Greenwich Mean Time

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For alternate meanings of "GMT", see GMT (disambiguation).

Greenwich Mean Time (GMT) is mean solar time at the Royal Greenwich Observatory in Greenwich, London, England, which by convention is at 0 degrees geographic longitude. Noon Greenwich Mean Time is not the moment when the Sun crosses the Greenwich meridian (and reaches its highest point in the sky in Greenwich) because of Earth's uneven speed in its elliptic orbit and its axial tilt. This event may be up to 16 minutes away from noon GMT (this discrepancy is known as the equation of time). The fictitious mean sun is the annual average of this nonuniform motion of the true Sun, necessitating the inclusion of mean in Greenwich Mean Time.

Time zones of Europe: blue - GMT or Western European Time, red - Central European Time, green - Eastern European Time, khaki - Moscow Time.

As Great Britain grew into an advanced maritime nation, English mariners kept their timepieces on GMT in order to calculate their longitude "from the Greenwich meridian". This did not affect shipboard time itself, which was still solar time. This combined with mariners from other nations drawing from Nevil Maskelyne's method of lunar distances based on observations at Greenwich, eventually led to GMT being used world-wide as a reference time independent of location. Most time zones were based upon this reference as a number of hours and half-hours "ahead of GMT" or "behind GMT".

Hourly time signals from Greenwich Observatory were first broadcast on 5 February 1924.

The daily rotation of the Earth is somewhat irregular (see ΔT) and is slowing down slightly. Atomic clocks represent a much more stable timebase. On 1 January 1972, GMT was replaced as the international time reference by Coordinated Universal Time (UTC), maintained by an ensemble of atomic clocks around the world. UT1 was introduced to represent "earth rotation time". Leap seconds are added to or subtracted from UTC to keep it within 0.9 seconds of UT1.

Although civil time, e.g., the Greenwich Time Signal in Great Britain, is now based on UTC, it is still popularly called GMT. It is also called Western European Time (WET).

Those countries marked in dark blue on the map at right use European Summer Time and advance their clock one hour in summer. In Great Britain, this is known as British Summer Time (BST); in Ireland it is called Irish Summer Time (IST). Those countries marked in light blue keep their clocks on UTC/GMT/WET year round.

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