GMT

(astronomy) Mean solar time at the meridian of Greenwich. Abbreviated GMT. Also known as Greenwich civil time; universal time; Z time; zulu time.

(Greenwich Mean Time) See UTC.

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Former term for Universal Time.

See the Introduction, Abbreviations and Pronunciation for further details.

time The clock scheme set to have 12:00 noon when the Sun is nominally overhead on the meridian through a defined point within the Observatory at Greenwich, but since 1972 effectively renamed Universal Time. (The ‘defined point’ was the centre of a precisely positioned telescope assigned to the purpose.)

Established, along with the Greenwich meridian as the datum line for longitude, by the International Prime Meridian Conference held in Washington in 1884, GMT formed the basis for the world-girdling scheme of time zones, generally set so that each spans 15° of longitude, with the time zone of Greenwich itself being from 0° to 15°W, and with an International Date Line following the 180° meridian except for minor deviations.

Hourly time zones, with GMT as the basis for them, and even Greenwich as the prime meridian, took years to be accepted. Germany dispensed with its multiple time zones only in the 1890s; France accepted GMT in 1911.

See equation of time for a discussion of the variation in the timing of noon.