A hack watch is a
mechanical watch
A mechanical watch is a watch that uses a clockwork mechanism to measure the passage of time, as opposed to quartz watches which function using the vibration modes of a piezoelectric quartz tuning fork, or radio watches, which are quartz wat ...
whose
movement offers a mechanism for stopping and setting the seconds hand of the watch, then restarting the watch the instant the time setting matches the time displayed by a reference timepiece.
Hack watches are used on ships for
astronomical sights for
navigation
Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...
and to synchronize the actions of personnel who may not be in direct communication (for example, personnel engaged in a military mission).
For navigational purposes, the hack watch is synchronized with the ship's
marine chronometer
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), or in the modern ...
. The use of a hack watch makes it easier to take sights, as the chronometer is normally in a fixed position in a ship – below decks and suspended in
gimbals
A gimbal is a pivoted support that permits rotation of an object about an axis. A set of three gimbals, one mounted on the other with orthogonal pivot axes, may be used to allow an object mounted on the innermost gimbal to remain independent of ...
to keep it level and protect it from the elements, while the hack watch is portable and can be carried on deck. Though not as accurate as the chronometer, the hack watch is accurate enough to be satisfactory over the relatively short time period between setting it from the chronometer and taking the sight.
For mission synchronization, several hack watches can be set alike, then set going at the same moment.
References
Nautical terminology
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