The experience of time, or of time duration, has received great attention in literature and philosophy. The experience varies among individuals and, because of its subjective nature, may appear to be inconsistent even in one individual. In scientific work, a numerical measure is used to order observations of events. If "now" is assigned the numerical value zero, then it is usual to assign earlier times negative values and later times positive values. Subsequently, to obtain a time scale, some periodic phenomenon which has repetitions occurring at a uniform rate that may be subdivided and counted must be used.
Modern Concept of Time
Before the 20th century it was assumed as self-evident that a single, universal, uniform time scale existed. For two events that are widely separated in space, it had been assumed that there is no difficulty in defining the meaning of the concept of simultaneity--namely, if to one observer the events appeared to occur simultaneously, then all other observers would agree that the events were indeed simultaneous. Albert Einstein, however, early in the 20th century, recognized that because of the universal constancy of the speed of light the measurement of time depends on the motion of the observer.
Consider events A and B separated in space, which appear simultaneous to one observer; to another observer, then, who is in motion relative to the first, the event A may occur before or after B, depending on the direction of the relative motion between the two observers. Thus, in the modern view, time is no longer absolute, but dependent on the relative motion of observers making the time measurements. According to the theory of relativity, time is but one aspect of a more general four-dimensional space-time continuum, which is the arena in which events occur in the universe. Time and space are different aspects of this underlying four-dimensional continuum. Frequently time is described as a fourth dimension.
Time Scales
From earliest times the rotation of the Earth (or the apparent location of the Sun in the sky) has been used to establish a uniform time scale. In order to specify a date, using the apparent motion of the Sun as a time scale, days must be counted from some reference date. In addition, a clock is used to measure fractions of a day.
Time derived from the apparent position of the Sun in the sky is called apparent solar time. Because of the eccentricity of the Earth's orbit around the Sun and the inclination of the Earth's rotation axis to the orbital plane, apparent solar time is not a uniform time scale. These effects can, however, be calculated and corrections applied to obtain a more uniform time scale called mean solar time. Universal time (UT0) is equivalent to mean solar time at the Greenwich Meridian (Greenwich Mean Time, or GMT). Observations of the apparent motion of a distant star may be used to obtain yet another time scale used in astronomy, called sidereal time.
Additional small deviations from uniformity of UT0 may be traced to small effects, such as the wandering of the Earth's polar axis and other periodic fluctuations of the Earth's rotation; accounting for these effects leads to additional, even more uniform, time scales (UT1 and UT2).
Ephemeris time is determined by the orbital motion of the Earth about the Sun and is not affected by fluctuations in the Earth's rotation. Astronomical observations may be used to determine ephemeris time to an accuracy of roughly 0.05 seconds, averaged over a nine-year period.
Atomic Timekeeping
The accuracy of timekeeping today has improved by ten or more orders of magnitude since the time of the Greeks. The invention of the quartz crystal oscillator and of the atomic clock makes possible the measurement of time and frequency more accurately than any other physical quantity. Thus, in addition to astronomical time scales, there are other time scales such as atomic time (AT), based on the microwave resonances of certain atoms in a magnetic field, which provide the most accurate and stable clocks known. Atomic time scales obtained by counting the cycles of an electromagnetic signal in resonance with cesium atoms have an accuracy of a few billionths of a second over short intervals of a minute or less.
Since about 1960 a number of laboratories around the world have cooperated in comparing their atomic time scales, leading to the formation of a weighted average of the various atomic time scales, which is now disseminated to the public as Universal Coordinated Time (UTC). In order to keep UTC in agreement with the length of the day, seconds are occasionally added to or deleted from the atomic time scale (a "leap second"). By international agreement, UTC is maintained within 0.7 seconds of the navigator's time scale, UT1.
Defining the Second
The advancement of precision in time measurement has resulted in the adoption of new, more precise, definitions of the second. Prior to 1956, one second was defined as the fraction 1/86,400 of the mean solar day. From 1956 to 1967, it was the ephemeris second, defined as the fraction 1/31556925.9747 of the tropical year at 00h 00m 00s 31 December 1899. The second is currently defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.
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Extract from the Grolier Encyclopedia