John Harrison was a self-taught English clockmaker whose marine timekeepers demonstrated that sufficiently accurate portable time could provide a practical method for determining longitude at sea.
Latitude could be found from celestial altitude with established methods, but longitude required knowing the time difference between a reference meridian and local apparent time.
The British Longitude Act of 1714 offered major financial rewards for methods meeting specified accuracy levels on long voyages.
Harrison first developed precision longcase clocks and then designed H1, a large marine timekeeper with balances intended to reduce the effect of ship motion.
H1 underwent a sea trial to Lisbon in 1736 and performed well enough to earn Harrison financial support for continued development.
H2 and H3 followed, but Harrison identified shortcomings and spent years refining the mechanisms instead of forcing them into formal trials prematurely.
His breakthrough came with H4, completed in 1759. It resembled an oversized watch and used a high-frequency balance derived from precision watchmaking rather than the architecture of H1–H3.
H4 performed successfully on a Jamaica trial in 1761–1762 and again in later testing. The Board of Longitude nevertheless demanded disclosure and reproducibility before granting the full reward Harrison believed he had earned.
Harrison’s dispute with the Board lasted years and involved his son William, Parliament, King George III, and rival astronomical methods such as lunar distances.
For TransitSatellite, the story should not say Harrison single-handedly made longitude easy overnight. H4 demonstrated the method, but marine chronometers became widespread only after later makers simplified and reduced the cost of reliable timekeepers.