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How a pocket watch solved the longitude problem.

Latitude could be read from the Sun's height, longitude couldn't: you need the time at your home port. Since Earth rotates 15 degrees per hour, each hour of difference from local time means 15 degrees east or west. Pendulum clocks failed at sea. So carpenter John Harrison built his H4 watch by 1761; it drifted only 5 seconds to Jamaica.

Useful: An accurate time signal is still the core of every position fix, GPS included.

Source: en.wikipedia.org

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