Ship's chronometer from HMS Beagle
A History of the World in 100 Objects
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The History of the Marine Chronometer
A marine chronometer is based on one invented in the mid-18th century by John Harrison. He finally cracked the problem of accurate timekeeping at sea, and for the first time made it possible for ships anywhere to establish their longitude. Before a ship set sail, its chronometer would be set to the local time in harbor, for the British this was usually Greenwich. Once at sea you could then compare the time at Greenwich with the time on board ship, wherever you were,. The difference between the two times gave you your longitude. There are 24 hours in the day, so as the earth rotates, every hour that the sun seems to move across the sky is one-
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