Greenwich Mean Time

Also called: GMT · Topic: Latitude, Longitude and Time · NCERT: Class 6, Ch 1 "Locating Places on the Earth"; Class 11, Ch 3 "Latitude, Longitude and Time"

Meaning

Greenwich Mean Time (GMT) is the mean (average) local time at the Prime Meridian (0°), which runs through the Royal Observatory at Greenwich, near London (United Kingdom). It was the world's reference time from 1884, and every country's standard time was given as hours ahead of or behind GMT. Today Coordinated Universal Time (UTC), which runs on atomic clocks, has replaced GMT as the world's civil time standard. The name "GMT" is still used in everyday speech.

Explanation

How local time leads to GMT

• Local time is set by the Sun. When the Sun is highest in the sky at a place, it is local noon there.
• Local time changes with longitude.
• The earth turns 360° in 24 hours.
• That is 15° in 1 hour, or 1° in 4 minutes.

• So every meridian has a slightly different local time.

• The world needed one starting line for time.

• Without it, each country's clock could not be compared with another's.

• The meridian through Greenwich was chosen as 0°. All longitudes are counted east or west from it.

• GMT is the local time on this 0° line. Every other standard time is counted from it:

• East of Greenwich, clocks are ahead of GMT (the Sun rises there earlier).
• West of Greenwich, clocks are behind GMT.

What "Mean" means

• "Mean" means average.
• The real solar day (noon to noon by the Sun) is a little longer in some months and a little shorter in others.

• GMT uses the average day, so every day is exactly 24 hours.

• GMT is still based on the earth's rotation.

• It is mean solar time at 0°, found by astronomical observation at the Royal Observatory, Greenwich.
• The earth's spin is uneven, so GMT changes slightly along with it.

How GMT became the world's reference

• International Meridian Conference, October 1884, Washington, D.C.:
• It adopted the meridian of the transit instrument (a telescope fixed to measure when stars cross the meridian) at the Royal Observatory, Greenwich, as the prime (zero) meridian [5].

• At this conference, GMT was adopted as the reference time for clocks around the world [7].

• Standard meridians are counted from Greenwich.

• A standard meridian is the meridian whose local time becomes a country's standard time.
• By convention it is a multiple of 7°30′, and 7°30′ of longitude = 30 minutes of time.
• So every country's offset from GMT comes out as a whole hour or a half hour.

From GMT to UTC

• Coordinated Universal Time (UTC) is the main world time standard today. It is the basis of all civil time across the world [7].
• It runs on atomic clocks (clocks that count the very regular vibrations of atoms, mostly caesium).
• The UTC second is defined by an atomic change in caesium. UTC comes from TAI (International Atomic Time) and is accurate to about 1 nanosecond (a billionth of a second) per day [7].

• The International Bureau of Weights and Measures (BIPM) coordinates it.

• Timeline: UTC began replacing GMT for civil use in the 1960s. It has had its present form since 1972.

• The leap second links the two kinds of time.
• Atomic clocks tick at a perfectly steady rate, but the earth's rotation does not. The braking pull of the Moon's gravity (tidal friction) is one cause of this [3].
• So atomic time slowly drifts away from time based on the earth's rotation (UT1).
• A leap second (one extra second added to UTC) keeps UTC within 0.9 second of earth-rotation time [6].
• Leap seconds were introduced in 1972 [6]. 27 have been added since then [3]. The last one was added on 31 December 2016.
• In 2022, the CGPM (General Conference on Weights and Measures) decided in Resolution 4 to stop adding leap seconds by 2035.

Where it occurs

• Greenwich, near London (United Kingdom): the Royal Observatory, through which the Prime Meridian (0°) passes. GMT is the mean time here.
• Washington, D.C. (USA): where the International Meridian Conference met in October 1884 and chose Greenwich as 0° [5].
• United Kingdom: GMT is still used there as winter time.
• India:
• The standard meridian is 82°30′E, which passes near Mirzapur (Uttar Pradesh).
• 82°30′ = 11 × 7°30′. 82.5° × 4 minutes = 330 minutes, so IST is 5 hours 30 minutes ahead of Greenwich.
• IST = GMT + 5:30 = UTC + 5:30.
• CSIR-National Physical Laboratory (CSIR-NPL), New Delhi, keeps IST [2][3]. It keeps IST traceable to UTC within ±2.9 nanoseconds, using five caesium clocks and two active hydrogen masers [2].

• India's last leap second was added at 05:29:59 IST on 1 January 2017, which was 23:59:59 UTC on 31 December 2016 [3].

• World: about 70 countries, including India, run atomic clocks at their national measurement institutes (NMIs) that feed into UTC [2].

Don't confuse with

• UTC (Coordinated Universal Time): GMT is based on the earth's rotation (mean solar time at 0°). UTC is based on atomic clocks (the caesium second). UTC is today's civil standard.
• Local time: local time is set by the Sun at any meridian and changes by 4 minutes per degree. GMT is the mean local time of one meridian, 0°.
• Standard time: standard time is one country's uniform clock time, taken from its own standard meridian (82°30′E for India). GMT is the world reference that every standard time is counted from.
• Prime Meridian: the Prime Meridian is a line of longitude (0°). GMT is the time kept on that line.

Prelims Hooks

• The International Meridian Conference (October 1884) met in Washington, D.C., not London. It fixed Greenwich as 0° [5] and adopted GMT as the world's reference time [7].
• GMT comes from the earth's rotation (mean solar time). UTC comes from atomic clocks (caesium). UTC has had its present form since 1972.
• "Mean" in GMT means average solar day, so every day is exactly 24 hours.
• East of Greenwich = ahead of GMT; west = behind GMT. IST (82°30′E, near Mirzapur, UP) = GMT/UTC + 5:30.
• Leap second: keeps UTC within 0.9 s of earth-rotation time [6]. 27 have been added since 1972 [3]. The last was on 31 December 2016. CGPM 2022 (Resolution 4) will stop them by 2035.
• Trap: the earth's rotation is uneven and slowly slowing down, while atomic time is steady. It is the earth that "loses" time, not the atomic clock.

Mains Points

• Why a world reference time was needed (GS-I): local time changes by 4 minutes for every degree of longitude. Railways, telegraph and trade needed one common zero. The 1884 choice of Greenwich gave every country a fixed base: its standard meridian, a multiple of 7°30′, gives a clean offset from GMT. This is a clear example of how longitude shapes the way the world is run.
• From earth time to atomic time: GMT followed the earth's uneven spin, while UTC (1972) follows atomic clocks. Leap seconds bridged the two. The CGPM's 2022 decision to end them by 2035 shows that steady, unbroken digital time now matters more than staying in line with the Sun, because a sudden extra second can upset computers, networks and satellite systems.
• Precise time as infrastructure (GS-III): IST is kept traceable to UTC by CSIR-NPL [2]. Telecom, banking and stock trading, power grids, air traffic control, navigation, 5G and cyber security all need exact time [2][4]. NavIC satellite clocks match IST within a few nanoseconds [2]. The Legal Metrology (Indian Standard Time) Rules, 2026 make IST the legal time reference under "One Nation, One Time" [4].

Related concepts

• Standard time
• Standard meridian
• Coordinated Universal Time
• Leap second

Read more

• Standard time, GMT and UTC