mean solar day

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Related to mean solar day: mean solar time
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Historically, the second was simply defined in terms of the rotation of the Earth as 1/86 400 of a mean solar day which means that the Earth's rotation was effectively the timebase.
Atomic clocks, in which an atom takes on the role of a pendulum, came into use a few decades ago as the ultimate in time-keeping accuracy, replacing astronomical observations of the mean solar day as the definition of the second.