Harrison's Sea Watch

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Reality: He never won the prize as such, but he was not left empty-handed. He received £4,315 from the Board of Longitude, £10,000 in 1765 and £8,750 from Parliament in 1773 — £23,065 in total, more than the headline prize.

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Reality: It was a clock problem. Latitude can be read from the sun’s height, but longitude requires knowing the time at a reference place at the moment of observation — and no clock could hold time on a rolling, damp, temperature-swinging ship.

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Reality: H4 is a large watch, not the boxed gimballed instrument the phrase "marine chronometer" usually calls to mind.