“GMT” stands for Greenwich Mean Time. It is the standard time in Britain. The UK puts the clocks back and forward by one hour in the year to account for the different lengths of days during the year. This takes Britain off GMT for the summer, in which period, it is on BST, which is British Summer Time. However, GMT continues to be recorded during this period because it is the standard time against which all other time zones are offset.
The Royal Observatory at Greenwich is within view of the pool of London from where Britain's trading ships left to travel the world. Distance over the globe is measured in latitutde and longitude which uses time as a basis for its calculation. It became important for all mariners to coordinate their ship’s clocks so that they could be sure it ran at the same time as that used by map makers. The Royal Observatory gives a signal every day at midday so that all ships in the harbour could synchronise their clocks. This time was carried all over the world by British ships and became the conventional standard time for navigation.
The word “mean” has several coincidental uses. In Greenwich Mean Time, it refers to the word’s use in statistics. “Mean” is another word for “average” and so Greenwich Mean Time is the average time measured at the Greenwich observatory. A day on earth is measured as the lapse between the sun’s appearance in the same position and hours are equal divisions of that period. However, the sun’s pace varies throughout the year and so the astronomers at Greenwich calculated the average length of the day, which is called “solar mean time.” This was the basis for GMT and the “mean” from “solar mean time” worked its way into the name.
Just as it was important that all ships carried the same time, so it became necessary to have a standard reference for time all over the world. This not only makes time standard, but it also sets a fixed point for all mapmakers when using coordinates of Latitude and longitude. GMT relates to Longitude which creates imaginary lines that run from the North Pole to the South Pole. The International Meridian Congress of 1884 decided to make the longitude line that passes through Greenwich the key starting point for measuring longitude. This line is called the prime meridian and it also dictates the time zones all over the world.
World time is no longer based on Greenwich solar observations. It is now based on an atomic clock which is still run by the Royal Observatory, but under the direction of the Bureau International des Poids et Mesures. Time is still expressed as it is used at Greenwich, but since 1972, the international standard is called UTC, which stands for Universal Time Coordinated. GMT is also sometimes called Zulu Time.