Time - Definitions and Standards

Definitions and Standards

Units of time
Unit Size Notes
yoctosecond 10−24 s
zeptosecond 10−21 s
attosecond 10−18 s shortest time uncertainty
in present measurements
femtosecond 10−15 s pulse time of ultrafast lasers
(100 as = 0.1 fs)
picosecond 10−12 s
nanosecond 10−9 s time for molecules to fluoresce
microsecond 10−6 s
millisecond 0.001 s
second 1 s SI base unit
minute 60 seconds
hour 60 minutes
day 24 hours
week 7 days Also called sennight
fortnight 14 days 2 weeks
lunar month 27.2–29.5 days Various definitions exist.
month 28–31 days
quarter 3 months
year 12 months
common year 365 days 52 weeks + 1-day
leap year 366 days 52 weeks + 2 days
tropical year 365.24219 days average
Gregorian year 365.2425 days average
Olympiad 4-year cycle
lustrum 5 years Also called pentad
decade 10 years
Indiction 15-year cycle
generation 17–35 years approximate
jubilee (Biblical) 50 years
century 100 years
millennium 1,000 years
exasecond 1018 s
cosmological decade varies
cosmological century varies
See also: Time standard and Orders of magnitude (time)

The SI base unit for time is the SI second. From the second, larger units such as the minute, hour and day are defined, though they are "non-SI" units because they do not use the decimal system, and also because of the occasional need for a leap second. They are, however, officially accepted for use with the International System. There are no fixed ratios between seconds and months or years as months and years have significant variations in length.

The official SI definition of the second is as follows:

The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

At its 1997 meeting, the CIPM affirmed that this definition refers to a caesium atom in its ground state at a temperature of 0 K. Previous to 1967, the second was defined as:

the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.

The current definition of the second, coupled with the current definition of the metre, is based on the special theory of relativity, which affirms our space-time to be a Minkowski space.

Read more about this topic:  Time

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