I. Definition and Basic Concepts

Greenwich Mean Time (GMT) is a time standard based on the former site of the Royal Observatory in Greenwich, London, with its core principle being the Prime Meridian (0° longitude) passing through the observatory as the starting point for global time calculation. Essentially, it is solar time based on Earth's rotation cycle (the average time for Earth to rotate once on its axis).

The "mean" in GMT stems from a correction to solar time: due to Earth's elliptical orbit and the tilt of its rotational axis, the length of true solar time (apparent solar time) varies slightly each day. GMT calculates the average value of annual solar time, resulting in a fixed 24-hour standard time unit that ensures the stability of time measurement.

In civilian contexts, GMT is often understood as the "benchmark for world time." Historically,各地 around the world determined local time by the difference from GMT (such as Beijing time being GMT+8, meaning 8 hours ahead of GMT). However, in modern scientific measurement, GMT's definition has gradually been supplemented and replaced by the more precise Coordinated Universal Time (UTC), although both are still considered approximately synonymous in most daily scenarios.

II. Historical Development

The formation of Greenwich Mean Time is closely related to the development of global transportation and communications. Its history can be divided into the following key stages:

1. Pre-19th Century: The Chaotic Era of Local Time Before the Industrial Revolution,各地 used local solar positions to determine "local time." For example, London and Paris differed by about 9 minutes, and London and New York by about 5 hours. This time chaos seriously hindered the development of railway transportation (requiring unified train schedules) and maritime navigation (requiring precise longitude calculations).

2. 1847: British Railways First Adopted GMT To solve the problem of chaotic railway timetables in the country, British railway companies first used the time at the Greenwich Observatory as the unified time for all British railways. This was the first large-scale standardization application of GMT, and subsequently, British postal and telegraph systems also adopted it.

3. 1884: International Meridian Conference Established Global Benchmark To unify global longitude and time standards, on October 1884, representatives from 25 countries convened in Washington for the "International Meridian Conference." The conference ultimately resolved:

   To designate the meridian passing through the Airy transit instrument at the Greenwich Observatory as the Prime Meridian (0° longitude), as the starting point for global longitude calculations; simultaneously establishing Greenwich time as the global "Universal Time" benchmark, from which各国 determine the time difference with GMT.

4. Mid-20th Century: GMT's Precision Limitations and the Rise of UTC With technological development (such as aviation, satellite communications, computer networks), people discovered that Earth's rotational period has slight fluctuations (affected by tides, atmospheric circulation, etc.), making GMT's precision unable to meet modern measurement needs. In 1967, the General Conference on Weights and Measures (CGPM) defined "Atomic Time (TAI)" based on atomic clocks, and in 1972 introduced "Coordinated Universal Time (UTC)" combining atomic time precision with Earth's rotation, gradually replacing GMT as the international standard time.

5. To Present: Civilian Retention and Symbolic Significance of GMT Although UTC has become the main time standard in scientific and official fields, GMT is still widely used in civilian contexts (such as British winter time still labeled as GMT, with some aviation and maritime sectors habitually using GMT terminology). Meanwhile, Greenwich's symbolic significance as the "starting point of time" makes it an important symbol in global time culture.

III. Relationship with Coordinated Universal Time (UTC)

GMT and UTC are two easily confused time concepts with both connections and clear distinctions. The core differences lie in "precision benchmarks" and "adjustment methods":

1. Core Connections

• UTC was formulated with the original intention to "inherit GMT's global time benchmark function." Therefore, in most daily scenarios (such as weather forecasts, flight schedules, mobile phone time), GMT and UTC can be considered "approximately equivalent," with a time difference usually of 0 (only showing brief differences during leap second adjustments).
• Most countries still define local time by "the time difference with UTC/GMT" (such as Tokyo time being UTC/GMT+9, Los Angeles time being UTC/GMT-8).

2. Key Differences

IV. Application Areas

Although GMT's scientific status has been replaced by UTC, it still has widespread applications globally, mainly in the following scenarios:

• Civilian Time Expressions Britain officially labels its time as "GMT" during the "winter time" period from October to March each year (switching to Daylight Saving Time BST, i.e., GMT+1, in summer). Some English-speaking countries (such as the US and Australia) still habitually use GMT to describe time zones in media and daily conversations (such as "event time GMT 14:00").
• Aviation and Maritime Fields In traditional air traffic control and maritime navigation, crew and pilots are accustomed to using GMT as a "universal time reference" to avoid time confusion caused by crossing time zones (although modern systems have gradually switched to UTC, GMT terminology is still common in operation manuals).
• Astronomical Observations and Historical Records In astronomical observations, some traditional observational data (such as stellar position records) still use GMT as the time benchmark. In historical documents and archaeological research, time annotations for global events (such as "August 15, 1945 GMT 0 hours, Japan announced surrender") often retain the GMT format to ensure consistency in cross-regional time comparisons.
• International Communications and Media International radio stations (such as BBC World Service) and global news agencies (such as Reuters and Associated Press) often simultaneously mark GMT time when reporting global events, facilitating time conversion for audiences in different time zones. Some international conferences also use GMT as "coordinated time" to avoid time zone misunderstandings.

V. Controversies and Limitations

1. Inherent Defect of Insufficient Precision

GMT is based on Earth's rotation cycle, but Earth's rotation speed gradually slows down due to tidal friction (caused by lunar gravity), changes in atmospheric circulation, crustal movements, etc. (approximately 1-2 milliseconds slower per century), with short-term fluctuations (such as El Niño phenomena causing temporary changes in rotation speed). This instability makes GMT unable to meet modern technology's demand for time precision - for example, satellite navigation systems (such as GPS) require nanosecond-level (10⁻⁹ seconds) time precision, while GMT's error can reach millisecond-level (10⁻³ seconds), far exceeding the allowable range.

2. Historical Controversy of "Western Centrism"

GMT's global benchmark status stems from Britain's colonial influence and industrial power in the 19th century. Some countries (such as Brazil and India) have proposed that "time benchmarks should be based on more neutral geographical or cultural coordinates," believing that GMT carries "Western-centric" coloration. Although this controversy has not changed GMT's historical status, it has promoted the subsequent "de-territorialization" design of UTC (UTC is not bound to any specific location, purely based on atomic clocks and scientific calculations).

3. Practical Problems Caused by Confusion with UTC

In civilian scenarios, most people cannot distinguish between GMT and UTC differences, which may lead to time interpretation errors. For example, when UTC undergoes leap second adjustments (such as adding 1 second to 23:59:60 after 23:59:59), some devices still labeled GMT may not synchronize adjustments, causing brief time deviations that could potentially affect scenarios dependent on precise time (such as financial transactions, network synchronization).

VI. Related Term Explanations

• Prime Meridian: The meridian at 0° longitude passing through the former site of the Greenwich Observatory, serving as the baseline for GMT and early global longitude calculations. Its global status was formally established at the 1884 International Meridian Conference.
• Atomic Time (TAI): A time standard based on the frequency of cesium-133 atomic ground state hyperfine transition, with precision reaching no more than 1 second error per million years, serving as the core precision source for UTC.
• Leap Second: To keep UTC synchronized with Earth's rotation (avoiding excessive deviation between UTC and GMT), the International Earth Rotation Service (IERS) may add 1 second (positive leap second) or subtract 1 second (negative leap second) after 23:59:59 UTC on June 30 or December 31 every 1-2 years. As of 2024, the world has cumulatively added 27 positive leap seconds.
• Universal Time (UT1): A time standard directly based on Earth's rotation, more precisely reflecting real-time changes in Earth's rotation than GMT. UTC controls the deviation from UT1 within ±0.9 seconds through leap second adjustments.
• Time Zones: The world is divided into 24 time zones by longitude, each spanning approximately 15°, with local time differing from GMT/UTC by integer hours (some regions by 30 minutes or 15 minutes, such as India at UTC+5:30), designed to address the living need that "the sun rises at different times in different regions."