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To determine "longitude by chronometer," a navigator requires a chronometer set to the local time at the Prime Meridian. Local time at the Prime Meridian has historically been called Greenwich Mean Time (GMT), but now, due to international sensitivities, has been renamed as Coordinated Universal Time (UTC), and is known colloquially as "zulu time".
The time is calculated based on the offset from UTC for the specified timezone taking into account whether daylight saving time is currently active in that timezone. The offset for each implemented timezone is calculated in a sub template (except for UTC).
The term "GMT" is also used as one of the names for the time zone UTC+00:00 and, [2] in UK law, is the basis for civil time in the United Kingdom. [ 3 ] [ a ] Because of Earth's uneven angular velocity in its elliptical orbit and its axial tilt , noon (12:00:00) GMT is rarely the exact moment the Sun crosses the Greenwich Meridian [ b ] and ...
An ephemeris is a table of the calculated positions of astronomical objects and various other data, usually for a specific time of the day, either noon or midnight. A uniform time measurement is needed to establish accuracy, and ephemerides will use variously Greenwich Mean Time, Universal Time or Ephemeris Time.
Time zone abbreviations for both Standard Time and Daylight Saving Time are shown exactly as they appear in the database. See strftime and its "%Z" field. Some of zone records use 3 or 4 letter abbreviations that are tied to physical time zones, others use numeric UTC offsets.
A marine chronometer is a precision timepiece that is carried on a ship and employed in the determination of the ship's position by celestial navigation.It is used to determine longitude by comparing Greenwich Mean Time (GMT), and the time at the current location found from observations of celestial bodies.
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 ...
The telegraphic distribution of time signals was made obsolete by the use of AM, FM, shortwave radio, Internet Network Time Protocol servers as well as atomic clocks in satellite navigation systems. Time signals have been transmitted by radio since 1905. [12] There are dedicated radio time signal stations around the world.