Marine Chronometers are highly accurate clocks kept aboard ships to aid in navigation. They were developed in the 18th century by John Harrison, and at the time were considered a major technical achievement.
Navigators needed to be able to determine longitude, which meant they needed a time standard that would work aboard a ship. Ordinary clocks were of no use at sea due to temperature changes and the ship's motion. A chronometer is set to Greenwich Mean Time (GMT), and the time difference between the chronometer and the ship's time were used to calculate the longitude of the ship.
Although considered a secondary means of navigation today, the chronometer is the only navigational method that is completely self-reliant, especially in the event of loss of power or radio communications