Nautical time
Encyclopedia
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.
The nautica time zone system is an ideal form of the terrestrial time zone
system for use on high seas
. Under the system time changes are required for changes of longitude
in one-hour steps. The one-hour step corresponds to a time zone
width of 15° longitude. The 15° gore that is offset from GMT or UT1 (not UTC) by twelve hours is bisected by the nautical date line into two 7.5° gores that differ from GMT by ±12 hours. A nautical date line is implied but not explicitly drawn on time zone maps. It follows the 180th meridian
except where it is interrupted by territorial waters
adjacent to land, forming gaps: it is a pole-to-pole dashed line.
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 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.
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 were to be vocalized using a phonetic alphabet
which pronounces the letter Z as Zulu, 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 World Time Zone Map 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 section sign
(§) for areas that do not have a legal standard time (the Greenland ice sheet
and all of Antarctica. The United Kingdom
specifies UTC−3 for the Antarctic Peninsula
, but no other country recognizes that).
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 1/4 of a minute of longitude at all latitudes and which is 1/4 of a nautical mile or 0.4 km at the equator but less at higher latitudes, varying by the cosine of the latitude. However, DUT can be added to UTC to correct it to within 50 ms of UT1, reducing the error to only 20 m.
A ship is required to adopt the standard time
of a country when it is in its territorial waters, but must revert to nautical time as soon as it leaves territorial waters.
In reality nautical times are used only for radio communication etc. Internally on the ship, e.g. for work and meal hours, the ship may use a suitable time of its own choosing. The captain is 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. Long distance going ships change time zone onboard at suitable times. Ships on short distance journeys do not change time zone at all, even if they go between different time zones, like between the UK and continental Europe. Passenger ships often use both time zones on signs. In time tables and communication with land, the land time zone has to be used.
Anglo-French Conference on Time-keeping at Sea
The Anglo-French Conference on Time-keeping at Sea was held in London in June 1917. The Conference established the nautical date line and adopted an ideal form of the terrestrial time zone system for use at sea. It recommended that time changes required by changes of longitude be made in one-hour...
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.
The nautica time zone system is an ideal form of the terrestrial time zone
Time zone
A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates , different places on the Earth need to have different clock times...
system for use on high seas
International waters
The terms international waters or trans-boundary waters apply where any of the following types of bodies of water transcend international boundaries: oceans, large marine ecosystems, enclosed or semi-enclosed regional seas and estuaries, rivers, lakes, groundwater systems , and wetlands.Oceans,...
. Under the system time changes are required for changes of longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
in one-hour steps. The one-hour step corresponds to a time zone
Time zone
A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates , different places on the Earth need to have different clock times...
width of 15° longitude. The 15° gore that is offset from GMT or UT1 (not UTC) by twelve hours is bisected by the nautical date line into two 7.5° gores that differ from GMT by ±12 hours. A nautical date line is implied but not explicitly drawn on time zone maps. It follows the 180th meridian
180th meridian
The 180th meridian or antimeridian is the meridian which is 180° east or west of the Prime Meridian passing through the Royal Observatory, Greenwich. It is common to both east longitude and west longitude. It is used as the basis for the International Date Line because it for the most part passes...
except where it is interrupted by territorial waters
Territorial waters
Territorial waters, or a territorial sea, as defined by the 1982 United Nations Convention on the Law of the Sea, is a belt of coastal waters extending at most from the baseline of a coastal state...
adjacent to land, forming gaps: it is a pole-to-pole dashed line.
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 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
International Date Line
The International Date Line is a generally north-south imaginary line on the surface of the Earth, passing through the middle of the Pacific Ocean, that designates the place where each calendar day begins...
, the nautical International Date Line follows 180° except where it is interrupted by territorial waters and the lands they border, including islands.
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 were to be vocalized using a phonetic alphabet
NATO phonetic alphabet
The NATO phonetic alphabet, more accurately known as the NATO spelling alphabet and also called the ICAO phonetic or spelling alphabet, the ITU phonetic alphabet, and the international radiotelephony spelling alphabet, is the most widely used spelling alphabet...
which pronounces the letter Z as Zulu, 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 World Time Zone Map by Her Majesty's Nautical Almanac Office (NAO), which extended the letters by adding an asterisk (*) or dagger
Dagger (typography)
A dagger, or obelisk. is a typographical symbol or glyph. The term "obelisk" derives from Greek , which means "little obelus"; from meaning "roasting spit"...
(†) for areas that do not use a nautical time zone, and a section sign
Section sign
The section sign , also called the "double S", "sectional symbol" or signum sectiōnis, is a typographical character used mainly to refer to a particular section of a document, such as a legal code. It is frequently used along with the pilcrow , or paragraph sign...
(§) for areas that do not have a legal standard time (the Greenland ice sheet
Greenland ice sheet
The Greenland ice sheet is a vast body of ice covering , roughly 80% of the surface of Greenland. It is the second largest ice body in the world, after the Antarctic Ice Sheet. The ice sheet is almost long in a north-south direction, and its greatest width is at a latitude of 77°N, near its...
and all of Antarctica. The United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
specifies UTC−3 for the Antarctic Peninsula
Antarctic Peninsula
The Antarctic Peninsula is the northernmost part of the mainland of Antarctica. It extends from a line between Cape Adams and a point on the mainland south of Eklund Islands....
, but no other country recognizes that).
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 1/4 of a minute of longitude at all latitudes and which is 1/4 of a nautical mile or 0.4 km at the equator but less at higher latitudes, varying by the cosine of the latitude. However, DUT can be added to UTC to correct it to within 50 ms of UT1, reducing the error to only 20 m.
A ship is required to adopt the standard time
Standard time
Standard time is the result of synchronizing clocks in different geographical locations within a time zone to the same time rather than using the local meridian as in local mean time or solar time. Historically, this helped in the process of weather forecasting and train travel. The concept...
of a country when it is in its territorial waters, but must revert to nautical time as soon as it leaves territorial waters.
In reality nautical times are used only for radio communication etc. Internally on the ship, e.g. for work and meal hours, the ship may use a suitable time of its own choosing. The captain is 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. Long distance going ships change time zone onboard at suitable times. Ships on short distance journeys do not change time zone at all, even if they go between different time zones, like between the UK and continental Europe. Passenger ships often use both time zones on signs. In time tables and communication with land, the land time zone has to be used.