Marine Glossary

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Zone Description

When referred to a watch, it is the number of whole hours between the watch time (WT) and Greenwich mean time (GMT). If your watch reads 13:04:20 as you hear GMT (also called universal coordinated time, UT or UTC) announced on the WWV broadcast as 21:04:00, then the ZD of your watch is +8 hours. You know this because you now know that you get GMT from your WT by adding 8 hours. As long as you don't change your watch time, this correction to the hours part will remain constant, even though the error in the seconds part of your watch time will increase as time goes by. In the example above, the watch error was 20 seconds fast. As long as you don't change your watch time, GMT is your watch time, corrected for the watch error, plus 8 hours, no matter what time of day you look at it, and no matter where on earth you are when you do look.

When referred to a place on earth (as in the zone time or standard time systems), the zone description describes the local time zone of the place, telling how many hours it differs from GMT. But in practical navigation there is little use for this conventional definition of the term. To navigate, the only time you need to know is GMT, and to get GMT you need to know only the time zone of your watch, not the time zone you happen to be in. Just forget that you are sailing across several time zones as you cross an ocean; it has no bearing on the important part of navigation. Midday will gradually slip an hour or two off of 12:00, but this is no problem at all compared to the time keeping problems you expose yourself to once you start changing your watch time. Once you get to your destination, set your watch to their time and go on from there.

See: sextant sight, standard time, time zone, universal time, watch error, watch rate, watch time, zone description, and zone time.

See also: Time Keeping in Navigation and Weather.

Abbreviation: ZD