Establishment
Prior to 1920, ships kept solar 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.[citation needed]
The establishment of nautical standard times, nautical standard time zones and the nautical date line were recommended by the Anglo-French Conference on Time-keeping at Sea in 1917. The conference recommended that the standard apply to all ships, both military and civilian. These zones were adopted by all major fleets between 1920 and 1925 but not by many independent merchant ships until World War II.
Letter suffixes
Around 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 can be vocalized using the NATO phonetic alphabet which pronounces the letter Z as Zulu, leading to the use of the term "Zulu Time" for Greenwich Mean Time, or UT1 from January 1, 1972 onward.
Zone Z runs from 7°30′W to 7°30′E longitude, while zone A runs from 7°30′E to 22°30′E longitude, etc.
These nautical letters have been added to some time zone maps, like the World Time Zone Map[2] published by HM Nautical Almanac Office (NAO), which extended the letters by adding an asterisk (*), a dagger (†) or a dot (•) for areas that do not use a nautical time zone (areas that have a half-hour or quarter-hour offset, and areas that have an offset greater than 12 hours), and a section sign (§) for areas that do not have a legal standard time (the Greenland ice sheet and Antarctica). The United Kingdom specifies UTC−3 for the claimed British Antarctic Territory.
Preference for GMT over UTC
In maritime usage, GMT retains its historical meaning of UT1, the mean solar time at Greenwich, which is empirically adjusted to track unpredictable variations in the Earth's rotational period. UTC, atomic time at Greenwich, makes these adjustments on a coarser granularity than GMT. Establishing latitude by local observations of solar position requires determination of the latitude on Earth where the Sun is directly overhead at the time when the observation is taken. Thus the coarseness of UTC in determining solar time makes it inaccurate in establishing the reference latitude of solar meridian, differing by as much as 0.9 seconds from UT1, creating an error of 1⁄4 of a minute of longitude at all latitudes and which is 1⁄4 nautical mile (460 m) at the equator but less at higher latitudes, varying roughly by the cosine of the latitude. However, the time correction DUT1 can be added to UTC to correct it to within 50 milliseconds of UT1, reducing the error to only 20 metres (66 ft).[citation needed]
Solar position can also be established by celestial observation of more distant stars taken at nighttime, but this also involves a calendrical correction due to the Earth's elliptical orbit around the Sun; establishing solar position by observations of bright solar objects, such as Venus (also with its own elliptical orbit), involves yet further complexity.
Today
In practice, nautical times are used only for radio communication, etc. Aboard the ship, e.g. for scheduling work and meal times, the ship may use a suitable time of its own choosing. The captain is permitted to change his or her clocks at a chosen time following the ship's entry into another time zone, typically at midnight. Ships on long-distance passages change time zone on board in this fashion. On short passages the captain may not adjust clocks at all, even if they pass through different time zones, for example between the UK and continental Europe. Passenger ships often use both nautical and on-board time zones on signs. When referring to time tables and when communicating with land, the land time zone must be employed.