Ship’s chronometer

A ship's latitude (declination from equator) can be determined from the position of the sun or north star. But to measure longitude, you need to know time. Pendulums clocks don't work on swaying ships. Spurred by a large prize from the British government, Harrison built highly accurate sea clocks.
 John Harrison England 1728 Ship’s chronometer

A ship's latitude (declination from equator) can be determined from the position of the sun or north star. But to measure longitude, you need to know time. Pendulums clocks don't work on swaying ships. Spurred by a large prize from the British government, Harrison built highly accurate sea clocks.