Evolution of Time Scales
NAVAL OBSERVATORY WASHINGTON DC
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Time scales have continued to evolve to meet the developing needs of users in modern times. Fifty years ago time based on astronomical observations of the Earths rotation was sufficient to meet the needs for time, both civil and technical. During that 50-year interval, however, not only have these scales been expanded, but the advent of practical atomic clocks has led to the development of time scales based on atomic frequency transitions. In addition, the development of modern space techniques and the improvement in observational accuracy by orders of magnitude have led to the improvement of the theoretical concepts of time and the resulting relativistic definitions of time scales. The following sections provide a brief description of the chronological development of time scales since the middle of the 20th century Mean Solar Time, Universal Time UT, Ephemeris Time ET, Atomic Time, Coordinated Universal Time UTC, Dynamical Time Scales, and the Future of Time Scales.
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