Calendars and time

Greenwich Mean Time

Direct answer

Greenwich Mean Time is mean solar time at the Greenwich meridian, historically used as a world time reference.

Calendars & TimekeepingThe Great Almanac DictionaryUpdated 2026-09-26

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At a glance

Found in: Calendars & Timekeeping
Related: Equation of Time, Terrestrial Time, Calendar, Julian Date

An engraved clock movement with an hourglass and a sundial — Calendar and clock · visual guide **Greenwich Mean Time**
Greenwich Mean Time is the historical mean-solar time reference at Greenwich.
A cycle or timeline schematic for comparing the named timekeeping concept.

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The story behind Greenwich Mean Time

British navigation, railways, telegraphy, and international meridian agreements spread Greenwich-based timekeeping.

Why Greenwich Mean Time matters

GMT remains common in public language, but precise modern work distinguishes UTC and UT1.

How to read Greenwich Mean Time

Greenwich Mean Time applies when a source explicitly uses GMT, then clarify its intended modern time scale.

Greenwich Mean Time in practice: an example

A historical nautical almanac observation may give Greenwich Mean Time where a modern dataset would specify UT1 or UTC.

Where readings of Greenwich Mean Time go wrong

It is a mistake to assume GMT is a fully precise synonym for UTC in technical calculations.

Sources and editorial method

Research references used for this entry include:

U.S. Naval Observatory: Universal Time - Official explanation of Earth-rotation time and the Universal Time family.
NIST: UTC(NIST) Time Scale - Official description of UTC realization, atomic time, and leap-second coordination.

Source: The Great Almanac Dictionary
Category: Calendars & Timekeeping
Last updated: 2026-09-26

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Equation of TimeThe Equation of Time is the seasonal offset between sundial time and mean clock time. Terrestrial TimeTerrestrial Time is the uniform geocentric time argument used for astronomical ephemerides. CalendarA Calendar organizes days into named and numbered cycles for reckoning time. Julian DateJulian Date is a continuous astronomical count of days and day fractions. Coordinated Universal TimeCoordinated Universal Time is the international reference standard for world time. Sidereal TimeSidereal Time is star-based rotational time used to orient the sky.

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