The longitude problem and the marine chronometer (John Harrison's H4)
A ship's timekeeper accurate enough to give longitude at sea: the difference between shipboard time and prime-meridian time is the longitude.
The British Parliament in 1714 offered a reward of up to 20,000 pounds, and such accuracy was thought unattainable: Newton in his memorandum for Parliament listed 'by a Watch to keep Time exactly' first and judged it not yet achievable. The English clockmaker John Harrison built H4, a timekeeper the size of a large pocket watch: a hardened steel balance, a spring against a bimetallic compensation curb, diamond pallets, a brass fusee movement. On the trial aboard the Deptford (sailed 18 November 1761, Jamaica 19 January 1762) the watch was 5.1 seconds slow; on the second trial of 1764 the error was 39.2 seconds over 47 days at sea.
The techniques that held such accuracy in an instrument that is rocked and alternately heated and cooled then passed into serial production: John Arnold and Thomas Earnshaw built more than two thousand marine chronometers in the late eighteenth and early nineteenth centuries.
Why this date
exact for the trial: the H4 timekeeper was built between 1755 and 1759, went to sea aboard the Deptford on 18 November 1761 and reached Jamaica on 19 January 1762. The reward was established by an Act of Parliament of 1714, the second trial to Barbados took place in 1764, and Harrison was paid by a separate Act of Parliament in June 1773. The axis value is 1761, the year of the first sea trial, not the year of construction nor the year of payment.
What it grew from
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The Longitude Act of 1714 and the Board of Longitude
influenced · Confidence: high
Harrison did not work in a vacuum or at his own expense: the successful trial of his first sea clock brought the Commissioners to their first minuted meeting, and from that followed a series of payments totalling over 5,000 pounds with which the next timekeepers, H4 among them, were built and tested. The Board also set the standard of accuracy by which the watch was judged.
Dispute: The link is called an influence rather than a hard dependence by deliberate choice. On Higgitt's account Harrison built his first sea clock before the Commissioners' first minuted meeting, and it was the trial of that clock which prompted the meeting: the Board paid for the continuation, not the beginning, of the work. Higgitt also notes that the Board's relationship with Harrison eventually broke down 'irrevocably', so the patronage was both a condition of the work and a drag on it.
Link sources
- Rebekah Higgitt, «Longitude», Encyclopedia of the History of Science, vol. 4, issue 1 (April 2021) (2021) Verified 20 Sep 2026
In Britain, the promising work of John Harrison, and successful trial of his first sea clock, prompted the first minuted meeting of the Commissioners of Longitude. This resulted in the first of a series of payments, totaling over £5000, given to Harrison to improve and test his next two sea clocks and sea watch, before they were considered for the large £10,000 or £20,000 rewards.
- «An Act for Providing a Publick Reward for such Person or Persons as shall Discover the Longitude at Sea» (Longitude Act), Royal Observatory Greenwich (1714) Verified 5 Oct 2026
to a reward, or sum of ten thousand pounds, if it determines the said longitude to one degree of a great circle, or sixty geographical miles
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
- Rebekah Higgitt, «Longitude», Encyclopedia of the History of Science, vol. 4, issue 1 (April 2021) (2021) Verified 20 Sep 2026
Other links
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The Nautical Almanac and the lunar-distance method in the mariner's hands
parallel to · Confidence: high
Two independent solutions to one problem, brought to completion in the same years and by the same institution. The chronometer carries prime-meridian time with it, the lunar method reads that time off the sky, and the Board of Longitude judged both to be workable.
Dispute: Historians disagree not about whether there were two solutions but about what to call their relationship. Higgitt writes: 'Some histories suggest the story of longitude is one of competition between astronomical and timekeeper methods', and immediately: 'Newer scholarship has instead emphasized complementarity', adding that 'Ultimately, where precision was needed or if there was any uncertainly, timekeepers and astronomical methods were used together', because a timekeeper required observations of local time and lunar observations required a watch. So parallel here is to be read as 'two independent lines solving one problem', not as 'rivals of which one survived'. The edge should not reproduce the popular scheme of Harrison against the astronomers: Higgitt states plainly that 'No one method or individual definitively "solved" the problem of longitude'.
Link sources
- Seb Falk, «Ships, Clocks & Stars: The Quest for Longitude», Science Museum Group Journal, Autumn 2014 (2014) Verified 20 Sep 2026
The exhibition presents each of these methods alongside their original proponents, explaining them succinctly before narrowing them down to the two eventual front runners: the timekeepers and the lunar-distance method.
- Seb Falk, «Ships, Clocks & Stars: The Quest for Longitude», Science Museum Group Journal, Autumn 2014 (2014) Verified 20 Sep 2026
Of course, even once the Board had agreed that both the chronometric and lunar-distance methods had been proved workable, the considerable task of development still lay ahead.
- Rebekah Higgitt, «Longitude», Encyclopedia of the History of Science, vol. 4, issue 1 (April 2021) (2021) Verified 20 Sep 2026
In the 18th century, both the lunar-distance and timekeeper methods were rewarded, promoted and developed.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
- Seb Falk, «Ships, Clocks & Stars: The Quest for Longitude», Science Museum Group Journal, Autumn 2014 (2014) Verified 20 Sep 2026
Node sources
- «An Act for Providing a Publick Reward for such Person or Persons as shall Discover the Longitude at Sea» (Longitude Act), Royal Observatory Greenwich / Board of Longitude (1714) Verified 5 Oct 2026
to a reward, or sum of ten thousand pounds, if it determines the said longitude to one degree of a great circle, or sixty geographical miles
- https://www.royalobservatorygreenwich.org/articles.php?article=1312 (1714) Verified 5 Oct 2026
One is, by a Watch to keep Time exactly
- «John Harrison and the Longitude problem», (Royal Observatory Greenwich) Verified 5 Oct 2026
They arrived in Jamaica on 19 January 1762, where the watch was found to be only 5.1 seconds slow!
- «John Harrison and the Longitude problem» Verified 5 Oct 2026
The watch's error was computed to be 39.2 seconds over a voyage of 47 days, three times better than required to win the £20,000 longitude prize.
- Royal Museums Greenwich, K1 Verified 5 Oct 2026
The hardened steel balance has a three turn blued steel spiral balance spring, of tapered form, with a long, somewhat straighter tail, acting against a bimetallic compensation curb, and a secondary 'isochronal curb pin.
- Royal Museums Greenwich Verified 5 Oct 2026
The timekeeper has a brass, one-day full plate fusee movement with four turned pillars, with a highly engraved slide plate, balance bridge and third wheel bridge.
- Falk S., «Ships, Clocks & Stars: The Quest for Longitude», Science Museum Group Journal (2014) Verified 5 Oct 2026
Arnold's and Earnshaw's large-scale production allowed more than 2,000 marine chronometers to be built in the late eighteenth and early nineteenth century
- «Longitude found: the story of Harrison's timekeepers», Royal Museums Greenwich (2024) Verified 5 Oct 2026
In 1761 the Commissioners gave permission for Harrison's son, William, to prepare for a voyage to Jamaica to trial the H4 timekeeper.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.