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 meridian prime 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
..... Click the link for more information. . In 1925 the numbering system was changed to make GMT equivalent to civil time civil 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.
..... Click the link for more information. at the prime meridian, and in 1928 the International Astronomical Union changed the designation of the standard time standard 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 .
..... Click the link for more information. of the prime meridian to universal time universal 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 ).
..... Click the link for more information. (UT), which is now in general use.

Greenwich Mean Time (GMT)

Former name for mean solar time of the longitude (0°) of the former Royal Observatory at Greenwich, England, or Greenwich meridian. GMT was used to avoid potentially confusing references to local time systems (zones). In accord with tradition, 0000 GMT (denoting the start of a solar day) occurred at noon. In 1925 the numbering system for GMT was changed so that the day (like the civil day) began at midnight. Some confusion resulted, and in 1928 the International Astronomical Union changed the designation of the standard time of the zero meridian to Universal Time, which remains in general use. The term GMT is still used for some purposes (including navigation) in English-speaking countries.

See UTC.

(time, standard)

Greenwich Mean Time - (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.