Inventors · 1693 – 1776

John Harrison

John Harrison (1693–1776) was a self-taught English carpenter who built the first clocks accurate enough to find longitude at sea. His pocket-sized H4, tested on a voyage to Jamaica in 1761–62, lost only about 5 seconds in 81 days.

Lived	1693 – 1776
Where	Lincolnshire and London, England
Famous clocks	H1, H2, H3 and the H4 sea watch
Prize	Longitude Act of 1714 offered up to £20,000
Where H1–H4 are now	Royal Observatory, Greenwich

A problem that sank ships

Sailors could find their latitude from the Sun and stars, but not their longitude: how far east or west they were. The answer was time. If you know the time at home and the local time where you are, the difference tells you your longitude. But no clock could keep accurate time on a rolling, damp ship. After a disaster in 1707 that sank four Royal Navy ships and drowned over 1,400 sailors, Parliament passed the Longitude Act of 1714, offering up to £20,000 for a solution.

Forty years at the workbench

Harrison, a carpenter with no formal training, built his first wooden clocks with gears made of oak that needed no oil. Backed by George Graham, he built H1 (1735), then H2 and H3, each taking years. Then he changed direction completely and built H4, a large pocket watch about 13 cm across. On its sea trial to Jamaica in 1761–62 it was off by only about 5 seconds.

A fight for the prize

The Board of Longitude, dominated by astronomers who favored a method using the Moon, kept raising the bar. Harrison was in his late seventies when King George III personally tested a copy of his watch and backed him. In 1773 Parliament awarded him £8,750. He died three years later. His clocks H1 to H4 survive at the Royal Observatory in Greenwich, and H1 still runs.

Questions people ask

What did John Harrison invent?

The marine chronometer: a clock accurate enough at sea to calculate longitude. His H4 watch passed its sea trial in 1761–62.

Did John Harrison get the longitude prize?

Not the official prize. After a long dispute, Parliament awarded him £8,750 in 1773, with the support of King George III.

Sources

Last updated October 18, 2026. Found an error? Tell us.