[Encyclios]

Notion of time

The notion of time and its measurement are based essentially on cyclical processes: the first proposal for the use of linear processes, more congruent with the philosophical concept of time, was advanced in 1715 by E. Halley, who indicated in the degree of salinity of the sea, increasing over time, an index of . Similarly, W. T. Kelvin indicated in the cooling of the Earth, and H. Helmholtz in the contraction of the Sun, indices of time.

The discovery of radioactivity provided the most precise linear process: the corresponding unit is the half-life of a radioactive element, that is, the time required by half of the nuclei of the element itself to disintegrate. Linear time scales proved particularly useful in devising methods for measuring large time intervals (e.g. carbon 14 method). Implicit in the use of radioactive natural clocks and in the development of ultraprecise clocks is the notion that atoms obey, in every place and time, the same physical laws: the possibility that the physical laws vary over time is, however, still subject to verification.

The notion of time, from the metrological point of view, should be examined in the two aspects of the time scale and the time unit; a time scale is an uninterrupted succession of phenomena that allows to establish a chronology, that is to assign a date to every other event; knowing the mechanics, that is the set of physical laws that concur to form the scale, dates can be expressed in uniform time, which in turn can be used for the interpretation of any other natural phenomenon.

The unit of time is the duration of the time interval separating two phenomena chosen once and for all along the time scale. The history of time measurement shows that the phenomena chosen to form the time scale were periodic natural phenomena; multiples or submultiples of the period of the phenomena themselves were adopted as the units of time.

The precision with which the units of time are determined, that is, the uniformity of the time scale, is a function of knowledge of the theory behind the phenomena forming the scale. For a long time, such phenomena were astronomical ones, related to the rotation of the Earth and its revolution around the Sun. The units of time (day, second, year, etc.) were derived through the comparison of numerous observations.

The time unit can be directly available, however, when there is a duration reproducible in every place and time: conditions of this kind are now satisfied with the use of clocks, in particular atomic clocks, which use as time unit the period of an appropriate atomic transition, chosen as a sample duration (the inverse of the sample duration is the sample frequency: for practical purposes is more convenient the use of frequencies, rather than time). With the latter it is possible to define a time scale, called atomic time, which is independent from the series of astronomical phenomena commonly considered for the evaluation of time.

The precision with which today we operate using an astronomical time scale, i.e. we date astronomical phenomena, is limited only by observation errors. It should be noted, however, that the theory of astronomical phenomena, in particular the movements of the Earth, is still imperfect, so the unit of time that derives from it is inaccurate: for homogeneity and convenience, in the dating of astronomical phenomena are therefore used conventional time scales.

The most commonly used are based on sidereal time and solar time: the first has as its unit the sidereal day, the second the solar day, defined by the value of the hour angle, respectively, of the stars (or a star appropriately chosen) and the Sun. They are both local times, that is, that their value, at the same time, depends on the position of the observer on Earth (longitude), from this variability arose the need to introduce time zones, 24 areas of the Earth in each of which is in force, conventionally, the same time.