Greenwich Mean Time
D2. Hydrography, tides, waves, bathymetry and marine geologyDefinition
Time standard at the prime meridian, formerly used for tidal predictions.
Greenwich Mean Time (GMT) is mean solar time at the Greenwich prime meridian, 0 degrees longitude, the historic basis for civil time and for tidal predictions before the adoption of Coordinated Universal Time. Tide tables once referenced lunar transit and high-water times to GMT, and the term still labels the zero time zone. UTC, kept by atomic clocks and steered to within 0.9 seconds of UT1 by leap seconds, has replaced GMT as the precise standard, though the two agree to under a second for practical navigation. Admiralty predictions are now given in UTC with a stated zone correction.
Source: IHO S-32 Hydrographic Dictionary; standard navigation references