Where does this come from, and can I check it?

John Harrison's H4 lost only seconds over an eighty-one day voyage to Jamaica. The figure is taken from Royal Museums Greenwich, which is linked in full on this page. Every fact on Mondo Knows names its source rather than repeating an unsourced claim.

Is this really a history fact?

Yes. It sits on the History shelf, one of 2,600 sourced facts grouped into space, animals, the human body, history, science and more. Category pages collect every fact in the same field.

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The links under the fact point to a quiz, game, puzzle or calculator on the same subject, and the shelf below shows related facts. The wording here is original, and quoted phrases are marked as quotations.

Mondo Knows Facts

A carpenter's clock solved the longitude problem

John Harrison's H4 lost only seconds over an eighty-one day voyage to Jamaica Sailors could find latitude easily but not longitude, because it required knowing the exact time at a reference meridian. Parliament offered twenty thousand pounds for a solution, and clockmaker John Harrison spent decades building sea clocks. His watch-like H4, tested in 1761 and 1762, kept time well enough to fix a ship's position. It sits in Mondo's history collection, one of 200 facts that each trace back to the reference below.

Where this comes from

Royal Museums Greenwich

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Tags: longitude, navigation, clocks

A carpenter's clock solved the longitude problem: the short answers

Where does this come from, and can I check it?: John Harrison's H4 lost only seconds over an eighty-one day voyage to Jamaica. The figure is taken from Royal Museums Greenwich, which is linked in full on this page. Every fact on Mondo Knows names its source rather than repeating an unsourced claim.
Is this really a history fact?: Yes. It sits on the History shelf, one of 2,600 sourced facts grouped into space, animals, the human body, history, science and more. Category pages collect every fact in the same field.
What else can I read about it?: The links under the fact point to a quiz, game, puzzle or calculator on the same subject, and the shelf below shows related facts. The wording here is original, and quoted phrases are marked as quotations.