Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London. 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.
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 UK, 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 not necessarily the moment when the noon sun crosses the Greenwich meridian (and reaches its highest point in the sky in 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 known as the equation of time). The fictitious mean sun is the annual average of this nonuniform 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 for numbering hours. The old astronomical convention (before 1 January 1925) was to refer to noon as zero hours, whereas the civil convention during the same period was to refer to midnight as zero hours. The latter convention is modern practice (on and after 1 January 1925) for astronomical as well as civil purposes. The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours.
As the United Kingdom grew into an advanced maritime nation, British mariners kept at least one chronometer on GMT in order to calculate their longitude from the Greenwich meridian, which was by convention considered to have longitude zero degrees (this convention was internationally adopted in the International Meridian Conference of 1884). Note that the synchronization of the chronometer on GMT did not affect shipboard time itself, which was still solar time. But this practice, combined with mariners from other nations drawing from Nevil Maskelyne's method of lunar distances based on observations at Greenwich, eventually led to GMT being used worldwide as a reference time independent of location. Most time zones were based upon this reference as a number of hours and half-hours "ahead of GMT" or "behind GMT".
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