The coordinates of celestial bodies are tabulated in almanacs along with Greenwich mean time and date. You as a navigator must record the GMT of each celestial observation to the nearest second to be able to get the information from your reference publications and plot an accurate celestial LOP. There are two methods of determining the correct time to the required accuracy on board ship, they are the chronometer and radio time signals.
Chronometers should be wound every 24 hours. On board ship the navigator or 2nd Mate usually does this. Also it is the Mates job to wind the chronometers, he also resets and winds all other standard wall clocks on board the ship.
Even a chronometer cannot keep exact time, but what distinguishes this instrument from a normal clock or watch is that its rate of gain or loss of time is constant. The difference between the time indicated on the chronometer or chronometer time, and the correct GMT at any instant is called the chronometer error, or CE. It is labeled either (F) or (S), depending on whether the chronometer time is ahead (faster than) or behind (slower than) GMT. Because chronometers are normally not reset on board ship, an accumulated error may become large in time. Example - if the chronometer lost three seconds each day, in one 30-day month it would have a chronometer time of 1 minute and 30 seconds behind or "slower" than GMT. The CE would be written (S) 01-30. If the chronometer read 01-02-03 at a particular time, the conversion to GMT would be.
C 01-02-03
CE + 01-30 (S)
GMT 01-03-33
Slow chronometer errors are always added to chronometer time to obtain GMT, and fast errors are always subtracted.
Chronometer should be checked for accuracy at least once each day, some ships keep a log called Timepiece Rate Book. This is done by checking the radio time signal. The error of the chronometer relative to GMT is observed and recorded into the Rate Book.
Difference in dates of observation.
The chronometer cannot be removed and brought topside when celestial observations are to be made, you have to use a small hand-held watch like a stopwatch this is called a comparing watch and is used to time actual observations.
A comparing watch differs from a normal stopwatch in that it usually has a sweep hand which can be stopped at the moment of observation, while the remainder of the watch movement continues in motion. After the time of the observation has been recorded, the indicator sweep can then be returned to run with the second sweep of the main movement, until you stop it again to record the next observation. If a comparing watch is not available, an ordinary stopwatch can be used as the timing watch.
When a round of celestial observations is to be made, the comparing watch is set as close as possible to either the local zone time (ZT) or to GMT, usually by reference to the chronometer. You can also use the radio time signal to set the watch, In either case the difference between watch time (W) and ZT or GMT is then found. This difference is the watch error,(WE). After recording the watch time of each celestial observation, fast watch errors are subtracted, and slow watch errors added, in order to obtain the correct ZT or GMT for each observation. If the watch is set to local zone time, the zone time of each observation is converted to GMT by applying the zone difference.
