TIME AS SYSTEMATIZED IN MODERN SCIENTIFIC SOCIETY

Time as systematized in modern scientific society Time measurement: general concepts Accuracy in specifying time is needed for civil, industrial, and scientific purposes. Although defining time presents difficulties, measuring it does not; it is the most accurately measured physical quantity. A time measurement assigns a unique number to either an epoch, which specifies the moment when an instantaneous event occurs, in the sense of time of day, or a time interval, which is the duration of a continued event. The progress of any phenomenon that undergoes regular changes may be used to measure time. Such phenomena make up much of the subject matter of astronomy, physics, chemistry, geology, and biology. The following sections of this article treat time measurements based on manifestations of gravitation, electromagnetism, rotational inertia, and radioactivity. Series of events can be referred to a time scale, which is an ordered set of times derived from observations of some phenomenon. Two independent, fundamental time scales are those called dynamicalbased on the regularity of the motions of celestial bodies fixed in their orbits by gravitationand atomicbased on the characteristic frequency of electromagnetic radiation emitted or absorbed in quantum transitions between internal energy states of atoms or molecules. Two time scales that have no relative secular acceleration are called equivalent. That is, a clock displaying the time according to one of these scales would notover an extended intervalshow a change in its rate relative to that of a clock displaying time according to the other scale. It is not certain whether the dynamical and atomic scales are equivalent, but present definitions treat them as being so. The Earth's daily rotation about its own axis provides a time scale, but one that is not equivalent to the fundamental scales because tidal friction inexorably decreases the Earth's rotational speed (symbolized by the Greek letter omega, w). There are also other sources of variation. Rotational time is needed for civil purposes, celestial navigation, and tracking of space vehicles. The decay of radioactive elements is a random, rather than a repetitive, process, but the statistical reliability of the time required for the disappearance of any given fraction of a particular element can be used for measuring long time intervals. Principal scales Numerous time scales have been formed; several important ones are described in detail in subsequent sections of this article. The abbreviations given here are derived from English or French terms. Universal Time (UT; the mean solar time of the meridian of Greenwich, England), Coordinated Universal Time (UTC; the basis of legal, civil time), and leap seconds are treated under the heading Rotational time. Ephemeris Time (ET; the first correct dynamical time scale) is treated in the section Dynamical time, as are Barycentric Dynamical Time (TDB) and Terrestrial Dynamical Time (TDT), which are more accurate than Ephemeris Time because they take relativity into account. International Atomic Time (TAI; introduced in 1955) is covered in the section Atomic time.