Greenwich Mean Time
The Royal Observatory in Greenwich, London, serves as the physical anchor for a time system that began as an astronomical calculation. Local mean time at this specific location counts from midnight, yet history shows it was once calculated differently. In the past, astronomers considered a day to start at noon while everyone else started theirs at midnight. This discrepancy created confusion because noon GMT is rarely the exact moment the Sun crosses the Greenwich Meridian. Earth's uneven angular velocity and axial tilt cause the event to occur up to 16 minutes before or after noon GMT. The term mean describes how noon GMT represents the annual average of these solar crossings. This arithmetic mean accounts for the word mean within the name Greenwich Mean Time.
Greenwich Mean Time was adopted across the island of Great Britain by the Railway Clearing House in 1847. Almost all railway companies followed suit by the following year creating what is now known as railway time. A legal case in 1858 held local mean time to be the official time instead. On the 14th of May 1880 a letter signed by Clerk to Justices appeared in The Times stating that Greenwich time was not yet legal time. Polling booths opened and closed at specific times like 8 13 p.m. without legal backing until later changes. Greenwich Mean Time was legally adopted throughout the island of Great Britain later in 1880. Hourly time signals from Greenwich Observatory were first broadcast by shortwave radio on the 5th of February 1924 at 17:30:00 UTC.
The daily rotation of Earth is irregular with a slowing trend making atomic clocks a much more stable timebase. On the 1st of January 1972 GMT as the international civil time standard was superseded by Coordinated Universal Time. This new system is maintained by an ensemble of atomic clocks around the world. Universal Time introduced in 1928 initially represented mean time at Greenwich determined in the traditional way. From the 1st of January 1956 this raw form of UT was re-labelled UT0 after decisions made by the International Astronomical Union in Dublin. William Markowitz initiated these refinements which eventually led to modern forms like UT1 and UT2. English speakers often use GMT as a synonym for UTC though precision requires distinguishing them.
Historically GMT has been used with two different conventions for numbering hours. The long-standing astronomical convention dating from Ptolemy referred to noon as zero hours. This contrasted with the civil convention referring to midnight as zero hours dating from the Roman Empire. The latter convention was adopted on and after the 1st of January 1925 for astronomical purposes resulting in a discontinuity of 12 hours. The instant designated as December 31.5 GMT in 1924 almanacs became January 1.0 GMT in 1925 almanacs. The term Greenwich Mean Astronomical Time was introduced to unambiguously refer to the previous noon-based astronomical convention. More specific terms UT and UTC do not share this ambiguity always referring to midnight as zero hours.
Legally the civil time used in the UK is called greenwich mean time without capitalization. An exception exists for periods when the Summer Time Act 1972 orders an hour's shift for daylight saving. The Interpretation Act 1978 section 9 provides that whenever an expression of time occurs in any Act it shall be held to be Greenwich mean time. Subsection 23 applies the same rule to deeds and other instruments. During the experiment of 1968 to 1971 British Standard Time covered all-year British Summer Time. BBC radio stations broadcast the six pips of the Greenwich Time Signal calibrated to UTC. Announcers on domestic channels declare the time as GMT or BST as appropriate while the World Service uses the term consistently throughout the year.
Several countries define their local time by reference to Greenwich Mean Time. Belgium decrees of 1946 and 1947 set legal time as one hour ahead of GMT. Ireland defines Winter Time as being the same as GMT and Standard Time as one hour in advance. Canada references standard time for provinces in relation to Greenwich time though some acts mention Greenwich Mean Time explicitly. Portugal sets legal time as one hour ahead except for the Azores which switch clocks during winter months. Burkina Faso, Ghana, Iceland, and Senegal use Greenwich Mean Time as standard time all year round. A monumental clock stands in Kumasi, Ghana marking this global reach across Europe Africa and the Americas.
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Common questions
What is Greenwich Mean Time and where does it originate?
Greenwich Mean Time is the time system anchored by the Royal Observatory in Greenwich, London. It serves as the physical basis for a calculation that began as an astronomical measurement of local mean time.
When was Greenwich Mean Time legally adopted across Great Britain?
Greenwich Mean Time was legally adopted throughout the island of Great Britain later in 1880 after being used by railway companies since 1847. A legal case in 1858 held local mean time to be official until this change occurred.
Why did Coordinated Universal Time replace Greenwich Mean Time on January 1st 1972?
Coordinated Universal Time replaced Greenwich Mean Time because Earth's daily rotation has an irregular slowing trend making atomic clocks a more stable timebase. This new system is maintained by an ensemble of atomic clocks around the world starting from the 1st of January 1972.
How many hours ahead or behind are countries like Belgium Ghana and Ireland relative to Greenwich Mean Time?
Belgium sets legal time one hour ahead of Greenwich Mean Time while Ghana and Iceland use it as standard time all year round. Ireland defines Winter Time as the same as Greenwich Mean Time and Standard Time as one hour in advance.
What historical convention difference exists between noon-based and midnight-based timekeeping at Greenwich?
The astronomical convention dating from Ptolemy referred to noon as zero hours while the civil convention dating from the Roman Empire refers to midnight as zero hours. The latter convention was adopted for astronomical purposes on and after the 1st of January 1925 creating a discontinuity of 12 hours.