Greenwich Mean Time

Greenwich Mean Time (GMT) is a time system originally referring to mean solar time at the Royal Observatory in Greenwich, London, which later became adopted as a global time standard. It is arguably the same as Coordinated Universal Time (UTC) and when this is viewed as a time zone the name Greenwich Mean Time is especially used by bodies connected with the United Kingdom, such as the BBC World Service, the Royal Navy, the Met Office and others particularly in Arab countries, such as the Middle East Broadcasting Center and OSN. It is the term in common use in the United Kingdom and countries of the Commonwealth, including Australia, South Africa, Nigeria, India and Malaysia, as well as many other countries in the Old World.

Before the introduction of UTC on 1 January 1972 Greenwich Mean Time (also known as Zulu time) was the same as Universal Time (UT) which is a standard astronomical concept used in many technical fields. Astronomers no longer use the term "Greenwich Mean Time".

In the United Kingdom, GMT is the official time only during winter; during summer British Summer Time is used. GMT is the same as Western European Time.

Noon Greenwich Mean Time is rarely the exact moment when the sun crosses the Greenwich meridian (and reaches its highest point in the sky at Greenwich) because of Earth's uneven speed in its elliptic orbit and its axial tilt. This event may be up to 16 minutes away from noon GMT (a discrepancy calculated by the equation of time). The fictitious mean sun is the annual average of this non-uniform motion of the true Sun, necessitating the inclusion of mean in Greenwich Mean Time.

Historically the term GMT has been used with two different conventions, sometimes numbering hours starting at midnight and sometimes starting at noon. The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours. Astronomers preferred the latter GMT convention in order to simplify their observational data so that each entire night was logged under a single calendar date.