Greenwich Mean Time (GMT) is both a historic time standard and the name of a modern time zone.
This article explains the difference and shows the current GMT clock.
Royal Observatory from afar.
Time at the Shepherd Gate Clock, at the Royal Observatory in Greenwich, UK
GMT is based on regular observation of the rotational velocity of the Earth at the Prime Meridian (0 degrees longitude). At noon,
when the Sun is directly above the Greenwich Meridian, the clock at the Royal Observatory shows
12:00.
The word 'mean' is used to point out that GMT is an average of all values recorded throughout
a year. This is due to the specifics of the Earth's movement round its axis and round the Sun —
the planet speeds up and slows down along its elliptical orbit, so a single 'solar noon' measured
on any given day would not match the next.
UTC is rectified to remain within 0.9 of a second of GMT
(leap second), with sub-second interval accuracy
as its prime objective.
The Prime Meridian line at the Royal Observatory, Greenwich — longitude 0°, the reference for
GMT. Photo released to the public domain (CC0) via Wikimedia Commons.
A common misconception about UK time
Many people believe that the United Kingdom runs on Greenwich Mean Time (GMT) all year
round — but this is only true during standard time, over winter.
From the last Sunday in March to the last Sunday in October the UK switches to British Summer Time (BST), which is GMT+1 — one hour ahead of GMT. So for roughly seven months
of the year, London is not on GMT.
Time zones and longitude 0
The Prime Meridian, longitude 0, is the reference
point for every time zone in the world. Offsets or time differences are generally written as
UTC/GMT plus or minus a number of hours, e.g. UTC+7 / GMT+7.
Every 15° of longitude represents one hour's difference in time (24 × 15 = 360, the degrees of a
circle). You can work out the time at any location on Earth if you know how many degrees it is
east or west of Greenwich.
GMT was the basis of the international civil time standard until 1972, when it was replaced by UTC (Coordinated Universal Time). UTC is kept within 0.9 of a
second of the Earth's actual rotation — UT1 — by the insertion of leap seconds. In the age of the internet these
adjustments are distributed via NTP, which is how we find out the exact time to the second.
Where it all began
GMT was originally set up to aid naval navigation when travel around the globe started to open up,
with the discovery of the New World (the Americas) in the fifteenth century. Read about the connection between accurate time-keeping, GMT and sea voyages.
Greenwich was a royal park and palace on a hill to the south of the River Thames east of London. In 1675 the race to create accurate maps for
navigators had begun, and Charles II offered the land to The Royal Society for Britain's first
national observatory. Christopher Wren was commissioned to design the domed building, and John
Flamsteed was appointed Astronomer Royal — British mapmakers began to set longitude lines from
Greenwich.
Railways and national time
With the introduction of the railways in the mid-nineteenth century, Britain needed a national
time system to replace the local time kept by major towns and cities. As Greenwich, due to the
Royal Observatory, had been the national centre for time since 1675, the choice was obvious.
Nevertheless, time as shown by the clocks at the Royal Observatory was not approved officially by
Parliament until 2 August 1880.
GMT was then adopted by the United States on 18 November 1883. The chosen moment was at noon, when
telegraph lines transmitted time signals to all major cities. Prior to that there were over 300
local times in the USA.
On 1 November 1884, the Greenwich Meridian was adopted universally at the International Meridian Conference in Washington
DC. As a result, Greenwich Mean Time became the time standard and the 24 time zones were created. GMT was replaced as
the international time standard in 1972 by UTC (Coordinated Universal Time), which is based on atomic
measurements.
Did you know?
Astronomical time starts at noon, while civil time starts at midnight — a convention that once
caused astronomers and clockmakers to disagree.
The Shepherd Gate Clock, installed in 1852, has a 24-hour dial and was the first clock to
show GMT directly to the public.
The Time Ball at the Observatory, now red, used to be black.
GMT was adopted as the legal time of Great Britain by the International Meridian Conference reference of 1884,
which chose Greenwich as the Prime Meridian of the world.
Iceland, Portugal, and several West African countries share GMT as their civil time zone
year-round — see the full list above.
@time_gadgets