The sextant and an accurate timepiece are the only two major instruments needed by the celestial navigator. In 1135, Englishman John Harrison developed the first chronometer that could be taken on board ship amidst the temperature changes and motion and yet tell extremely accurate time. By the end of the eighteenth century, such instruments were being constructed with more reasonable size, weight and price. By the early twentieth century, time ball signals activated by telegraph and low power audible signals in major ports enabled the navigator to check the accuracy of his chronometer. Now, accurate and inexpensive quartz chronometers as well as radio time signals make the establishment of time an easy task. Time and longitude are directly related. Without time, longitude by normal methods cannot be established correctly. Considering the earth to be encircled by 360° of longitude with a rotation of 24 hours of time, we find
that one zone or hour of time corresponds to 15° of longitude, in other words, one hour of rotation of the earth corresponds to 15° angle of rotation of the earth. Reducing this even further, one second of time corresponds to one quarter of a minute of longitude, equal to one quarter of a mile (roughly) at the equator, less of a distance as latitude increases. An error of time, is a error of longitude
A system of time zones has been established worldwide, most zones being one hour or 15° wide. In some cases the actual boundaries vary to coincide with state borders or island groupings. Everyone within the zone keeps the same time called zone time or ZT. The centers of each zone have longitudes evenly divisible by fifteen such as 0°, 15°, 30°, 45°, etc. The edges of the zones extend 7 1/2 to each side of the center. Therefore, the zone whose center is 60° would include longitudes from 52 1/2 degree's to 67 1/2 degree's.
For purposes of celestial navigation, zone time must be converted to Greenwich Mean Time, time kept at the zone whose center is 0° or the Prime Meridian. This is done by means of the Zone Description or ZD, a number obtained by dividing your longitude by 15 and rounding the answer off to the nearest whole number.
If the longitude is west, the ZD is positive. If the longitude is east, the ZD is negative. For example, if your longitude were 50° W, dividing by 15 would give 3.33. Rounding to the nearest whole number would give 3 as a ZD. It would be a +3 since longitude is west. This means that GMT is three hours later than your zone time. If your longitude were 129° E, dividing by 15 would give 8.6. Rounding to the nearest whole number would give 9, a negative ZD since longitude is east. If addition or subtraction of the ZD to your ZT puts your GMT over 24h or less than OOh, a date change must be made. For example, if zone time is 1500 on 3 April, ZD is +11, this makes a GMT of 0200 on 4 April. If ZT is 0400 on 7 November, ZD is -7, this makes a GMT of 2100 on 6 November. In the cases where daylight time is used instead of standard time, a negative one hour (-l hour) is applied to the ZD to get GMT.
Watch error or WE is the error that your timekeeper varies from true zone time. For instance, if your chronometer is 10 seconds slow, the error should be added to the time indicated to give true zone time. A fast error should be subtracted from the chronometer reading to give true zone time. A chronometer doesn't necessarily have to tell the exact time. If its rate of loss or gain, say one second per day, is consistent then the known error can be applied to get true zone time. In the absence of a chronometer, a stopwatch can be used that is set to a radio time signal. In this case, the time the stopwatch is started is added to the stopwatch reading at the time of sextant sight to obtain true zone time. The watch time or WT is the time read on your timepiece to the nearest second at the time your sextant sight is taken. Below is a example
WATCH TIME (W.T.) IS THE READING OF YOUR WATCH AT THE INSTANT YOU MAKE THE SEXTANT READING. THE WATCH IS SET TO THE STANDARD TIME OF THE TIME ZONE THE BOAT IS IN.
ZONE TIME (Z.T.) IS THE CORRECT TIME OF THE ZONE YOU'RE IN.
WATCH ERROR (W.E.) IS THE AMOUNT OF TIME THE WATCH IS SLOW (S) OR FAST (F). IF THE WATCH IS SLOW, YOU ADD THE ERROR. IF IT'S FAST, SUBTRACT.
1.WATCH TIME OF SIGHT WAS 0812 AND 15 SECONDS. WATCH ERROR WAS 12 SECONDS (F).
W.T. 08h-12m-15s
W.E. -12s (F)
Z.T. 08h-12m-03s
