Editing Navigation (section)

====Marine chronometer====
{{main|Marine chronometer}}
[[File:Breguet marine clock-CnAM 16767-1-IMG 1525-white.jpg|thumb|right|Breguet marine chronometer]]
In order to accurately measure longitude, the precise time is required of a sextant sighting (down to the second, if possible) which is then recorded for subsequent calculation. Each second of error is equivalent to 15 seconds of longitude error, which at the equator is a position error of .25 of a nautical mile, about the accuracy limit of manual celestial navigation. The spring-driven marine chronometer is a precision timepiece used aboard ship to provide accurate time for celestial observations.<ref name="bow269">Bowditch, 2002:269.</ref> A chronometer differs from a spring-driven watch principally in that it contains a variable lever device to maintain even pressure on the mainspring, and a special balance designed to compensate for temperature variations.<ref name="bow269"/> A spring-driven chronometer is set approximately to Greenwich mean time (GMT) and is not reset until the instrument is overhauled and cleaned, usually at three-year intervals.<ref name="bow269"/> The difference between GMT and chronometer time is carefully determined and applied as a correction to all chronometer readings.<ref name="bow269"/> Spring-driven chronometers must be wound at about the same time each day.<ref name="bow269"/>

[[Quartz clock#Accuracy enhancement|Quartz crystal marine chronometers]] have replaced spring-driven chronometers onboard modern ships because of their greater accuracy.<ref name="bow269"/> They are maintained on GMT directly from radio time signals.<ref name="bow269"/> This eliminates chronometer error and watch error corrections.<ref name="bow269"/> Should the second hand be in error by a readable amount, it can be reset electrically.<ref name="bow269"/> The basic element for time generation is a quartz crystal oscillator.<ref name="bow269"/> The quartz crystal is temperature compensated and is hermetically sealed in an evacuated envelope.<ref name="bow269"/> A calibrated adjustment capability is provided to adjust for the aging of the crystal.<ref name="bow269"/>

The chronometer is typically designed to operate for a minimum of one year on a single set of batteries.<ref name="bow269"/> Observations may be timed and ship's clocks set with a comparing watch, which is set to chronometer time and taken to the bridge wing for recording sight times.<ref name="bow269"/> In practice, a wrist watch coordinated to the nearest second with the chronometer will be adequate.<ref name="bow269"/> A stop watch, either spring wound or digital, may also be used for celestial observations.<ref name="bow269"/> In this case, the watch is started at a known GMT by chronometer, and the elapsed time of each sight added to this to obtain GMT of the sight.<ref name="bow269"/> All chronometers and watches should be checked regularly with a radio time signal.<ref name="bow269"/> Times and frequencies of radio time signals are listed in publications such as [[Radio Navigational Aids]].<ref name="bow269"/>