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.
Earth time measurement can compare to Sun {solar time}. Varying orbital speeds and varying seasons cause Sun-referenced Earth-rotation period to average four minutes longer than star-referenced rotation period. Sun-referenced Earth-rotation period can be ahead 16 minutes in early November or behind 14 minutes in early January.
Years {anomalistic year} can be time from Earth perihelion to next perihelion, which is five minutes longer than sidereal years, because major axis of Earth elliptical orbit moves around Sun in same direction as Earth moves.
Years {sidereal year} can be time between same positions with respect to fixed stars. Time {sidereal time} can be relative to fixed stars.
Years {tropical year} can be time from first point of Aries to next first point of Aries, which is 20 minutes shorter than sidereal year, because first point moves around Sun in opposite direction to Earth motion.
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Date Modified: 2022.0225