Description: A "chronometer" is a catch-all term for any object designed to tell time. Pocketwatches, grandfather clocks, water clocks, and more can all be called "chronometers". However, when studying navigation, particular focus should be given to marine chronometers.
Telling time onboard a ship is difficult for many reasons. Determining the localtime can be done via sunrise and sunset tables, but determining the time in a more absolute way (for instance, the time in London) is very difficult. Sand glasses are too small, too inaccurate, and must be turned too often. Pendulum-based clocks do not keep accurate time as a ship pitches and rolls on the waves. Water clocks were likewise inaccurate, and often quite heavy. Harrison's style of windable clock was the eventual answer to these problems.
Usage Dates: Harrison's clock in 1741 was the first accurate marine chronometer, but attempts were made as early as 1673. Mechanical chronometers mostly went out of use in the late 20th century as electronic clocks became cheap and accurate.
Henry Sully's marine chronometer from 1716. This model was unable to keep precise time on board a ship.
A beautiful boxed marine chronometer from the Harvard Museum of Historical Scientific Instruments.
Close-up on the same boxed chronometer. Visible are hour and minute marks, a countdown showing when the clock will need to be wound, a separated second hand, and a maker's mark.
Wikipedia's pages on the Marine Chronometer
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