{{unreferenced, date=June 2016 The Anglo-French Conference on Time-keeping at Sea was a conference held in London in June 1917.

History

The Conference established the

nautical date line Nautical time is a maritime time standard established in the 1920s to allow ships on high seas to coordinate their local time with other ships, consistent with a long nautical tradition of accurate celestial navigation. Nautical time divides the ...

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 French (french: français(e), link=no) may refer to: * Something of, from, or related to France ** French language, which originated in France, and its various dialects and accents ** French people, a nation and ethnic group identified with Franc ...

and

American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the "United States" or "America" ** Americans, citizens and nationals of the United States of America ** American ancestry, pe ...

. 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.

Current usage

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 Standard time is the synchronisation of clocks within a geographical region to a single time standard, rather than a local mean time standard. Generally, standard time agrees with the local mean time at some meridian that passes through the r ...

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 Universal Time (UT or UT1) is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle with ...

(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. This includes fixed installations such as oil rigs. For example, the Norwegian Ekofisk oil rigs are located in international water at longitude 3°E but use Norwegian time.

History

Current usage

See also