A marine chronometer is a timekeeper precise enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation. When first developed in the eighteenth century it was a major technical achievement, as accurate knowledge of the time over a long sea voyage is necessary for navigation without electronic or communications aids. The first true chronometer was the life work of one man, John Harrison, spanning 31 years of persistent trial and error that revolutionized naval (and later aerial) navigation as the Age of Discovery and Colonialism hit a new gear. The term chronometer (apparently coined in 1714 by Jeremy Thacker, an early competitor for the prize set by the Longitude Act in the same year) is used more recently to describe wristwatches tested and certified to meet certain precision standards. Timepieces made in Switzerland may only display the word 'Chronometer' if certified by the COSC.

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