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A chronometer (more fully, marine chronometer) is an accurate timepiece used to establish a ship's longitude. For this purpose, the chronometer is set to show the time at an agreed fixed point (usually Greenwich). This is then compared with the ship's local time as determined by observations, such as the officers' daily verification of noon by means of the sextant or quadrant. The difference between the two times can easily be converted into degrees; one hour represents one twenty-fourth of the earth's circumference, or fifteen degrees. Thus, if the daily observation shows that the sun is at its highest at a time when the chronometer reads 2 p.m., it follows that the ship is two hours or thirty degrees west of Greenwich. In turn, the distance in degrees can be converted into nautical miles once the ship's latitude is known; one degree represents 60 nautical miles at the equator, diminishing to zero at the Poles. (In practice there are other factors which have to be taken into account for an accurate calculation of longitude, notably the rate of the chronometer - that is its overall tendency to run fast or slow - and the difference between solar time and mean time.)

The first practical marine chronometer was completed in 1759 by the English self-taught craftsman John Harrison (1696-1778). Abandoning the large table-top chronometers on which he had been working with considerable success since the 1730s, Harrison built this machine (his 'No. 4') in the form of a large pocket-watch, and many early chronometers followed this example. However, it was realised from the first that it was advisable to keep the position of the instrument unchanged as far as possible, and so it became common by about 1790 to house the chronometer in a wooden (usually mahogany) box. John Arnold (1734-1799), who was the first to put chronometers into regular production, favoured an octagonal mahogany case; the brass movement had to be lifted out and turned upside-down for winding. Jack Aubrey favoured Arnold's chronometers. Arnold's younger contemporary Thomas Earnshaw developed a more sophisticated case in which the movement was suspended in gimbals, like those used for the ship's compass, so that it would remain horizontal in all conditions; this pattern had emerged by about 1810, became standardised by 1840, and was followed with very little change until electronic navigational methods superseded the mechanical chronometer in the 1970s.

By 1800 the Admiralty had acquired a stock of chronometers, and a captain bound for a long voyage could apply for one of these. However, a conscientious navigator such as Jack would commonly provide two or three additional chronometers from his own resources, so that each could act as a check on the others.

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