The sextant and an accurate timepiece are the only two major instruments needed by the celestial navigator. Time and longitude are directly related. Without time, longitude by
normal methods cannot be established correctly. Considering the earth to be 360° of longitude with a rotation of 24 hours of time, that one zone or hour of time corresponds to 15° of longitude; 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 degrees to each side of the center. The zone whose center is 60° would include longitudes from 52 1/2 to 67 1/2 degrees.
For 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. If your longitude were 50° W, dividing by 15 would give 35/15. 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 89/15. 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 00h, a date change must be made. 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 h) is applied to the ZD to get GMT. If ZD were 8h for standard time, it would be +7h for daylight time. If ZD were -3 h for standard time, it would be -4h for daylight time.
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.
