The Forgotten Genius of John Harrison and the Longitude Problem

Until the middle of the eighteenth century, no ship at sea reliably knew where it was. Latitude was easy: the angle of the noon sun above the horizon, corrected for the date, gave you the answer to within a few miles by 1700. Longitude was a different problem entirely. There was no celestial reference for east-west position. The Earth rotates uniformly under the heavens, so determining where you are along a line of latitude requires comparing local time (which you can measure from the sun) to time at some fixed reference longitude. This is a clock problem.

The trouble is that pendulum clocks, the most accurate timepieces of the seventeenth century, did not work at sea. The roll of the ship interfered with the pendulum. Temperature changes through latitude bands caused metal parts to expand and contract, throwing off the rate. Humidity affected the wood. Salt air corroded everything. The best ships' clocks of 1700 lost or gained ten minutes a day, which corresponds to roughly a hundred and fifty miles of longitudinal error per day at the equator. After a six-week voyage, you could be a thousand miles from where you thought you were.

The Scilly Disaster

The political event that turned the longitude problem from a scientific curiosity into an urgent state matter was the wreck of Sir Cloudesley Shovell's fleet on the Isles of Scilly in October 1707. Four warships ran aground in fog because their longitude was wrong by about a hundred miles. Around two thousand sailors died, including the admiral. It was one of the worst peacetime maritime disasters in British history, and it was caused by a navigation error that better timekeeping would have prevented.

The Longitude Act of 1714 followed. Parliament offered up to £20,000 (roughly £4 million in today's money) to anyone who could devise a method of determining longitude at sea to within half a degree, which corresponds to about two minutes of time. A Board of Longitude was established to evaluate submissions. The prize attracted Newton, Halley, Whiston, and most of the astronomical establishment of Europe.

The two camps

The mainstream view was that the solution had to be astronomical. The "lunar distances" method, championed by Newton and the Royal Astronomers, used the moon's motion across the fixed stars as a celestial clock. By measuring the angle between the moon and a chosen star, comparing it to tables of predicted angles at Greenwich, and doing some spherical trigonometry, you could deduce the time at Greenwich and from there your longitude.

This approach had two defects. First, the calculations were brutal: a competent ship's officer needed about four hours of computation per fix, with logarithm tables, working in conditions that did not favor concentration. Second, it required clear nights and a clear horizon. In poor weather, no fix was possible.

The minority view was the chronometer view: build a clock accurate enough to keep Greenwich time at sea, and the problem becomes trivial. Local solar noon minus Greenwich-time-at-noon, in hours, gives longitude in degrees by direct conversion (one hour equals fifteen degrees). The catch was that no clock existed that could do this. The chronometer route was thought, by the leading astronomers, to be impossible.

Harrison

John Harrison was born in 1693 in Yorkshire, the son of a carpenter. He had no formal training in horology or natural philosophy. He learned woodworking from his father and clockmaking from a borrowed copy of Saunderson's lectures on mathematics, which he is said to have copied out by hand. By his mid-twenties he was building wooden tower clocks of unprecedented accuracy, including one for the stable yard at Brocklesby Park that survives, still running, three centuries later.

The wooden clocks were the seedbed of his later innovations. Wood is a curious material for clockwork: it does not need lubricating, because it can be made of self-lubricating species like lignum vitae, which excretes its own oils. Wooden gears, if cut along the grain, are stronger than metal of equivalent thickness in the dimensions that matter and lighter. Most of all, wood barely changes dimension with temperature compared to brass. Harrison's wooden clocks did not need oil, and they kept time well across hot and cold.

Around 1730, Harrison began working on a marine chronometer. His first attempt, now called H1, was completed in 1735. It weighed seventy pounds, used cross-balanced springs instead of a pendulum to immunize against the ship's motion, and incorporated a "gridiron pendulum" of his own invention to compensate for thermal expansion. He took it on a sea trial to Lisbon in 1736. It performed well enough that the Board of Longitude awarded him £500 to continue.

H2, H3, and the long disappointment

H2 (1739) was a refinement of H1, with a more compact balance system. Harrison decided not to test it at sea, citing a flaw in the design he wanted to fix. H3 was the project that ate two decades of his life. It was a more ambitious instrument, with bimetallic temperature compensation (an early use of the bimetallic strip) and an antifriction system using caged ball bearings that he had to invent himself, since none existed. H3 was never quite finished to Harrison's satisfaction. He worked on it from 1740 until 1755.

The interesting twist is what happened next. Harrison realized, after twenty years of work on large chronometers, that the answer was probably not a large machine at all. It was a watch. A pocket watch could be made from materials of more uniform quality, could be insulated from temperature changes more effectively, and could be tested more cheaply.

H4

H4, completed in 1759, looked like a large pocket watch about five inches across. Harrison delivered it for sea trial in 1761. On a voyage to Jamaica that took eighty-one days, H4 lost five seconds. This is roughly one part in a million; in nautical terms, a longitude error of a single nautical mile, against a prize threshold of thirty miles. The voyage proved the chronometer method.

The Board of Longitude refused to award the prize.

The reasons given were various. The trial had not been long enough. The watch's performance might have been a fluke. The original act required that the method be reproducible by other makers, and only Harrison knew how to build one. Harrison was required to disclose his methods to other watchmakers, then to build a copy (H5), then to be tested again. Each demand was met. Each was followed by another.

The astronomers on the Board, including the new Astronomer Royal Nevil Maskelyne, had a competing interest. Maskelyne had been preparing the first Nautical Almanac, the volume of pre-computed lunar distance tables that would make the rival method practical. The Board's footdragging on Harrison kept the chronometer route from being declared the winner before the lunar distance method could establish itself in parallel.

The intervention of the King

Harrison was eighty when the dispute came to a head. His son William petitioned King George III directly. The King had H5 tested in his own private observatory at Kew, watched it run for ten weeks, and reportedly told Harrison: "By God, Harrison, I will see you righted." Royal pressure produced a parliamentary act in 1773 that awarded Harrison most (though not all) of the prize money. He was eighty when he received it. He died three years later.

What it took

The interesting thing about Harrison's story is that the technical achievement is in some ways the smaller part. The longitude problem was solved by a self-taught carpenter who spent forty years on a single piece of equipment. He did this against a scientific establishment that thought his approach was futile, against a board of evaluators that resented his progress, and against a competing approach (the lunar distance method) that was being institutionally favored.

What it required was not just engineering insight but the kind of stubborn persistence that does not accept "we will not give you the prize" as a final answer for the third time, the fifth time, the tenth time. Harrison kept building, kept showing that the watch worked, kept submitting it for trial. The institutional machinery eventually gave way, but it took a king.

The chronometer that came out of this work transformed navigation. By the 1820s, marine chronometers were standard issue on Royal Navy ships. The shipwrecks of the Scilly type, where a fleet runs aground because nobody knows where it is, became uncommon. The chain of consequences runs forward to the modern world: sea trade became safer and cheaper, merchant fleets grew, the global economy of the nineteenth century was made possible in part by knowing, on a heaving ship in fog, what time it was at Greenwich.

Harrison's H4 sits today in the Royal Observatory at Greenwich. It still keeps time when wound. The Board of Longitude no longer exists.