Measuring time {time measurement} requires standard vibration or rotation frequency. Fundamental time unit is one second {standard second} {SI second} (s), which is 9,192,631,770 periods of unperturbed microwave transition between two cesium-133 ground-state hyperfine levels, as measured by atomic clocks {Coordinated Universal Time} (UTC).
year
Standard second can depend on Earth motion around Sun. Tropical year 1900 has 31,556,925,974 seconds {ephemeris second} {ephemeris time} {Newtonian time}. Gravity predicts solar-system object positions at distinct times. Ephemeris time uses actual number of days {universal time} (UT0) or universal time corrected for Earth-axis wobble {navigator's time} (UT1) (UT2).
derived units
60 seconds {minute, time} (min). 60 minutes {hour} (hr). 24 hours {day}. 7 days {week}. 14 days and nights {fortnight}. 4 1/3 week or 28 to 31 days {month}. 365 days {year} (yr). 366 days every fourth year {leap year}. 10 years {decade}. 100 years {century, time}. 1000 years {millennium}. 1,000,000 years {eon}.
minimum
By quantum mechanics, minimum time is Planck time, 10^-43 seconds, where time becomes quantized.
maximum
Maximum time is age of universe, 10^21 seconds.
Physical Sciences>Physics>Measurement>Kinds>Time
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Date Modified: 2022.0224