A marine chronometer is a precision timepiece that is carried on a ship and employed in the determination of the ship's position by celestial navigation. It is used to determine longitude by comparing Greenwich Mean Time (GMT) and the time at the current location found from observations of celestial bodies. When first developed in the 18th century, it was a major technical achievement, as accurate knowledge of the time over a long sea voyage is necessary for navigation, lacking electronic or communications aids. The first true chronometer was the life work of one man, John Harrison, spanning 31 years of persistent experimentation and testing that revolutionized naval (and later aerial) navigation and enabling the Age of Discovery and Colonialism to accelerate.
The term chronometer was coined from the Greek words χρόνος (chronos) (meaning time) and meter (meaning measure) in 1713 by William Derham. It has recently become more commonly used to describe watches tested and certified to meet certain precision standards. Timepieces made in Switzerland may display the word "certified chronometer" or "officially certified chronometer" only if certified by the COSC (Official Swiss Chronometer Testing Institute).