to anyone devising a reliable means of measuring longitude in all conditions. Great scientists like Sir Isaac Newton thought the task impossible. Others thought it would surely be solved using existing methods which simply needed refinement. But none succeeded. Clockmaker John Harrison developed several prototypes over several decades, starting with a large, all-wooden sea watch and ending with something that, resembling a large pocket watch, kept remarkably accurate time on a rocking ship, over great distances, in weather conditions both fair and foul. Calling his invention a Sea Clock, this self-taught, working-class carpenter's son succeeded where elite scientists had not.

Self-Educated and Fascinated with Timepieces

Harrison was born on April 3, 1693, the first of his parents' five children, in Foulby, a village in North Yorkshire, England. When he was seven years old, the family moved southeast to Lincolnshire, settling in the village of Barrow. The elder Harrison, an accomplished joiner skilled in the intricate joining of pieces of wood in crafting furniture, was a carpenter in the employ of a local estate. As a child of six, it is said that John Harrison was confined to bed with a serious illness when someone gave him a watch to play with. He spent hours examining its workings and listening to its rhythms, leading to a lifelong passion for clockmaking as well as music.

John Harrison

Georgios Kollidas/

Harrison followed in his father's footsteps as a woodworker, excelling in the finer aspects of joinery, and in his spare time he learned clock repair. As his skills grew, he began building wooden clocks, completing his first longcase (or grandfather) clock in 1713 at the age of 20. His love of music also flourished, and he later became the choirmaster at his parish church in Barrow. Harrison married in 1718, was widowed in 1726, and married again soon after. Both his wives were named Elizabeth.

Gained Expertise in Precision Instrumentation

Harrison's clocks continued to incorporate mechanical innovations over the years. His gridiron pendulum used alternating brass and iron rods to balance each other's expansion and contraction. His grasshopper escapement— a kind of rocker mechanism that captures and releases the clock's driving power through pendulum motion—improved on traditional designs and also needed no lubrication, a welcome upgrade.

With his success in land-based clocks, Harrison believed that he had a good chance at designing a clock that could go to sea and keep accurate time, solving the longitude problem. Shipboard clocks were subjected to wild fluctuations in temperature, pressure, and humidity as well as the tilt and motion of the vessel while under sail. To be a reliable aid to navigation, a sea clock would have to be accurate in every condition. Designing such a clock was a daunting problem, given the delicate balance in a clock's mechanism and the many forces at play.

The Longitude Problem

Sea clocks were one of several proposed solutions to the longitude problem. Navigating an oceangoing vessel required that the captain know the ship's coordinates: where it is on an east-west axis, or longitude, relative to where it is on a north-south axis, or latitude. Mariners knew how to find latitude by determining the altitude of the sun at noon—its highest point—using a table of its positions during the day and using the stars at night. However, longitude was impossible to accurately determine when far out at sea. As a result, mariners would sometimes first sail to the correct latitude of their destination, then turn east or west, checking their latitude periodically as they steered along their course. While traveling vast oceanic distances, however, time as well as risk were added to a voyage by using this method. The Method of Lunar Distances—calculating one's position based on sightings of the moon—was also viewed as promising, but it required extensive, detailed calculations over the course of a voyage.

In her book Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time, Dava Sobel explained that, to calculate longitude, a ship's captain had to know simultaneously the exact time the ship reached a specific location as well as at its home port or another place of known longitude. With these two points of reference, the captain could “convert the hour difference into geographical separation. Since the earth takes 24 hours to revolve 360 degrees, one hour marks 1/24th of a revolution, or 15 degrees. And so each hour's time difference between the ship and starting point marks a progress of fifteen degrees of longitude to the east or west.” Announced by the Board of Longitude, the Longitude Prize would be awarded to the person whose measurement method could, on a transatlantic voyage, successfully identify a longitudinal position within half a degree, or two minutes' time.

To give longitude calculations uniformity, it was decided that a fixed coordinate should be used in calculations of longitude, and this coordinate was determined in 1675, when King Charles II founded the Royal Observatory at Greenwich. For British subjects competing for the prize, “Greenwich Time” would be part of their calculations. Due to Harrison's success, Greenwich Time—now called the UTC or Universal Time Code—is used to this day.

Built Series of Sea Clocks

In 1730, Harrison took his first shot at competing for the Longitude Prize. He designed a wooden marine clock which, based on his land clocks, took advantage of another clockmaker's design which solved the problem of temperature fluctuations. Harrison's clock, a large, heavy tabletop model, also solved the problem caused by a ship's pitching and rolling motion through the addition of an internal balancing wheel. All he needed now was financial backing in order to build his sea clock.

Harrison traveled to London and presented his design ideas to Edmond Halley, Astronomer Royal for the British government. Halley referred him to George Graham, a leading clockmaker, who was so impressed by Harrison's ideas that he loaned him money to build the clock. Five years later, Harrison returned to London with his first sea clock. Designated “H1,” it featured an ingenious “linked balance” mechanism in which any change in motion from one side would be compensated for by the other. The Board of Longitude was so impressed with the clock that they ordered a test at sea in 1736. On a short voyage to Lisbon, Portugal, the clock bested the calculations of the ship's navigational officer by 60 miles. Harrison was granted a stipend to build a second model for transatlantic testing.

Harrison's second sea clock, “H2,” was a rugged and compact version of H1 and was ready for testing in three years. Britain was then at war with Spain, however, and the clock's advanced technology was considered too important to risk a voyage in which it might fall into enemy hands. Harrison received another stipend and the chance to improve on his H2 while waiting out the war. His “H3” took 17 years to build and featured two innovations that outlived the clock: the bimetallic strip and the caged roller bearing, both which are now used in machine applications. At the time, however, the inventor was frustrated that H3 did not perform as precisely as he knew it should. Unfortunately, the solution lay in principles of physics that would not be adequately understood for another 200 years.

Created His Masterpiece, the Sea Clock

Another insight came about after Harrison moved to London in 1758. Here the top craftsmen were building pocket watches every bit as precise as his sea clocks, but in a far smaller package. New types of steel had by now been developed that had the strength needed for small, precision metal components. Harrison's new project, the H4, incorporated many features of his Jeffries-built pocket watch within a larger, 5.2-inch form and its mechanism utilized the new steel and diamonds.

The H4 was completed in 1761 and testing began with a land trial followed by a round-trip voyage from Portsmouth, England to Kingston, Jamaica. Harrison, then age 68, sent his son William to oversee the shipboard test and results were well within the two-minute requirement set by the Board of Longitude. When William returned to report success, the board was unconvinced and dismissed the results as a matter of luck. After the board demanded another trial, John and William Harrison objected but to no avail and a new voyage was arranged, to Barbados. This time, another competitor for the Longitude Prize was on board: the Reverend Nevil Maskelyne, who employed the Method of Lunar Distances. While Maskelyne's results were also within the two-minute requirement, achieving them was highly labor intensive, requiring near-constant hand calculations that made such a solution impracticable.

In 1765, when both results were presented, the Board of Longitude remained stubbornly unconvinced that the H4's success was reproducible. In fact, Maskelyne was now the Astronomer Royal, and as Harrison's rival he remained critical of the chronometer method. The controversy reached Parliament, which offered Harrison a £10,000 advance on the £20,000 prize, under certain conditions, with the balance remitted upon duplication of the sea clock by other watchmakers. Pressed to name a clockmaker to do the job, Harrison suggested Larcum Kendall, a clockmaker who had helped him on the H4. Harrison was also required to submit all four of his clocks, as well as the plans for his H4, to the board for long-term testing. (Painfully for Harrison, Maskelyne oversaw the delivery of his four prototypes to Greenwich, during which one of them was damaged.) Lastly, Harrison was required to make two copies of the H4 himself and turn them over for testing.

Petitioned the King

Three years later, Harrison and his son William, together with Kendall, presented copies of the H4 to the Board of Longitude, asking that these fulfill the requirement for two copies for testing. The board had these sea clocks tested but insisted that the Harrisons, rather than Kendall, build a second promised copy. By this point, a now-elderly Harrison was in no mood to tolerate further delays, viewing the board as unsympathetic and intransigent. “The affair of Longitude was now become nothing but Parties,” he later wrote regarding this time. He decided to appeal directly to King George III, whom he felt would appreciate his predicament. The king was sympathetic and for ten weeks tested his own copy of the clock against Harrison's newest (called the H5), finding it accurate to a fraction of a second per day. The king told Harrison to petition Parliament for the final prize, threatening the legislative body with a personal appearance if they did not act.

In 1773, when Harrison was 80 years old, Parliament awarded him an additional £8,750, resulting in a lifetime payment of something more than the promised award money. His marine chronometer was acknowledged a success and quickly put to use. Among the famous early adopters were Captain James Cook and Lieutenant William Bligh, who used K1 and K2 (Kendall's copies of the H4), respectively, on their famous voyages. Cook returned from the South Seas in 1776, reporting high accuracy from his chronometer. It is uncertain whether Harrison heard of Cook's successful report before his death a year later, at age 83, on March 24, 1776.

Innovations Affirmed by History

While Harrison's sea clocks continued to be improved upon after his death, his design principles influenced the best builders for generations. In his final years, Harrison authored a book taking his rivals to task and claiming that he possessed the means to build the most accurate land clock ever. The book's claim was generally criticized as outlandish and absurd, and Harrison died before he could prove it. Two hundred years later, however, a Harrison expert and clockmaker built two versions of the clock laid out in the plans that led to Harrison's claim. The second of these clocks was acquired by the National Maritime Museum in Greenwich, England. Museum staff tested it for 100 days in 2015, with an outstanding result: Harrison's clock only lost 5/8ths of one second during that span of time. Had the clock been built in 1762, at the time Harrison perfected the H4, and run continuously, it would have been slow by only nine minutes and 43 seconds by September of 2017. The Guinness Book of World Records certified Harrison's clock as “the most accurate mechanical clock with a pendulum swinging in free air.”


Betts, Jonathan, Time Restored: The Harrison Timekeepers and R.T. Gould, the Man Who Knew (Almost) Everything, Oxford University Press, 2006.

Sobel, Dava, Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time, Bloomsbury, 1995.


Guardian (London, England), April 18, 2015, Robin McKie, “Clockmaker John Harrison Vindicated 250 Years after ‘Absurd’ Claims.”


The Pirate King, john.htm (September 17, 2017), Jonathan Betts, “John Harrison: English Super-Genius Invented the Tools and Methodology to Determine a Ship's Longitude at Sea.”

University of Cambridge Digital Library, (September 17, 2017), Katy Barrett, “John Harrison: Account of John Harrison and His Chronometer.”□

(MLA 8th Edition)