Latitude was easy, longitude was not
A navigator at sea can work out latitude, the distance north or south of the equator, by measuring the angle of the Sun at noon or of a known star above the horizon. Longitude, the distance east or west, is harder because there is no fixed feature in the sky to mark it. The Earth turns once in 24 hours, which is 15 degrees of longitude per hour (360 divided by 24), or one degree every four minutes. So if a navigator knows the local time on board (from the Sun) and also knows the time at a reference place such as home port at that very moment, the difference between the two times gives the longitude.
The difficulty was knowing the home time. Pendulum clocks of the day did poorly on a rolling deck and with changes in temperature and humidity. Without a reliable way to carry the time of a reference port, ships relied on dead reckoning, estimating position from speed, heading and elapsed time, and errors accumulated over weeks.
The cost of getting it wrong
Errors in longitude cost lives and cargo. A frequently cited disaster is the loss of several Royal Navy ships on the Isles of Scilly in 1707, in which Admiral Sir Cloudesley Shovell died. It is often linked to navigation uncertainty, though the causes were complex. Several European states had already offered rewards for a solution, including Spain, the Dutch Republic and France.
In 1714 the British Parliament passed the Longitude Act, creating a Board of Longitude with the power to give awards. The top prize, reported as 20,000 pounds, required a method that could find longitude within half a degree after a voyage to the West Indies. Half a degree equals two minutes of time (half of four minutes per degree), and near the equator it corresponds to about 30 nautical miles. A clock aiming for that accuracy after a six-week voyage had to lose or gain only a few seconds a day.
Two possible answers
There were two main ideas. One was astronomical: the lunar distance method, in which a navigator measures the angle between the Moon and a bright star or the Sun, then looks up the result in tables to find the time at a reference meridian. It needed careful observation, difficult calculation, and accurate tables. Tobias Mayer’s lunar tables and the work of the Astronomer Royal Nevil Maskelyne made the method practical, and the Nautical Almanac, first published in 1767, supported it.
The other was mechanical: build a clock that keeps time at sea. Many scientists, including some of the leading figures of the era, doubted that this was possible. Both approaches were tested over decades, and each had its strengths.
John Harrison
John Harrison, a self-taught carpenter and clockmaker from Yorkshire, took on the mechanical approach. He designed clocks with parts that avoided the need for lubrication, used combinations of materials to cancel out the effect of temperature, and balanced the movement so that the ship’s motion would not disturb it. His first sea clock, now called H1, was completed in the mid-1730s and tested on a voyage to Lisbon and back in 1736. It was large and weighed well over 30 kilograms. It performed well enough to encourage the Board of Longitude to fund further work, though the trial was not a formal test of the prize requirements.
H2 and H3 followed. H3 took many years to complete, and Harrison then turned to a very different design that looked like an oversized pocket watch, now called H4, finished by 1759. H4 was tested on a voyage to Jamaica in 1761 and 1762 and then to Barbados in 1764. Reports say that it lost only a few seconds over the long passage to Jamaica, which was better than the prize required. Even so, the Board demanded more tests, wanted the design to be disclosed and copied, and required other makers to produce working copies. Harrison received payments along the way, and he was eventually granted a final settlement in 1773 after the king reportedly intervened, though he never received the formal prize in the way he had hoped.
Copies, rivals and later makers
A watchmaker named Larcum Kendall made a copy of H4, known as K1. Captain James Cook took a similar timekeeper (K1) on his second voyage of 1772 to 1775, and his reports of its performance supported the case for sea-going timekeepers.
Other makers worked in parallel. In France, Pierre Le Roy and Ferdinand Berthoud designed their own timekeepers with different mechanisms. In Britain, John Arnold and Thomas Earnshaw developed simpler designs, including the detent escapement, that could be made more quickly and cheaply. Earnshaw’s layout became close to the standard form of a marine chronometer, and both Arnold and Earnshaw helped to turn the device from a one-off masterpiece into a product.
A tool in the navy and merchant fleets
By the early 1800s, Royal Navy ships carried chronometers, often more than one so the readings could be compared and the failure of a single instrument noticed. The instruments were kept in gimbaled boxes to keep them level and wound at the same time every day. Navigators checked the rate against a known time signal whenever they reached a port, using the observatory’s time ball or a signal. Later, wireless telegraphy allowed ships to receive time signals directly from shore stations.
For the great sailing ships of the age, along with later clipper ships, chronometers were part of a toolkit that also included sextants and lunar distances. The chronometer did not make the lunar method vanish overnight; for a time both were taught, since chronometers were expensive, and an observer with good tables could still check them.
What it meant
The marine chronometer made it possible to plot courses and charts with much more confidence. It supported surveying, trade and exploration, and it helped standardize maps by giving every chart a more accurate grid. Chronometers stayed in use for more than two centuries, and ships still carry them as backup. Today, satellite navigation has replaced them as the main method, though the principle is similar: measure time very accurately and use it to work out where you are.