A 'never failing guide'
The marine chronometer is a portable clock that can keep accurate time at sea. It is used by navigators to establish longitude — how far east or west somewhere is.
English clockmaker John Harrison invented the chronometer in the mid-1700s, finding a way to keep accurate time on a rolling ship at sea without a pendulum. His invention revolutionised navigation — but he had to fight to claim the Board of Longitude's prize.
Right: This portrait of John Harrison, English clockmaker, was painted by artist Thomas King in 1767. Courtesy: Science Museum, London, United Kingdom/The Bridgeman Art Library.
On his second Pacific voyage in 1772 Cook tested a copy of Harrison's fourth chronometer design, made by London clockmaker Larcum Kendall. Initially sceptical, Cook was eventually won over by the instrument, which he called his 'never failing guide'. By comparing high noon on board to Greenwich time on the chronometer, Cook could quickly determine his longitude, helping him locate and chart coastlines, and set the Resolution's course.
Left: K1, the chronometer tested by Cook on his second and third Pacific voyages. Courtesy: National Maritime Museum, London.