A marine chronometer is a precision timekeeper carried aboard a ship and used to work out the ship’s position. Navigators compare the Greenwich Mean Time it keeps with local time at the ship to find longitude. Before such timekeepers, sailors had no way to calculate longitude, so they could not be sure where they were.
How does a marine chronometer work?
Most modern chronometers use a detent escapement. A small detent holds the escape wheel still, so the balance can swing with almost no interference. This escapement needs no oil, which gives it a big advantage over other designs. It became the standard for marine chronometers and for other high-precision mechanical timekeepers. Temperature was a second problem. To correct for it, most chronometer balances carried bimetallic strips. As the temperature changed, the strips moved small weights closer to or farther from the centre of the balance.
How was a marine chronometer used on board?
The chronometer hangs in gimbals inside its box. The gimbals keep it lying flat, with the dial facing up, even when the ship tilts. The chronometer itself stayed below deck and was not carried about. Instead, navigators used it to set a hack watch. The hack watch was less accurate, but it could be taken on deck, where the navigators made their sightings.
Where it comes from
The English clockmaker John Harrison solved the problem of accuracy with his small H4 chronometer in 1761. The detent escapement came from France, where Pierre Le Roy invented it in 1748. John Arnold and Thomas Earnshaw improved on his design. In 1780 they developed and patented simpler detached spring detent escapements. The word itself appears in 1684 in Arcanum Navarchicum, a theoretical work by Matthias Wasmuth, a professor at Kiel.
Not to be confused with
Today the word chronometer is also used for watches that have been tested and certified to meet precision standards. That is a separate meaning. A certified watch of this kind is not a marine chronometer. The marine chronometer is a ship’s instrument: a precision timekeeper hung in gimbals inside its box, kept below deck and used to find longitude. The two share a name but are different things.
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