Anglo-French Conference on Time-keeping at Sea
Encyclopedia
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 steps. This recommendation was adopted between 1920 and 1925 by all major fleets, including British
, French
and American
. The rules applied to almost all naval ships and to many non-naval ships. Nevertheless, up to the Second World War, the old practice of keeping local apparent time prevailed on many independent merchant ships.
The nautical date line is implied but not explicitly drawn on time zone maps. It follows the 180° meridian
except where it is interrupted by territorial waters
adjacent to land, forming gaps: it is a pole-to-pole dashed line. Ships are required to adopt the standard time
of a country when they are within its territorial waters, but must revert to international time zones (15° wide pole-to-pole gores) as soon as they leave territorial waters. 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.
In reality nautical time zones 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.
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
in June 1917. The Conference established the nautical date line and adopted 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 at sea. It recommended that time changes required by 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 ....
be made in one-hour steps. This recommendation was adopted between 1920 and 1925 by all major fleets, including British
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...
, French
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
and American
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. The rules applied to almost all naval ships and to many non-naval ships. Nevertheless, up to the Second World War, the old practice of keeping local apparent time prevailed on many independent merchant ships.
The nautical date line is implied but not explicitly drawn on time zone maps. It follows the 180° 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. Ships are 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 they are within its territorial waters, but must revert to international time zones (15° wide pole-to-pole gores) as soon as they leave territorial waters. 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.
In reality nautical time zones 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.