Marine chronometer is the name given to a large watch primarily designed for keeping time on boats. Marine chronometers have a range of special features:1) They are almost always fitted with a detent escapement and their direct-drive hand jumps every half second.2) They feature a fusee - a special device designed to regulate the force of the mainspring.3) The marine chronometer is mounted on a "cardan" suspension, arranged in such a way that the chronometer is positioned horizontally, whatever the movements or position of the boat.It is an extremely high-precision instrument that is primarily used to determine longitude at sea. Some chronometers are regulated to sidereal time. The principle of longitude determination is, very briefly, as follows:the boat leaves port with its chronometer regulated to Greenwich Mean Time. It also carries special tables ("Connaissance des Temps", Nautical Almanac, etc.). These tables indicate, for each day of the year, the time at which the sun and certain stars cross the Greenwich Meridian.To determine longitude, the navigating officer uses his chronometer to observe the time at which any star, or the sun, passes his own meridian. The difference between the time of the star's passage and the time at which it is set to cross the Greenwich Meridian, indicates, in hours, minutes and seconds, the longitudinal difference between Greenwich and the point of observation.Smaller watches known as deck watches are also used in navigation.