Longitude clock no. 24

Until the 18th century, sailors navigated by ‘dead reckoning’, calculating the latitude by taking measurements using the North Star or the height of the Sun with the aid of astrolabes, sextants or octants. In the 1730s, nautical clocks joined these navigation instruments. On departure, the marine clock is set to the time. By measuring the difference between the time indicated on this clock and the time ‘observed’ (for example with a sundial), sailors could determine the longitude and thus calculate their precise position at sea. In 1769 Ferdinand Berthoud created a clock of hitherto unknown precision that varied by only two seconds a day despite corrosion, temperature changes and the rolling of the ship. Between 1760 and 1787 Berthoud made forty-five nautical clocks and longitude watches, thereby giving the French navy the means to rival its British adversary. His collection was acquired by Louis XVI in 1782 and allocated to the Conservatoire in 1807.