Definition:Time/Unit
Definition
The units of measurement of time are universal in all systems of measurement.
Second
The second is the SI base unit of time, and also therefore of the MKS system.
It is also the base unit of time for the FPS and CGS systems.
The second is defined as:
- 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 rest at $0 \ \mathrm K$.
Derived units of time include:
Minute
The minute is a derived unit of time.
- $1$ minute $= 60$ seconds.
Hour
The hour is a derived unit of time.
| \(\ds \) | \(\) | \(\ds 1\) | hour | |||||||||||
| \(\ds \) | \(=\) | \(\ds 60\) | minutes | |||||||||||
| \(\ds \) | \(=\) | \(\ds 60 \times 60\) | \(\ds = 3600\) | seconds |
Day
| \(\ds \) | \(\) | \(\ds 1\) | day | |||||||||||
| \(\ds \) | \(=\) | \(\ds 24\) | hours | |||||||||||
| \(\ds \) | \(=\) | \(\ds 60 \times 24\) | \(\ds = 1440\) | minutes | ||||||||||
| \(\ds \) | \(=\) | \(\ds 60 \times 60 \times 24\) | \(\ds = 86\, 400\) | seconds |
There are longer time periods, all of which are more or less approximate:
Week
- $1$ week $=7$ days.
Fortnight
Month
The month is measured only approximately in duration, and is not used as a scientific measure.
- $1$ month is between $28$ and $31$ days, depending on which month it is.
Lunar Month
The lunar month is defined as the length of time between $2$ new moons.
- $1$ lunar month $\approx 29 \cdotp 530588$ days
or:
Season
The season is measured only approximately in duration, and is not used as a scientific measure.
- $1$ season $= 3$ months, although exactly which months fall into which season depends on the locale and the culture.
Quarter-Year
The quarter-year is defined as:
- $91$ days
or
Year
| \(\ds \) | \(\) | \(\ds 1\) | year | |||||||||||
| \(\ds \) | \(\approx\) | \(\ds 365 \cdotp 24219 \, 878\) | days | |||||||||||
| \(\ds \) | \(\approx\) | \(\ds 365 \cdotp 24219 \, 878 \times 24 \times 60 \times 60\) | seconds |
That is:
| \(\ds \) | \(\) | \(\ds 1\) | year | |||||||||||
| \(\ds \) | \(\approx\) | \(\ds 365\) | days, | |||||||||||
| \(\ds \) | \(\) | \(\ds 5\) | hours, | |||||||||||
| \(\ds \) | \(\) | \(\ds 48\) | minutes, and | |||||||||||
| \(\ds \) | \(\) | \(\ds 45 \cdotp 9747\) | seconds |
It is defined to be equal to the orbital period of Earth round the sun.
Decade
- $1$ decade $=10$ years.
Century
- $1$ century $=100$ years.
Millennium
- $1$ millennium $=1000$ years.
Great Year of Plato
| \(\ds \) | \(\) | \(\ds 1\) | great year of Plato | |||||||||||
| \(\ds \) | \(=\) | \(\ds 12 \, 960 \, 000\) | days | |||||||||||
| \(\ds \) | \(\approx\) | \(\ds 36 \, 000\) | years (of $360$ days) |