1707 – 1773
Who solved the longitude problem?
John Harrison, a self-taught English carpenter and clockmaker, solved it with an accurate sea clock. His H4 watch, finished in 1759, lost only about 5 seconds on an 81-day voyage to Jamaica in 1761–62. Comparing local time with a clock keeping home-port time gives longitude: every 4 minutes of difference is 1 degree.
Key facts
| The rule | 1 hour of time difference = 15° of longitude; 4 minutes = 1° |
|---|---|
| Error from a 1-second clock error | about 0.25 nautical miles at the equator |
| Scilly naval disaster | 1707, four warships wrecked, about 1,400–2,000 sailors lost |
| Longitude Act | 1714, prize up to £20,000 for accuracy within half a degree |
| Harrison's sea clocks | H1 (1735), H2, H3, H4 (1759) |
| H4 trial | 1761–62, lost about 5 seconds in 81 days |
| Rival method | Lunar distances, Nautical Almanac first published 1767 |
Finding your latitude at sea is relatively easy: measure the height of the sun at noon or the Pole Star at night. Longitude, your east–west position, was the great unsolved problem of navigation, and it was really a problem about time.
Why longitude needs a clock
The Earth turns 15 degrees every hour. If you know the exact time at your home port, and you can tell local time from the sun, the difference gives your longitude. Local noon three hours after London noon means you're 45 degrees west of London. But the clock has to be very good: an error of just one minute puts you about 15 nautical miles off at the equator. Pendulum clocks were useless on a heaving deck, and no other clock came close.
The cost of not knowing
Ships sailed blind, often following a line of latitude to a known coast and then running along it. Mistakes were deadly. In 1707, a British fleet under Admiral Sir Cloudesley Shovell misjudged its position and struck rocks off the Isles of Scilly; four warships sank and somewhere between 1,400 and 2,000 sailors died. In 1714 Parliament passed the Longitude Act, offering up to £20,000, a fortune at the time, for a practical method accurate to within half a degree.
Harrison's clocks
John Harrison (1693–1776) was a carpenter from Lincolnshire with no formal training, who had already built wooden clocks with remarkably low friction. He spent the next four decades on sea clocks. H1 (1735) was a large brass machine with linked balances that cancelled out the ship's motion. H2 and H3 refined the idea, and H3 introduced the bimetallic strip to correct for temperature and the caged roller bearing.
Then Harrison changed direction. H4, completed in 1759, was a large pocket watch about 13 cm across, with a fast-beating balance, jewelled bearings and temperature compensation. On its trial voyage to Jamaica in 1761–62 aboard HMS Deptford, it lost only about 5 seconds in 81 days, comfortably within the prize's terms.
The fight for the prize
The Board of Longitude, dominated by astronomers, was slow to accept that a watch could beat astronomy. A rival method, lunar distances, worked out longitude by measuring the moon's position against the stars, and the Astronomer Royal Nevil Maskelyne championed it with the first Nautical Almanac in 1767. Harrison received payments over the years, and after an appeal to King George III, Parliament awarded him £8,750 in 1773. He never received the formal prize.
What came next
Captain James Cook took Larcum Kendall's copy of H4, called K1, on his second Pacific voyage and praised it highly. Cheaper chronometers by John Arnold and Thomas Earnshaw followed, and by the 1800s most ocean-going ships carried one. The idea of a precise reference time shared across the world, and a single prime meridian to measure from, led directly to the time zones we use today.
Harrison's H1 to H4 survive and can be seen at the Royal Observatory, Greenwich.
Questions people ask
How do you find longitude with a clock?
Compare local time, found from the sun, with the time at a reference place kept by an accurate clock. Each hour of difference is 15 degrees of longitude; each 4 minutes is 1 degree.
Did John Harrison get the £20,000 prize?
Not formally. He received a series of payments from the Board of Longitude and £8,750 from Parliament in 1773, which together came to roughly the prize amount, but the Board never declared him the winner.
Where are Harrison's clocks now?
H1, H2, H3 and H4 are on display at the Royal Observatory in Greenwich, London. Most of them still run.
Sources
Last updated October 18, 2026. Found an error? Tell us.