A Bunch of Stuff on GMT

by Michael

Greenwich Mean Time (GMT) is also known as Zulu Time
and UTC (Universal Time Coordinated). The standard by which all World
Time is set was agreed at the 1884 International Meridian Conference
at Washington DC, USA placed Greenwich on the Prime Meridian (Zero

So…you might as why does the Prime Meridian (Zero Longitude) pass
through Greenwich? See below…

The International Meridian Conference

Washington DC, USA – October 1884

dates back to October 1884. At the behest of the President of the
United States of America 41 delegates from 25 nations met in Washington,
DC, USA for the International Meridian Conference.

At the Conference the following important principles were established:

1 – It was desirable to adopt a
single world meridian to replace the numerous one’s already
in existence.
2 – The Meridian passing through
the principal Transit Instrument at the Observatory at Greenwich
was to be the ‘initial meridian’.
3 – That all longitude would be
calculated both east and west from this meridian up to 180.
4 – All countries would adopt a
universal day.
5 – The universal day would be a
Mean Solar Day, beginning at the Mean Midnight at Greenwich
and counted on a 24 hour clock.
6 – That nautical and astronomical
days everywhere would begin at mean midnight.
7 – All technical studies to regulate
and extend the application of the decimal system to the division
of time and space would be supported.

Resolution 2, fixing the Meridian at Greenwich was passed 22-1 (Santo
Domingo voted against), France & Brazil abstained.

What is Greenwich Mean Time (GMT)?

Greenwich Mean Time or GMT is the time standard against which all
other time zones in the world are referenced. It is the same all year
round and is not effected by Summer Time or Daylight Savings Time.
GMT was originally set-up to aid naval navigation when the globe started
to open up with the discovery of the “New World” (America) in the
fifteenth Century.

It was not forced on to “land-lubbers” until the introduction of the
railways (railroads) in the mid nineteenth century. In the 1840 ‘s
with the introduction of the railways (railroads) there was a need,
in Britain, for a national time system to replace the local time adopted
by major towns and cities. Greenwich was the national centre for time
and had been since 1675. It was not adopted officially by Parliament
until 2 August 1880.

Greenwich Mean Time (GMT) was adopted by the United States (USA) at
noon on 18 November 1883 when the telegraph lines transmitted time
signals to all major cities. Prior to that there were over 300 local
times in the USA.

Greenwich Mean
Time (GMT) was adopted universally on the 1 November 1884 when the
International Meridian Conference (see above) in Washington, DC,
USA met. From then the International Date Line was drawn up and
24 time zones created.

Today, GMT is used as the UK’s civil time, or UTC. But to navigators,
GMT has referred to “UT1”, which directly corresponds to the rotation
of the Earth, and is subject to that rotation’s slight irregularities.
It is the difference between UT1 and UTC that is kept below 0.9s
by the application of leap seconds.

What is Co-ordinated Universal Time (UTC)?

Replaced Greenwich Mean Time (GMT) as the World standard for time
in 1986. It is based on atomic measurements rather than the earth’s
rotation. Greenwich Mean Time (GMT) is still the standard time zone
for the Prime Meridian (Zero Longitude).

The definition of a “second” is also directly related to GMT.
What is the definition of a second?

Originally based on the Earth’s rotation this has been found to
be irregular and has in more recent times been replaced by atomic

The overhead sun at noon can arrived 16
minutes 18 seconds early or up to 14 minutes 28 seconds late. In
fact only 4 days per year have 24 hours in them. These days occur
on, or about 25 December, 15 April, 14 June and 31 August. The rest
are longer or shorter as measured from the overhead midday sun.

A more precise measure of time was required.
So there have been many definitions of the second throughout time:

1884 – International Meridian Conference
(Washington DC): based on a mean solar day at Greenwich, England.
1956 – Ephemeris Time: Based on lunar
1958 – Atomic Time: Based on clock
data from numerous countries.
1967 – Atomic Time: based on radiation
patterns of the caesium element.
1972 – Co-ordinated Universal Time (UTC) introduced.
1984 – Dynamical time – based on motion
of celestial bodies.
1986 – UTC replaced GMT.

Despite time being measured highly accurately by atomic means the
earth time stills rules. Should earth time and atomic time get out
of step scientists adjust time by subtracting or adding “leap seconds”
on the last day of June or December.

I hope this post contains some useful information for everyone.