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A chronometer is a clock designed to have sufficient long-term accuracy that it can be used as a portable time standard on a vehicle, usually in order to determine longitude by means of celestial navigation.

Until the mid 1750s, navigation at sea was an unsolved problem. Navigators could determine their latitude by measuring the sun's angle at noon. However to find their longitude, they needed a portable time standard that would work on a ship. Conceptually, at local high noon they could compare the chronometer's time to determine their longitude.

In modern practice, a navigational almanac and trigonometric sight reduction tables permit navigators to measure the Sun, Moon, visible planets or any of 57 navigational stars at any time of day or night.

The problem of the clock was difficult. At the time, the best clocks were pendulum clocks, and the rolling of a ship at sea caused these to be inaccurate. John Harrison, a carpenter, developed a clock based on a pair of counter-oscillating weighted beams connected by springs, whose motion was not influenced by gravity or the motion of a ship. His chronometers H1-H3 were all of this design but were large and heavy, and required to be suspended from a beam in a ship.

He finally solved the problem with his H4 chronometer, essentailly a large 5" diameter watch, winning a prize from the British Admiralty. His design used a temperature-compensated balance wheel. This method remained in use till microchips reduced the cost of a quartz clock to the point that electronic chronometers became commonplace.

The crucial problem was to find a resonator that remained unaffected by the motions of a ship at sea. The balance wheel solved that problem. Balance wheels for chronometers used bi-metallic strips to move small weights toward and away the center of the wheel, in order to adjust the period of the balace wheel for the temperature of the chronometer.

The other crucial problem was that the energy of most spring materials changes with temperature. A special alloy of steel was eventuallly developed, just to solve this problem. Additionally, this spring had to be given a special oval shape.

The escapement[?] drives the balance wheel, usually from a gear train. It's the part that ticks. Escapements have a locking state, and a drive state. In the locking state, nothing moves. The motion of the balance wheel switches the escapement to drive, and the escapement then pushes on the wheel for a brief part of the wheel's cycle.

The escapement is the part of a clock most prone to wear, because it moves the fastest. The efficiency of an escapement's design, that is, how much energy is converted into resonant motion, directly affects the accuracy of a clock, and long a clock can operate between windings.

A chronometer's escapement is usually designed to minimize the energy and time required to unlock the escapement, so that it affects the resonant frequency of the oscillator as little as possible.

Quartz clocks and atomic clocks have made mechanical chronometers obsolete, although some custom watchmakers can still produce them. The techniques used to mass-produce mechanical chronometers are now lost. The techniques to construct mechanical chronometers may soon be lost.

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