Nautical time zones

Nautical time zones
Before 1920, all ships kept local apparent time on the high seas by setting their clocks at night or at the morning sight so that, given the ship's speed and direction, it would be 12 o'clock when the Sun crossed the ship's meridian (12 o'clock = local apparent noon). During 1917, at the Anglo-French Conference on Time-keeping at Sea, it was recommended that all ships, both military and civilian, should adopt hourly standard time zones on the high seas. A ship within the territorial waters of any nation would use that nation's standard time. The captain was permitted to change his ship's clocks at a time of his choice following his ship's entry into another time zone—he often chose midnight. These zones were adopted by all major fleets between 1920 and 1925 but not by many independent merchant ships until World War II.

Time on a ship's clocks and in a ship's log had to be stated along with a "zone description", which was the number of hours to be added to zone time to obtain GMT, hence zero in the Greenwich time zone, with negative numbers from −1 to −12 for time zones to the east and positive numbers from +1 to +12 to the west (hours, minutes, and seconds for nations without an hourly offset). These signs are opposite to those given below because ships must obtain GMT from zone time, not zone time from GMT. All zones were pole-to-pole staves 15° wide, except −12 and +12 which were each 7.5° wide, with the 180° meridian separating them. Unlike the zig-zagging land-based International Date Line, the nautical International Date Line follows 180° except where it is interrupted by territorial waters and the lands they border, including islands. About 1950, a letter suffix was added to the zone description, assigning Z to the zero zone, and A–M (except J) to the east and N–Y to the west (J may be assigned to local time in non-nautical applications; zones M and Y have the same clock time but differ by 24 hours: a full day). These were to be vocalized using a phonetic alphabet which included Zulu for GMT, leading sometimes to the use of the term "Zulu Time". The Greenwich time zone runs from 7.5°W to 7.5°E Longitude, while zone A runs from 7.5°E to 22.5°E Longitude, etc.

These nautical letters have been added to some time zone maps, like the map of Standard Time Zones by Her Majesty's Nautical Almanac Office (NAO), which extended the letters by adding an asterisk (*) or dagger (†) for areas that do not use a nautical time zone, and a double dagger (‡) for areas that do not have a legal standard time (Greenland's ice sheet and all of Antarctica. The United Kingdom specifies UTC-3 for the Antarctic Peninsula, but no other country recognizes that). They conveniently ignore any zone that does not have an hour or half-hour offset, so a double dagger (‡) has been co-opted for these zones in the list of time zones.

In maritime usage, GMT retains its historical meaning of UT1, the mean solar time at Greenwich. UTC, atomic time at Greenwich, is too inaccurate, differing by as much as 0.9 s from UT1, creating an error of 0.4 km in longitude at the equator. However, DUT can be added to UTC to correct it to within 50 ms of UT1, reducing the error to only 20 m.