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John Harrison

1693 – 1776
English Horologist and Inventor
Exploration Ranking 19th of 26
Harrison's marine chronometer of 1759, improved from his first attempt in 1735. Stamp from Great Britain, 1993.

John Harrison invented the first practical marine chronometer (ship’s clock) in 1735, which was revolutionary for navigation, because it enabled navigators to compute accurately their longitude (distance east or west from a point) at sea. Before Harrison, latitude (distance north or south of the equator) could be estimated but estimates of longitude were usually very inaccurate. Harrison’s invention was also transformative for cartographers because it allowed them to create accurate world maps for the first time.


Determining Latitude and Longitude at Sea


The central question of navigation on the seas was the determination of the ship’s latitude and longitude at any given time. The first of these was not too difficult. One’s latitude, in the northern hemisphere, was equal to the altitude of the north celestial pole, and this was marked, approximately, by Polaris, the pole star. A good approximation of the latitude was found simply by taking the latitude of that star, although because in the fifteenth century Polaris was about 3-1/2 degrees from the pole, appropriate adjustments needed to be made.


An alternate method of finding latitude, especially when sailing close to or south of the equator, was by observation of the sun. The zenith distance of the sun at local noon is equal to the latitude minus the sun’s declination. The ancient Greeks used an astrolabe, a metal disk on which navigators could measure the height of the sun or a star above the horizon. Medieval Arabs perfected this device, adding tables and formulas that revealed the navigator’s latitude. By the 1500s, mariners replaced the astrolabe with the cross-staff, a simpler tool that provided the same information. The cross-staff was an early version of the quadrant, which was later developed into the sextants and octants that are still in use today.


Navigators of the fifteenth century also had accurate tables of the sun’s declination for any day of the year, so they needed only to take a reading of the sun’s altitude at noon. This altitude was, of course, the highest altitude of the day and could be determined by finding the shortest shadow of a standard pole. These tools allowed mariners to determine latitude in most regions near and north of the equator. They were not as useful south of the equator until the 1600s, when astronomers began including southern constellations in their star charts.


            Determining Longitude


The determination of longitude was much more difficult. For centuries mariners had no easy, reliable way to find this information. Using a method called dead reckoning, a navigator could estimate longitude with rough calculations of speed and direction. Dead reckoning was useful over short distances, but a small error on a long ocean voyage could lead a ship hundreds of miles off course, possibly into unknown and dangerous waters.


Knowing the difference between the longitudes of two places is equivalent to knowing the difference between their local times, because 15 degrees of longitude is equivalent to one hour. Theoretically, if one had a clock set to the time at a place of known longitude and could determine when, on that clock, local noon occurred at one’s current location, the difference in time could enable one to make a determination of longitude.


Alternatively, one could compare the known time of an astronomical event, such as an eclipse of the moon, at the place of known longitude with its local time at one’s current location. Unfortunately, these methods could not work given the current state of knowledge of the moon’s motion or of the accuracy of timekeeping devices. The clocks in use were simply not precise enough, especially if operated on the moving decks and in the changing temperatures of a ship at sea. For example, when Columbus attempted to determine longitude on his second voyage to America in 1494 using an eclipse of the moon, his error was about 18 degrees or one hour and 20 minutes. This example is somewhat misleading because Columbus had remarkable success just using dead reckoning and amazing intuition on how to read the seas and the weather.


Columbus estimated time and distances with simple devices such as a rope or buoy or landmark. He was an intuitive master of the most ancient form of navigation. All his maps and charts and painfully acquired formal education—so impressive, yet so misleading—were of little use to him. He relied on his instincts and experience concerning tides and wind; the color of the sea and composition of clouds mattered more to him than the mathematical calculations of the era’s leading cosmographers. They had never gone to sea, but Columbus had. His dead reckoning proved so accurate that he had already sailed from Spain to the New World without incident the very first time, and, incredibly, with no loss of life.(1) And each time after that, he improved his course based on experience rather than theory. Nonetheless, Columbus was the exception as many succeeding mariners met their doom after miscalculating their longitude.

Harrison Sets to Work

On October 22, 1707, four homebound British warships ran aground at the Scilly Isles near the southwestern tip of England with the loss of nearly 2,000 men because the Admiral and his navigators had misjudged the ships’ longitude. The British government then offered a reward of 20,000 pounds (approximately $4,780,000 in today’s currency)(2) for a method of accurately determining longitude at sea within thirty miles.

John Harrison set himself the task of producing a mechanical timepiece that would give the correct time at London while a ship was at sea, making it proof against the yawing, pitching and rolling of the vessel which invariably knocked existing clocks out of time. The chronometer also had to be proof against the effects of differential climate, humidity, corrosion, friction and variations in gravity. Harrison’s work evinced a lifetime of dedication. Altogether he built five chronometers, two of which were tested at sea.


Many of the improvements that this former carpenter and viola player, bell tuner and choirmaster included in each clock have gone on to become essential components of modern precision machinery: Harrison created the encaged roller bearing which became the predecessor to the ball bearing and led to the foundation of huge modern corporations such as Timken and SKF. The bimetallic stripe, invented solely by Harrison in an attempt to compensate for changes in temperature in his H3 timekeeper, is still used in scores of common essentials: in thermostats, toasters, electric kettles, and their like. The stripe combines brass and steel, in which the different rates of shrinkage and expansion of the two metals cancel each other out. He also designed an extraordinary escapement mechanism, the ticking heart of the clock, that had no sliding parts (and hence no friction, either) and that is still known as a grasshopper escapement because one of the components jumps out of engagement with the escape wheel, just as a grasshopper jumps suddenly out of the grass.

By 1735 he dispensed with the pendulum and replaced it with two mutually correcting balances. This was the H1 chronometer, which should at least have won one of the Board of Longitude’s minor prizes. But Harrison was always bedeviled by jealousy from rival watchmakers and the powerful lobby of professional astronomers. The sole problem with Harrison’s spring-driven chronometers was that the early pedestal-mounted models were too heavy and cumbersome to be practicable on board ship. Finally, in 1759, he produced his masterpiece, the famous H4, a convenient timepiece in watch form. The British government reluctantly rewarded Harrison most of the prize money after his series of increasingly accurate timepieces survived numerous trials both on land and at sea.(3) 


Harrison’s Legacy

By 1770, the Harrison chronometer was standard issue for the British Navy. A copy of H4, made by another clock maker, Larcum Kendall, and named the K1, was taken by James Cook on his second voyage to the South Pacific in 1772-75. Cook highly praised its accuracy compared to the use of lunar tables. By 1815 there were an estimated 5,000 chronometers in use on ships worldwide; now mariners could crisscross or even circumnavigate the globe, confident that they could always determine their location with reasonable accuracy.(4) By allowing ships to know their positions and plot their voyages with efficiency, accuracy, and precision, these clocks and their successors enabled the making of untold trading fortunes. In addition the fact that the Harrison clocks were British-invented and their successor clocks firstly British-made helped Britain in the heyday of her empire to become, for more than a century, the undisputed ruler of all the world’s oceans and seas.(5) Precise-running clockwork made for precise navigation; precise navigation made for maritime knowledge, control, and power.

Prime Meridian

Before 1884, most nations in the world designated an arbitrary meridian as the prime meridian for use on their land maps and nautical charts. Similarly, each country set its clocks by the meridian that it had designated as the prime meridian. Thus, each country had a different standard of time and of location. This situation made it difficult to coordinate travel and business activities.


In 1884 the International Meridian Conference met in Washington, D.C. Representatives from 25 nations came together to designate a single meridian as the prime meridian so that maps and times could be standardized around the globe.

The meridian that runs through the Royal Observatory at Greenwich, England, was chosen as the prime meridian. The prime meridian marks 0' longitude.

The designation of the Greenwich meridian as the standard prime meridian allowed an international time standard to be established. The meridian exactly opposite the prime meridian was designated as the International Date Line, and it marks the place where each new day officially begins at midnight. At that same moment, on the opposite side of Earth, the prime meridian marks noon of the previous day.

So by the late nineteenth century, most of the world’s sailing ships used sea charts that listed Greenwich, England, as the “prime meridian.” That means zero degrees longitude—the imaginary line from the North Pole to the South Pole—runs through Greenwich. If navigators aboard ships had clocks that told them exactly what time it was at Greenwich, and they checked the positions of the sun, moon and stars where they were—and if they had reliable sea charts that told them where the celestial (heavenly) bodies were supposed to be—they could get an accurate fix on their locations.


It is worth noting that John Harrison’s clockworks enjoyed perhaps only three centuries’ worth of practical usefulness. Nowadays, the brassbound chronometer in a ship’s chart room, just like the sextant kept in its watermark morocco box, is a thing more decorative than essential. Time signals of impeccable accuracy now come across the radio. The digital readout of longitude and latitude coordinates come to a ship’s bridge from a Global Positioning System’s (GPS) interrogation of faraway satellites.


(1) Laurence Bergreen, Columbus: The Four Voyages 1492-1504 (New York, 2012), p. 30.

(2) Eric W. Nye, Pounds Sterling to Dollars: Historical Conversion of Currency, accessed August 12, 2019, https:/

(3) Dava Sobel, Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time (New York, 1995), pgs. 132, 149. Of the total prize money of 20,000 pounds, Harrison received 10,000 pounds in the late autumn of 1765 and an additional 8,750 pounds in June 1773 which nearly totaled the remainder of the longitude prize due Harrison, but it was not the coveted prize. Rather the second amount was a bounty awarded by the benevolence of Parliament—in spite of the Board of Longitude, instead of from it.

(4) Ibid., p. 163. 

(5) Simon Winchester, The Perfectionists: How Precision Engineers Created The Modern World (New York, 2018), p. 31.


Key Reference


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

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