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Before reliable clocks existed, sailors faced a dangerous problem: they could find their latitude, or how far north or south they were, by measuring the Sun and stars, but they had no accurate way to find their longitude, or east-west position. The key, eventually realised, was time. Because the Earth turns at a steady rate, knowing the exact time at a fixed home port and comparing it with local time at sea reveals how far east or west a ship has travelled. The challenge was building a clock that kept perfect time despite the rolling, damp, and changeable conditions aboard a ship. Solving that puzzle transformed ocean navigation.