What is "marine chronometer"

A marine chronometer is a timepiece that is precise and accurate 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 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 (apparently coined by the Greek words chronos [meaning time] and meter [meaning counter] 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 watches tested and certified to meet certain precision standards. Timepieces made in Switzerland may display the word 'chronometer' only if certified by the COSC (Official Swiss Chronometer Testing Institute).