Greenwich Mean Time

Greenwich Mean Time (GMT) ismean solar time atRoyal Greenwich ObservatoryGreenwich near London, England, which by conventionat 0 degrees geographic longitude. Theoretically, noon Greenwich Mean Time ismoment whenSun crossesGreenwich meridian (and reaches its highest point inskyGreenwich). Because ofEarth's uneven speedits elliptic orbit, this event may be up16 minutes off apparent solar time (known asanalemma); but thisaveraged out overyear throughuse ofmean sun.

The daily rotation ofEarthsomewhat irregular (see Delta-T)is slowing down. Therefore, GMTnot used as official clock time anymore. Nowadays,official clock timemeasured by atomic clocksis known as Coordinated Universal Time (UTC). By using leap seconds, UTCkept within 0.9 seconds from GMT.

Hourly time signals from Greenwich Observatory were first broadcasted on February 5, 1924.

See also sidereal time, solar time, BPM, CHU, VNG, WWV