Editing Greenwich Mean Time (section)

== History ==
Mean solar time is calculated by averaging the apparent movement of the Sun across the sky over the course of a year. This removes the daily variation found in apparent solar time and produces a consistent twenty-four-hour day.

The Royal Observatory was established at Greenwich in 1675. Astronomical observations made there allowed Greenwich mean solar time to be calculated and distributed. During the nineteenth century, Greenwich time became increasingly important as railways and telegraph networks required a common time instead of separate local times.

The International Meridian Conference met in Washington, D.C., in 1884. The conference selected the Greenwich meridian as the international prime meridian. GMT subsequently became the main reference for international civil time and the calculation of time differences between locations.

[[Coordinated Universal Time]] replaced GMT as the principal international civil time standard in 1972. GMT continued to be used as a named standard time and as a historical timekeeping term.