Harrison radically re-thought his design and produced H4, a timekeeper that resembled a large pocket watch.

BBC: Longitude clock comes alive

Yet, 200 years ago, H4's accuracy would have seemed, as it did to the astronomers, simply unbelievable.

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"What it does is it brings things into a more realistic perspective, " says Neale Smiles, business director of military training company H4 Global.

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"What H4 was doing is still current today because in GPS, for example, accurate time standards are required for navigation, " said Jonathan Betts.

BBC: Longitude clock comes alive

Then Economou switched to legal research, renaming his companyH5 in tribute to 18th-century English clockmaker John Harrison, whose fourth chronometer, commonly known as "H4, " enabled modern sea navigation.

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In the meantime Captain Cook had embarked on his second voyage of discovery with a copy of H4, which he referred to as "our faithful guide through all the vicissitudes of climates".

BBC: Longitude clock comes alive

When H4 was taken on a testing voyage across the Atlantic in 1764, it established the longitude of Barbados to within 9.8 nautical miles (18.1 km) of what we know it to be today.

BBC: Time to make history