Longitude and time are related, without time longitude by normal methods cannot be established correctly. The earth is a circle 360° with a rotation of approx. 24 hours of time, one zone or hour of time is 15° of longitude, one hour of rotation of the earth is 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 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. Everyone within the zone keeps the same time called zone time or ZT. The centers of each zone have longitudes 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. The zone whose center is 45° would include longitudes from 37 1/2 degree's to 52 1/2 degree's.
In 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 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. If your longitude was 40° W, dividing by 15 would give 2.66. Rounding to the nearest whole number would give you 3 as a ZD. It would be a +3 since your longitude is west. This means that GMT is three hours later than your zone time. If your longitude was 153° E, dividing by 15 would give you 10.2. Rounding to the nearest whole number would give you 10, a negative ZD since your longitude is east. If addition or subtraction of the ZD to your ZT puts your GMT over 24h or less than 00h, a date change must be made. If daylight time is used instead of standard time, a negative one hour (- l hour) is used to the ZD to get your GMT. For example, if ZD were 6h for standard time, it would be +5h for daylight time. If ZD were - 4h for standard time, it would be - 5h for daylight time.
Watch error (WE) is the error that your time varies from true zone time. If your chronometer is 12 seconds slow, the error should be added to the time to give true zone time. A fast error should be subtracted from the chronometer reading to give true zone time. A chronometer doesn't have to tell the exact time. If its rate of loss or gain is one second per day, and is consistent then the known error can be applied to get true zone time. If you don't have 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 (WT) is the time read on your time piece to the nearest second at the time your sextant sight is taken.
Example
Watch Time (WT) 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 your vessel is in.
Zone Time (ZT) is the time of the zone you are in.
Watch Error (WE) 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.
