Greenwich Mean Time
"
Greenwich Mean Time" (
GMT) is a term originally referring to
mean solar time at the
Royal Greenwich Observatory in
Greenwich in
London. It is now often used to refer to
Coordinated Universal Time (UTC) when this is viewed as a
time zone, although strictly UTC is an
atomic time scale which only approximates GMT in the old sense. It is also used to refer to
Universal Time (UT), which is the
astronomical concept that directly replaced the original GMT.
Noon Greenwich Mean Time is not necessarily 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.
Historically the term "GMT" has been used with two different conventions for numbering hours. The old astronomical convention was to refer to noon as zero hours, whereas the civil and more modern convention is to refer to midnight as zero hours. The more specific terms "UT" and "UTC" do not suffer this ambiguity, always referring to midnight as zero hours.
As the United Kingdom grew into an advanced maritime nation, British mariners kept their timepieces on GMT in order to calculate their
longitude "from the Greenwich meridian", which was by convention considered to have longitude zero degrees. 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".
Greenwich Mean Time was adopted across
Great Britain by the
Railway Clearing House in
1847, and by almost all railway companies by the following year. It was gradually adopted for other purposes, but a legal case in
1858 held "
Local Mean Time" to be the official time.[
1] This changed in
1880, when GMT was legally adopted throughout Great Britain. GMT was adopted on the
Isle of Man in 1883,
Jersey in 1898 and Guernsey in 1913. Ireland adopted Greenwich Mean Time in 1916, supplanting
Dublin Mean Time.[
2]
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.
The international
prime meridian is no longer precisely the Greenwich meridian, but remains close to it.
Although civil time, e.g., the
Greenwich Time Signal in the
United Kingdom, 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 above use
European Summer Time and advance their clock one hour in summer. In the United Kingdom, this is known as
British Summer Time (BST); in the Republic of Ireland it is called Irish Summer Time (IST). Those countries marked in light blue keep their clocks on UTC/GMT/WET year round.
*
Coordinated Universal Time (UTC)
*
British Summer Time (BST)
*
Universal Time*
Nautical mile*
Sidereal time*
Solar time*
Swatch Internet Time*
BPM*
CHU*
VNG*
WWV*
Western European Time*
Central European Time*
Eastern European Time*
Moscow Time*
Sandringham Time*
Indian Standard Time*
Greenwich Mean Time *
NIST - World Time Scales *
International Earth Rotation and Reference Systems Service*
World Clock - Current Local Times
