Marine chronometer

Marine chronometer
A marine chronometer by Charles Frodsham of London, shown turned upside down to reveal the movement. Chronometer circa 1844-1860.
Classification	Clock
Industry	Transportation
Application	Timekeeping
Powered	No
Inventor	John Harrison
Invented	1761

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 was vital for effective 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.

The term chronometer was coined from the Greek words χρόνος (chronos) (meaning time) and meter (meaning measure). The 1713 book Physico-Theology by the English cleric and scientist William Derham includes one of the earliest theoretical descriptions of a marine chronometer.^[1] It has recently become more commonly used to describe watches tested and certified to meet certain precision standards.