John Harrison and the Marine Chronometer
A carpenter from northern England spent most of his life building clocks that could keep time on a rolling ship. The fourth one fits in your hand.
Why longitude needed a clock
For centuries, sailors could not reliably work out their longitude, their position east or west, once they were out of sight of land. Getting it wrong could be deadly. In 1707 four British warships were wrecked off the Isles of Scilly and up to 2,000 men died, a disaster that showed how badly navigators needed a better method.
The key is time. The Earth turns 360 degrees in 24 hours, so local time shifts by one hour for every 15 degrees of longitude. If a navigator knows the local time from the Sun, and also knows the time at a home reference point, the difference gives the ship's longitude.
The hard part was carrying home time to sea. A clock on a ship is rocked by the waves and exposed to changing temperatures, so a sea clock had to shrug off both. That is exactly the problem Harrison spent his life on.
In 1714 Parliament passed the Longitude Act, offering money for a practical way to find longitude at sea. The top prize, as the University of Cambridge's Plus magazine puts it, was a massive £20,000 for its time.
See also: compare the time in two places at once on OnlineClock.app.
A carpenter who taught himself clocks
John Harrison was born in 1693. Royal Museums Greenwich describes him as a carpenter by trade who taught himself clockmaking. In the mid-1720s he designed precision longcase clocks that kept time to about a second a month.
The Science Museum Group dates his earliest surviving clock to 1713. He learned of the Longitude Act in 1726, by which time he was already building unusually accurate clocks in Lincolnshire, and in 1749 the Royal Society awarded him its Copley Medal for his experimental work.
That success made him think he could build a portable clock accurate to within three seconds a day. He took his plans to London, where the Astronomer Royal, Edmond Halley, sent him to the leading clockmaker George Graham.
H1, H2 and H3: the sea clocks
Harrison built his first marine timekeeper, now called H1, in Barrow upon Humber, probably with help from his brother James, and brought it to London in 1735. Instead of a pendulum it used two linked swinging balances, an arrangement designed so that a ship's motion would not throw it off.
In May 1736 H1 sailed to Lisbon aboard HMS Centurion and came home on the Orford. On the return voyage, Harrison correctly identified the coast as the Lizard rather than Start Point, showing the ship was about 60 miles from where its officers believed.
The commissioners overseeing longitude agreed to give Harrison £500 in 1737, half at once and half once he had built an improved clock. H2 was finished within two years but never sent to sea, because Harrison found a fundamental flaw in it.
Work on H3 started in 1740 and dragged on for 19 years through many changes. By the time it was done, Harrison had realized that a much smaller timekeeper might be the answer.
H4: the watch that solved it
In the early 1750s the watchmaker John Jefferys made a pocket watch to Harrison's design (Royal Museums Greenwich gives both 1751–52 and 1753). It convinced Harrison that a small, fast-beating balance could keep better time at sea than his big clocks.
The result was H4. Its movement is inscribed John Harrison and Son, 1759, and Royal Museums Greenwich gives its weight as 1.45 kilograms and its dial as 102 millimeters across. It looks like an oversized pocket watch but ticks five times a second, and its large, fast-swinging balance is hard for a ship's motion to disturb. The Science Museum Group dates its completion to 1760.
In 1761 Harrison's son William took H4 on a voyage to Jamaica. Using it, he predicted landfall at Madeira and was proved right. The commissioners still judged that trial insufficient, and relations with the Harrisons soured.
A second trial followed, to Barbados. The astronomer Nevil Maskelyne went ahead to make independent observations, and Harrison sailed with H4 in March 1764. In February 1765 the board found that H4 had kept time within the strictest limits of the 1714 Act.
The long fight for the prize
Winning the test did not mean winning the money. The board recommended £10,000 once Harrison had demonstrated how H4 worked, and a further £10,000, less what he had already been paid, only when other makers could reproduce it. A new Longitude Act made that law in May 1765.
From May 1766 Maskelyne tested H4 at the Royal Observatory for ten months, and it performed poorly in his published results. Harrison disputed them, and relations grew bitter.
Harrison appealed to King George III and then to Parliament. Plus magazine records that he received the remaining money in 1773, and he died three years later, in 1776. Royal Museums Greenwich notes that the payment came as a reward from Parliament for services to the nation rather than as the prize itself, and that it was less than Harrison felt he was owed.
From Harrison's watch to the marine chronometer
Harrison's timekeepers were brilliant but expensive, and ships did not adopt them overnight. Other makers simplified the idea and brought the cost down, and Royal Museums Greenwich says marine chronometers came into much wider use in the early 1800s.
The principle never changed. A navigator compared the chronometer's home time with local time from the Sun, and the difference gave the ship's longitude, exactly as the Longitude Act's framers had hoped.
Royal Museums Greenwich describes Harrison's marine timekeepers as arguably the most important ever made. All four belong to its collection and are usually shown at the Royal Observatory Greenwich, which has announced a temporary closure for restoration, so check before you travel to see them.
See also: how satellite navigation uses precise time on OnlineClock.app.
Questions people ask
What was the longitude problem?
Ships could not reliably find their east-west position at sea. Because local time changes by one hour per 15 degrees of longitude, solving it required a clock that kept accurate home time at sea.
What are H1, H2, H3 and H4?
John Harrison's four marine timekeepers. H1 to H3 were large sea clocks; H4, dated 1759, is a large watch and the one that passed the official trial.
Did John Harrison win the longitude prize?
Not formally. He received large sums in stages, the remainder in 1773 after appealing to King George III and Parliament, but never the prize in the form he believed he had earned.
Where can I see Harrison's clocks?
H1 to H4 belong to Royal Museums Greenwich and are usually displayed at the Royal Observatory Greenwich in London. The Observatory has announced a temporary closure for restoration, so check its website before visiting.