Before the introduction of UTC on 1 January 1972, Greenwich Mean Time (also known as Zulu time) was the same as Universal Time (UT), a standard astronomical concept used in many technical fields. Astronomers no longer use the term "Greenwich Mean 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 there, 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. Noon is the annual average (i.e. mean) time of this event, prompting 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 night was logged under a single calendar date.