Subdivisions of the day include the hour (1/24 of a day), which was further subdivided into minutes and finally seconds. Units originally derived from this base include the week at seven days, and the fortnight at 14 days. Earth-based: the time it took for the earth to rotate on its own axis, as observed on a sundial.Moon-based: the month was based on the moon's orbital period around the earth.Year-based units include the olympiad (four years), the lustrum (five years), the indiction (15 years), the decade, the century, and the millennium. Sun-based: the year was the time for the earth to revolve around the sun.Historically units of time were defined by the movements of astronomical objects. It is defined by taking the fixed numerical value of the cesium frequency Δ ν Cs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s −1." "The second, symbol s, is the SI unit of time. The exact modern definition, from the National Institute of Standards and Technology is:
The base unit of time in the International System of Units (SI) and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom. Table showing quantitative relationships between common units of timeĪ unit of time or midst unit is any particular time interval, used as a standard way of measuring or expressing duration.
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