# Time and Its Measure: Historical and Social Implications
## 1 Introduction  
Time is one of the seven base units of the International System (SI), and the \"second\" (s) is the unit adopted as the standard for its measurement. The unit of time was initially defined as 1/86,400 of the average solar day, but the irregularities in the Earth's rotation motion made the definition no longer satisfactory. Therefore, in 1960, the XI General Conference of Weights and Measures (CGPM) adopted a fraction of the time of the Earth's revolution around the Sun in the year 1900 as the unit. Later, in the XIII CGPM (1967), the \"Atomic Standard Time\", based on the cesium atom transition between two energy levels, led to the current definition of the unity of time: _the second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium 133 atom_. Basically, when an electron in the _cesium_ 133 atom passes from one energy level to another, it emits (or absorbs) radiation (a wave phenomenon) with a constant frequency, passing 9,192,631,770 oscillations in 1 s [1]. Other very accurate resonance frequencies are those of _rubidium Rb87_ and _hydrogen maser_, as evident in Table 1, where frequencies are compared with those of a mechanical pendulum and a quartz watch. This definition, as specified in the 1997 CGPM meeting, is based on a cesium atom not perturbed by black-body radiation, a condition that is obtained at a temperature of 0 K (\(t=-273.15\)\({}^{\circ}\)C). The frequencies obtained from atomic clocks at room temperature should therefore be corrected for the offset induced by electromagnetic background noise. Note that, from a metrological point of view, there is no substantial difference between the definitions of \"time\" and \"frequency\", which, mathematically, are one the inverse of the other (\(f=1/t\)). The time scale is duodecimal with multiples and submultiples, partly sexagesimal and partly decimal (1 h = 60 min = 3600 s... tenths of s, hundredths of s, thousandths of s). Note again that, with the 2019 redefinition of the SI base units, \"time\" remains the (maybe the only) fundamental unit in the SI, defined only in terms of physical constants and used in the definitions of the following: in \"meter (m)\", together with the speed of light in a vacuum \(c\); in \"kilogram (kg)\", in the \"light intensity unit (cd)\" and in \"Kelvin (K)\", together with the speed of light \(c\) and the Planck constant \(h\); and, finally, in the \"unit of electric current (A)\", together with the elementary charge \(e\)[2]. In Italy, the national time scale is made at INRIM by a set of independent cesium atomic clocks and is compared, via satellite, with the time scales of other countries. It is maintained within \(\pm\)100 ns of the international reference UTC (Unified Time Coordinated). The unit of time is realized with a relative standard uncertainty of 1 \(\times\) 10\({}^{-13}\). However, the progress of research allows us to have, today, atomic clocks (based on strontium or ytterbium atoms) that are wrong 1 s every 5 billion years, that is, one error in a time greater than the age of the Earth and the solar system (4.5-4.6 billion years). Time measurements can be relative or absolute. Absolute time is the calendar one, usually linked to historical or religious events to such an extent that, even today, there is no universal time scale because each society has tried to adopt one by imposing it on the others. In the definitions of absolute time, physiologically, there is \"an error of zero\" due to personal choice (the beginning of creation for Chinese people or for Jewish people), an error that, in the Christian calendar, could be \"attenuated\", referring not to the birth of [PERSON] (an event that does not find certain references) but to his crucifixion, which instead finds confirmation in the documents of Rome ([PERSON]) [3].