Greenwich Mean Time

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Greenwich mean time

or

Greenwich meridian time

(GMT), the former name for mean solar time at the original site of the Royal Observatory in Greenwich, England, which is located on the prime meridianprime meridian,
meridian that is designated zero degree (0°) longitude, from which all other longitudes are measured. By international convention, it passes through the original site of the Royal Observatory in Greenwich, England; for this reason, it is sometimes called the
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. In 1925 the numbering system was changed to make GMT equivalent to civil timecivil time,
local time based on universal time. Civil time may be formally defined as mean solar time plus 12 hr; the civil day begins at midnight, while the mean solar day begins at noon.
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 at the prime meridian, and in 1928 the International Astronomical Union changed the designation of the standard timestandard time,
civil time used within a given time zone. The earth is divided into 24 time zones, each of which is about 15° of longitude wide and corresponds to one hour of time. Within a zone all civil clocks are set to the same local solar time.
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 of the prime meridian to universal timeuniversal time
(UT), the international time standard common to every place in the world, it nominally reflects the mean solar time along the earth's prime meridian (renumbered to equate to civil time).
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 (UT), which is now in general use.

Greenwich mean time

(GMT) The mean solar time on the longitude (0°) at Greenwich, in the UK, reckoned from midnight. In 1928, on the recommendation of the IAU, GMT became known as universal time (UT) for scientific purposes; the version coordinated universal time, available since 1972 from broadcast signals, is now generally known as GMT.

Greenwich mean time

[′gren·ich ′mēn ¦tīm]
(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 (GMT)

Greenwich Mean Time (GMT)click for a larger image
The arc of the equatorial measured westward intercepted between the Greenwich antimeridian and the meridian of the mean sun. Also called Zulu time.

Greenwich Mean Time

, Greenwich Time
mean solar time on the 0? meridian passing through Greenwich, England, measured from midnight: formerly a standard time in Britain and a basis for calculating times throughout most of the world, it has been replaced by an atomic timescale

Greenwich Mean Time

(time, standard)
(GMT) The local time on the Greenwich meridian, based on the hypothetical mean sun (which averages out the effects of the Earth's elliptical orbit and its tilted axis). GMT is the basis of the civil time for the UK.

In 1925 the reference point was changed from noon to midnight and it was recommended that the term "Universal Time" should be used for the new GMT.

Authorities disagreed on whether GMT equates with UT0 or UT1, however the differences between the two are of the order of thousandths of a second. GMT is no longer used for scientific purposes.
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