The Invention of the Chronometer

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The Invention of the Chronometer


Locations on Earth are determined by a gridwork of lines, one set marking distance north or south of the equator and the other marking distance east and west of the Prime Meridian, running through Greenwich, England. For centuries, determining one's longitude, that is, one's position east or west of Greenwich, was nearly impossible, leading to the loss of life, ships, and property. This problem was finally solved by John Harrison (1693-1776), an Englishman, with his development of a highly precise clock called a chronometer. This invention revolutionized travel by sea, with repercussions that lasted until the 1990s.


Today, for a few hundred dollars, virtually anyone can purchase a hand-held unit that, by detecting signals from an artificial constellation of satellites, will obligingly display one's latitude, longitude, and altitude with a precision of a few meters or less. The entire surface of the Earth has been mapped photographically, gravitationally, and recently, with space-borne radar to an unprecedented level of detail. It is difficult to remember that, until very recently, nobody on Earth knew where they were with this degree of precision. In fact, as late as the 1980s, before completion of the Global Positioning System (GPS), most ships at sea knew their positions to within only a few kilometers or more, and many still located themselves by celestial observations.

We live in a world that relies on GPS to tell us where we are at any time. Hikers carry GPS receivers in case they lose the trail, cars have GPS receivers to help navigate the streets of strange cities, and scientists use GPS to clock the uplift of Mount Everest. This accessibility of accurate geographic information is unprecedented in human history. Although the concepts of latitude and longitude have been with us for about two millennia (and we have been able to measure latitude for nearly that length of time), our ability to determine longitude is a recent accomplishment.

In fact, latitude measurements require only a simple instrument that can measure the distance above the horizon the Sun appears to be at noon, or the distance above the horizon a given star is seen to be. As we travel away from the Equator, the Sun drops lower in the sky while the pole stars climb higher. With but a smattering of math, this elevation can be turned into a latitude. Longitude is much more difficult and, for centuries, proved nearly intractable.

Because sailors could not accurately determine their location east or west on a map, early navigation often consisted of sailing north or south to a specific latitude and then striking out east or west along that imaginary line until one's destination was reached. Longitude was guessed, based on the captain's estimate of ship's speed, currents, wind speed, and other factors, but the captains were often wrong. In 1707, a wrong guess led to the sinking of four British man-of-war and the loss of 2,000 men when their fleet ran aground on islands just off the coast of Britain. In 1741, another wrong guess led the HMS Centurion to spend two additional weeks at sea on a ship struck by scurvy. By the time Centurion found port, 250 men had died. Eighty of those men died in the two weeks Centurion was delayed because her captain didn't know whether to turn east or west.

The longitude problem was recognized at the dawn of the Age of Discovery, in the early 1500s. Early suggestions were to use the Moon, the stars, or the position of Jupiter's satellites as clocks, but the practicality of making precise astronomical observations from the rolling deck of a ship proved insurmountable, and the positions of these bodies could not be predicted with a high degree of accuracy in that era. Other suggestions were to use clocks, if only they could be made sufficiently precise.

The Earth turns 360 degrees in 24 hours. In one hour, it will turn 15 degrees, and it will turn one degree in four minutes. One degree of longitude at the equator is about 60 nautical miles, or 70 "standard" mi (112 km). To measure longitude with any degree of certainty required a clock that was accurate to within a few seconds per day. And this degree of precision was required under ever-changing conditions of temperature, humidity, rolling of the vessel, air pressure, and more. Mechanical clocks were simply not up to the task, leading to the early interest in the heavens.

In the middle part of the eighteenth century, John Harrison (1693-1776) solved the longitude problem by constructing a series of chronometers that kept time more accurately than any previous such devices. Running without lubrication, changing temperatures could not cause grease or oil to thicken or thin. Neither could temperature cause expansion or contraction of moving parts, because Harrison coupled different metals to overcome such effects. On several test voyages, Harrison's chronometers kept nearly perfect time, especially his most famous clock, the H4 (built in 1759). In spite of this, many years were to pass before, in 1775, his chronometers were acknowledged to have solved the longitude problem—a problem that had stumped some of the greatest minds in Europe for nearly two centuries.


The impact of Harrison's chronometer on seafaring can hardly be overstated. However, it affected more than just ships at sea. National commerce benefited, and, somewhat less tangibly, conquering the longitude problem gave humanity a little more control over its world.

The most obvious impact, of course, was on shipping. Captains were finally freed from dead reckoning and experience to determine their positions. Instead, they could do so scientifically, objectively, and with a high degree of precision. Because the captains now knew where they were with some degree of accuracy, they could better plan their landfalls and could ration their supplies more appropriately. The numbers of ships running aground did not, of course, drop to zero, if only because uncharted lands still existed. However, with more accurate maps of the world and a better idea of their ships' location on the globe, captains could now predict when they would reach port, determine the best course to take for repairs, or calculate their speed for the day. All of these, in turn, gave a much better idea of the amount of time left at sea, letting captains parcel out food to make it last until landfall.

In addition to these benefits, captains could now search for the most efficient routes across the oceans. Since they were no longer stuck with running along a line of latitude until they ran into their target, they could cut diagonals across the oceans, making for port in the shortest time following the most direct routes. Not only did this help speed up ocean travels, it also allowed a wider variety of shipping paths. This, in turn, helped reduce loss of shipping due to pirates or enemies because the shipping lanes were suddenly less predictable. Gone were the days when an enemy fleet would sit on top of the most widely used east-west parallel of latitude, waiting for a commercial or treasure fleet. Instead, with more routing options, there were simply too many paths to watch and attacks of this sort began to die off. This didn't happen immediately, of course, and attacks by privateers, warships, and pirates never ended entirely. However, their number dwindled and became less important as time went on.

This drop in shipping losses, of course, helped the treasuries of all sea-faring nations. Losing fewer ships, they had fewer ships to replace and fewer new crews to train. Faster land-fall meant goods could reach market more quickly, giving a faster turn-around on money invested in a load of cargo from overseas. Fewer commercial shipping loses meant more money reaching the coffers of ship owners and investors, more tax revenue for the government, and more spending by everyone making a profit on the voyage.

Finally, this was but one more step in man's understanding and control of Earth. The longitude problem was of fundamental importance to the seafaring nations, and most of them offered large monetary prizes at various times for anyone who could solve the problem. News of possible breakthroughs caught the public's attention, and the public followed these events with some degree of interest. Those with family members or friends at sea had a more pressing interest, but in nations like Britain, the Netherlands, and Portugal, the entire country followed such issues because of the long and proud dependence on the sea for national sustenance and pride. Even nations like France and Spain were interested, having a proud seafaring tradition themselves. However, these land powers did not have as much at stake as did the smaller, sea faring countries, and their publics did not take the same interest in the problem.

When a solution to the longitude problem was finally announced, there were many who rejoiced because their loved ones, friends, or themselves were now at much less risk. At the same time, national voyages of discovery were continuing to set forth, intent on visiting and mapping all the lands of the globe. Armed with Harrison's chronometers and the imitations that quickly followed, these explorers could now chart islands, harbors, and other important features with unprecedented accuracy. This, in turn, gave their fellow sailors a much better idea of where to turn for supplies, repairs, or shelter, in addition to allowing the mapping of Earth in much better detail.


Further Reading


Andrews, William. The Quest for Longitude. Cambridge: Harvard University Press, 1996.

Sobel, Dava and William Andrews. The Illustrated Longitude. Walker and Company, 1995.

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The Invention of the Chronometer

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