What is GMT?
GMT is the abbreviated form of Greenwich Mean Time. It is also known under the name Greenwich Meridian time. The name owes it origin to the place Greenwich in London. The Greenwich Meridian Time is measured from the Royal Observatory in London. The longitude from GMT is assumed to be zero. It is the arithmetic mean of the time the earth takes to reach from one noon to the next. The average is being calculated because the Earth rotates with a non uniform speed in its elliptical orbit.
Astronomers have replaced the term GMT with Zulu Time for their technical use. All time zones in the world are measured from the Greenwich Meridian. Hence it is also called the Prime Meridian. The concept of Greenwich Mean Time is used mainly in the United Kingdom. But it is used officially only during the winter season. Countries such as Iceland depend on Greenwich Mean Time through out the year. In a time zone map GMT will occupy the centre position. GMT serves as a basis for time zones all over the world. The time of the day is being set by the World time zone.
As Britain became a maritime nation it was necessary for mariners to measure the longitudes from Greenwich even though it did not have any effect on their shipboard time. Due to the non uniform rotation of the earth which slows down at times the instrument at the Royal Observatory in London was considered to be inaccurate. Now atomic clocks are used to measure the official time zone. At present a new concept known as UTC or Co-ordinated Universal Time is being used. But still the UTC based time system is known under the name Greenwich Mean Time. UTC based system in Europe is also known as Western European Time or WET.BST or British Summer Time is also in use in the European countries.