History of the metric system levels of the krypton 86 atom as the new standard for the metre. The standard metre artefact was retired. In 1960, Giorgi's proposals were adopted as the basis of the "Système International d'Unités" (International System of Units), the SI. This initial definition of the SI included six base units, the metre, kilogram, second, ampere, degree Kelvin and candela, and sixteen coherent derived units. The evolution of the SI after its publication in 1960 has seen the addition of a seventh base unit, the "mole", and six more derived units, the "pascal" for pressure, the "gray", "sievert" and "becquerel" for radiation,
Unit of time Unit of time A unit of time or time unit is any particular time interval, used as a standard way of measuring or expressing duration. 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. The exact modern definition, from the National Institute of Standards and Technology is: Historically units of time were defined by the movements of astronomical objects. These units do not have a consistent relationship with each other and require intercalation. For example, the