Marine chronometer
Completion of John Harrison's H4 longitude watch
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Original prompt & settings (JSON) ↗It looked like an oversized pocket watch and weighed 1.45 kilograms. Its balance ticked five times a second — audibly faster than a typical eighteenth-century watch.
A watch instead of a clock
Royal Museums Greenwich, which holds H4, records it as John Harrison's completed 1759 longitude watch and dates the object to that year. The movement itself carries the inscription John Harrison and Son A.D. 1759.
Harrison began it in 1755, after a smaller watch made to his design by John Jefferys showed what a watch-sized timekeeper could do. His long-running H3 was struggling to reach the required accuracy, so he changed form entirely: a stable, high-frequency balance in a case 165 by 124 by 28 millimetres, with a 102-millimetre dial.
The 1760 problem
No exact completion day is established. The Science Museum Group chronology says Harrison produced H4 in 1760, and another historical account says it was completed and shown to the Board that year.
The difference may reflect physical completion against presentation or final adjustment. The movement inscription supports the custodian's 1759 dating.
Completed is not proven
Finishing the watch did not demonstrate that it worked at sea. Its first transatlantic trial did not begin until November 1761, carried by Harrison's son William, and evidence of performance followed in 1761–1762.
Nor was it the first marine timekeeper: Harrison's earlier H1 had made a successful Lisbon voyage in 1736. H4 was a unique, difficult-to-reproduce prototype, not yet the standardized chronometer of the nineteenth century.
Sources
Researched 23 Aug 2026 4 sources 3 audit passes, 1 issue open Date disputed
How this was checked
Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →
Date disputed. Sources disagree about when this happened.
What the sources leave uncertain
- The object's custodian dates H4's completion and manufacture to 1759, supported by the movement inscription 'John Harrison and Son A.D. 1759.' However, the Science Museum Group chronology says Harrison 'produced' H4 in 1760, and another historical account says it was completed and shown to the Board in 1760. The difference may reflect physical completion versus presentation or final adjustment.
- No exact completion day is established by the consulted sources.