The Greenwich Prime Meridian

The Greenwich Prime MeridianTransit Circle in the Royal Observatory, even if
 the exact location of the line may move to either
 side of Airy's meridian.
The Royal Observatory in Greenwich is the home 
of Greenwich Mean Time (GMT) and the PrimeThe line in Greenwich represents the Prime
Meridian of the world. Many years ago in theMeridian of the World - Longitude 0º. Every
1920's my great Aunt Hilda traced our family treeplace on Earth is measured in terms of its
back to the Kings and Queens of England fromdistance east or west from this line. The line itself
the 7th. Century. This basically means I amdivides the eastern and western hemispheres of
related to most of the British Royal Family goingthe Earth - just as the Equator divides the
back 1500 years and this has made me a greatnorthern and southern hemispheres.
fan of English and British Icons including the history 
of GMT.The Greenwich Meridian was chosen as the Prime
 Meridian of the World in 1884. There were
Greenwich Mean Time (GMT) is a term originallyForty-one delegates from 25 nations who met in
referring to mean solar time at the RoyalWashington DC for the International Meridian
Observatory in Greenwich, London. It isConference. By the end of the conference,
commonly used in practice to refer toGreenwich had won the prize of Longitude 0º
Co-ordinated Universal Time (UTC) when this isby a vote of 22 to 1 against (San Domingo), with
viewed as a time zone, especially by bodies2 abstentions ( France and Brazil – what a
connected with the United Kingdom, such as thesurprise the French was against us Brits.!! ). There
BBC World Service, the Royal Navy, the Metwere two main reasons for choosing Greenwich
Office and others, although strictly UTC is anas the Prime meridian:
atomic time scale which only approximates GMTThe first was the fact that the USA had already
with a tolerance of 0.9 second. It is also used tochosen Greenwich as the basis for its own
refer to Universal Time (UT), which is a standardnational time zone system.
astronomical concept used in many technical fieldsThe second was that in the late 19th century,
and is referred to by the phrase Zulu Time.72% of the world's commerce depended on
In the UK, GMT is the official time only duringsea-charts which used Greenwich as the Prime
winter; during summer British Summer time ( BSTMeridian.
) is used. GMT is substantially equivalent to 
Western European Time.The decision, essentially, was based on the
 argument that by naming Greenwich as Longitude
In 1884 the Prime Meridian was defined by the0º, it would be advantageous to the largest
position of the large "Transit Circle" telescope innumber of people. Therefore the Prime Meridian
the Observatory's Meridian Building. The transitat Greenwich became the centre of world time,
circle was built by Sir George Biddell Airy, the 7thand was the official starting point for the new
Astronomer Royal, in 1850. The cross-hairs in theMillennium.
eyepiece of the Transit Circle precisely defined 
Longitude 0° for the world. As the earth's crustIn the future, when we Earthlings set out to
is moving very slightly all the time the exactexplore the solar system GMT will still be used as
position of the Prime Meridian is now moving verythe Point Of Earth time. As an example the Earth
slightly too, but the original reference for theGMT will be called: E-GMT.
prime meridian of the world remains the Airy