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Nautical time zones

Nautical time zones are a system of time zones used by navigators and mariners to determine their position at sea. The system is based on the standard time zones used on land, but it is adjusted to account for the curvature of the Earth and the fact that the Earth rotates around its axis.

Since the 1920s, a nautical standard time system has been in operation for ships on the high seas. Nautical time zones are an ideal form of the terrestrial time zone system. Under the system, a time change of one hour is required for each change of longitude by 15°. 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.

In the nautical time zone system, the Earth is divided into 24 time zones, each 15 degrees wide, with the Prime Meridian (0 degrees longitude) and the International Date Line (180 degrees longitude) as the boundaries between the time zones. The time zones are numbered from 0 to 23, with time zone 0 corresponding to the Prime Meridian and time zone 12 corresponding to the International Date Line.

Nautical time zones are used by navigators and mariners to determine their longitude, which is the angular distance from the Prime Meridian. To determine their longitude, navigators and mariners use a nautical almanac, a book containing tables of the positions of the sun, moon, and other celestial bodies at different times of the year. A navigator or mariner can determine their longitude and position at sea by comparing the position of a celestial body with the tables in the nautical almanac.

Nautical time zones are used in conjunction with standard time zones to determine the time of day at a particular location accurately. For example, a vessel located in the eastern part of the Atlantic Ocean, which is in time zone 0, would use Greenwich Mean Time (GMT) as its reference time. In contrast, a vessel located in the western part of the Pacific Ocean, which is in time zone 12, would use UTC+12 as its reference time.

A ship within the territorial waters of any nation would use that nation’s standard time but would revert to nautical standard time upon leaving its territorial waters. The captain is permitted to change the ship’s clocks at a time of the captain’s choice following the ship’s entry into another time zone. The captain often chooses midnight. Ships going in shuttle traffic over a time zone border often keep the same time zone to avoid confusion about work, meal and shop opening hours. Still, the timetable for port calls must follow the land time zone.

Timezones by regions