John Harrison and the Clock That Solved Longitude

For centuries, sailors could find latitude from the sun and stars but not longitude — east-west position — and the ignorance killed. After a navigational blunder wrecked a Royal Navy squadron on the Scilly Isles in 1707 with the loss of nearly two thousand men, Britain’s Parliament passed the Longitude Act of 1714, offering up to £20,000 — a colossal fortune — for a practical method accurate to half a degree on a voyage to the West Indies.

The principle was long understood: longitude is time. Earth turns 15 degrees per hour, so a navigator who knows both local time (from the sun) and the time at a reference port can convert the difference straight into degrees east or west. The obstacle was carrying the reference time: pendulum clocks died at sea, and no watch of the era could hold accuracy through months of rolling decks, salt air, and temperature swings. The learned consensus favored astronomy instead — the intricate “lunar distance” method of reading time from the moon’s position among the stars.

John Harrison, a self-taught Yorkshire clockmaker, bet on machinery. Across four decades he built four marine timekeepers: H1, a gleaming spring-balanced machine immune to the pendulum’s failings; H2 and H3, each solving problems the last revealed, introducing inventions like the bimetallic temperature compensator and caged roller bearings still in use today. Then came the leap: H4, finished in 1759, abandoned the large-machine approach entirely for a five-inch watch of exquisite precision. On its 1761–62 trial to Jamaica it lost mere seconds — comfortably inside the prize’s demand.

The reward came grudgingly. The Board of Longitude, dominated by astronomers including the Astronomer Royal Nevil Maskelyne — champion of the rival lunar method — demanded retrials, disclosure, and duplication, paying in reluctant installments. Only in 1773, after the aged Harrison petitioned King George III himself (“By God, Harrison, I will see you righted!”), did Parliament grant the balance of his due.

Copies of H4’s principles, made affordable by later makers, became the marine chronometer that guided Cook’s voyages and a century of navigation — Greenwich time carried in a box, the reason the world’s zero meridian runs through that observatory today.

Every longitude line on a modern map, and every offset in the world clocks at itsdately.com, still speaks Harrison’s equation: time, converted into where you are.

The irony of the contest is that both sides won: Maskelyne’s lunar tables, published as the Nautical Almanac from 1767, gave navigators a cheap astronomical method for decades, while chronometers — at first costing as much as a small ship’s cargo — fell in price until every vessel carried several. Captains ran the methods against each other, and the sea itself judged: by the mid-nineteenth century the chronometer’s convenience had retired the moon.

Harrison’s machines survive as working celebrities. H1 through H4 are displayed at the Royal Observatory in Greenwich — the first three still running, wound daily by curators — and H4, too precious to run, rests under glass a few steps from the meridian line its descendants made meaningful. Dava Sobel’s book Longitude carried the story to millions, completing the vindication Parliament granted so grudgingly.

Related on itsdately: How Sundials Work (and How to Read One Correctly) · Before Mechanical Clocks: Water Clocks, Hourglasses, and Candle Time · The Oldest Working Clocks in the World

Further reading: Royal Museums Greenwich — home of Harrison’s H1–H4