What was greenwich mean time originally referred as?

Answer 1

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,[1] 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 asUniversal 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.[2]

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 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 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, 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.