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Greenwich Mean Time was formulated in the 1800’s in order to deal with the confusion that existed when each country decided its own solar time. This was especially important with the increase in traveling and train schedules. In 1884, at the Meridian Conference, 27 countries decided to implement a system that is practically the same one that we have today. They based their system on one formulated by a Canadian railway planner and engineer, Sir Sanford Fleming.
In this system, there are 24 lines that go from the North to the South called meridians of longitude. They are spaced 15° apart. The first line, known as the prime meridian, is located in Greenwich, England. It is 0° longitude. Greenwich Mean Time (GMT) refers to solar mean time at the Royal Observatory in Greenwich, England. Solar mean time is calculated using the angle of the mean Sun – a fictional Sun used to compute time. Since Britain was an advanced maritime nation, they had long used Greenwich as a point of longitude to calculate their longitude in relation to the Greenwich meridian. Due to its long history, the world decided to accept it as the prime meridian as well.
Different countries have legally defined their local time in reference to Greenwich Mean Time. These countries include the United Kingdom, Belgium, the Republic of Ireland, and Canada. Although GMT is used all year round in Iceland, it is only used during the winter in Ireland and the UK. During the summer, British Summer Time (BST) is used. This is also known as Greenwich Daylight Savings Time (GDT), which corresponds to daylights savings time in the United States. The Greenwich Mean Time is also known as the Universal Time (UT).
The issue of time zones is made more complicated because the zones are not always split along the longitudinal lines. They are also divided along political boundaries. This means that although it may be one time in the one country, if you head north or south while staying in the same longitude, the time may change.
GMT is used for a variety of things. For example, many emails between people from different time zones include a reference to GMT, explaining how many hours that person’s time zone is from GMT. On the International Space Station, they use GMT. The US Government National Weather Service and the Weather Channel both use weather maps that use GMT. These are just a few of many ways that the GMT is used today.
Astronomy Cast has an episode on Earth.