These 2 turning points (different definition of a second and the introduction of leap seconds) ‘forced’ GMT to be the same as UTC based on what seemed a gradual, tacit convention. In 1972 leap seconds were introduced to synchronize UTC time with solar time. UTC’s second is far more precise than GMT's original second. Unlike GMT which is based on solar time and originally calculated a second as a fraction of the time it takes for the Earth to make a full rotation around its axis, UTC calculates a second as “the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom”. UTC essentially appeared in 1960, GMT being the ‘main thing’ until then.
Literature and history are a bit ambiguous.
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