Time Realisation (metrology)
1 time 1.1 direct measurement of solar time 1.2 timekeepers 1.3 time generators time the realisation of time has gone through 3 phases. during both first , second phases, man used solar time—during first phase, realisation of time observing earth s rotation using such devices sundial or astrolabe. during second phase actual timing devices such hourglasses or clocks used. if user needed know time-of-day rather elapsed time, clocks synchronized astronomical time. third phase made use of clocks sufficiently accurate measure variations in earth s rotation—such clocks taking on rotation of earth prime measure of time. direct measurement of solar time sundials , astrolabes timekeepers accuracy of clocks time generators radiation frequency & si