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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsJohn Harrison’s H4 was a watch-like marine timekeeper designed to preserve the time at a reference meridian during a voyage. By comparing that time with local time at sea, navigators could calculate longitude. Its sea trials showed that a compact mechanical watch could meet the Longitude Act’s demanding accuracy limit—but the prize process that followed was contested and did not end with a simple £20,000 payout.
How H4 addressed the longitude problem
By 1700, sailors could determine latitude more reliably than longitude. Longitude depended on knowing the time at a known meridian, then comparing it with local time aboard ship. A difference of one hour corresponds to 15 degrees of longitude, so a reliable reference clock could turn a time comparison into an east–west position. Royal Museums Greenwich describes the problem as urgent in an era of expanding international trade and dangerous shipwrecks.
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H4 was intended to carry that reference time through a voyage. The navigator could read the timekeeper and compare it with local time determined from the Sun or stars. This approach avoided having to calculate Greenwich time from a sequence of celestial observations every time a position was needed, but it depended on a clock that could keep time despite temperature changes and a ship’s motion.
What made H4 different from Harrison’s earlier sea clocks?
H1, H2 and H3 were large mechanical sea clocks. H2 did not go to sea trial, and H3’s balance arrangement did not deliver the timekeeping accuracy Harrison sought. H4 was a major change in form: it looked like an oversized pocket watch, but it was built as a precision marine timekeeper, not an ordinary watch.
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Around 1751–52, Harrison commissioned watchmaker John Jefferys to make a watch with a new type of balance. Harrison incorporated that balance into H4 after it performed well. Royal Museums Greenwich says H4 ticks five times per second, describing its rapid ticking as the clue to its operation.
The design combined a high-energy balance with mechanisms intended to limit the effects of movement and temperature. Royal Museums Greenwich says the balance was less affected by a ship’s motion. NIST’s technical history describes a heavily modified verge escapement with diamond pallets and a spring-and-balance oscillator compensated for temperature changes. These features distinguish H4’s approach; they are not a complete account of its engineering.
What happened on H4’s sea trials?
Jamaica, 1761
The Commissioners authorized William Harrison’s Jamaica trial in 1761. Royal Museums Greenwich recounts that William used H4 to predict an earlier landfall at Madeira than the crew expected. After the voyage, the Commissioners considered the test insufficient, and relations between the Harrisons and the Board deteriorated.
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NIST’s 2011 account reports that H4 lost 1 minute 54.5 seconds over the 147-day Jamaica voyage after rate adjustment. The adjustment itself became part of the dispute, so that figure should be understood as an adjusted result rather than a simple uncorrected voyage loss.
Barbados, 1764
Harrison sailed with H4 on its second sea trial to Barbados in 1764. Nevil Maskelyne had gone ahead and determined Barbados’s longitude using observations of Jupiter’s satellites, providing a comparison for the timekeeper’s result. NIST reports that the voyage lasted 46 days and that H4’s average estimated error was 39.1 seconds, corresponding to less than 10 nautical miles of longitude.
In February 1765, the Board considered the Barbados trial and confirmed that H4 had kept time within the strictest limit in the 1714 Longitude Act: accuracy sufficient to determine longitude within 30 nautical miles. Meeting that test did not automatically settle the award, because the Board’s proposed terms and the Harrisons’ interpretation of the original Act differed.
Why the £20,000 prize did not mean a straightforward win
The Longitude Act offered a top prize of £20,000 for a method meeting the 30-nautical-mile standard. The Board proposed £10,000 after Harrison demonstrated H4’s principles, with the remaining £10,000 dependent on other makers producing similar timekeepers. Those terms became law in the Longitude Act of 10 May 1765. The Harrisons believed the full reward was already due under the original Act.
The accounts of payments distinguish the headline prize from what Harrison received. NIST’s 2011 history says Harrison received £10,000 when H4 was allowed to be copied in 1765, then a further £8,750 in parliamentary compensation in 1773. Royal Observatory Greenwich’s history separately describes a £7,500 award later in 1765 as bringing the total received for the completed H4 to £10,000. The figures reflect different stages and descriptions of the award process; they do not support saying that Harrison simply collected the full £20,000 prize after the Barbados trial.
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Greenwich testing and continued disagreement
On 5 May 1766, Astronomer Royal Nevil Maskelyne received H4 from the Board for further testing at the Royal Observatory. The test lasted ten months, and H4 performed poorly in that setting. Maskelyne published results that Harrison challenged. The dispute continued even after the successful Barbados voyage and the Board’s finding that H4 met the Act’s strictest accuracy limit.
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How H4 compared with lunar-distance navigation
H4 and the lunar-distance method offered different ways to find longitude through time. H4 carried a timekeeper preserving reference time. Lunar navigation measured the Moon’s angular distance from a star or the Sun, then used published tables and calculations to determine Greenwich time and compare it with local time.
| Approach | What the navigator did | Main practical demand |
|---|---|---|
| H4 timekeeper | Carried reference time aboard ship and compared it with local time. | Required a precision instrument to keep working accurately through the voyage. |
| Lunar distances | Measured the Moon’s angular distance from a star or the Sun, then used tables to calculate Greenwich time. | Required celestial observations and lengthy calculations; Royal Observatory Greenwich says Maskelyne could take up to four hours to complete them. |
Neither approach was initially fully practicable at broad scale, according to Royal Observatory Greenwich’s history. H4 demonstrated the potential of a sea-going timekeeper, while lunar distances offered a route based on observations and calculation rather than carrying a clock with reference time.
Where can you see the original H4?
The original H4 is held and displayed at the Royal Observatory Greenwich in its Time and Longitude gallery, according to Royal Museums Greenwich. The gallery calls it “the most important timekeeper ever made”—the museum’s characterization of the artifact, not a neutral consensus ranking. Check the museum’s current visitor information before planning a visit.
Quick Recap
Sources
- Royal Museums Greenwich: John Harrison and H4
- Royal Museums Greenwich: The longitude problem and Time and Longitude gallery
- NIST: Longitude and chronometers (2011)
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