Everyday use of time in one form or another is a common experience for everyone throughout their lives. The availability of a means to measure the passage of time with the required accuracy is taken for granted. However, the concepts on which those time scales are based and the requirements for accuracy in many applications can be both sophisticated and complex. Time is thus not a simple subject.

During the twentieth century the variability of the Earth ’ s rotational speed was established. The basis for time that had served for so many centuries was no longer adequate to meet the more demanding needs. A search for the defi nition and introduction of a uniform second and a uniform time scale followed, leading to the introduction of Ephemeris Time based on the orbital motions of solar system bodies. At the same time, atomic clocks were being developed, which offered a more convenient and accurate basis for time. Time measurement progressed from scales based on the rotation of the Earth to those based on atomic physics. In addition, improvements in the accuracy of predicting planetary positions required the introduction of dynamical time scales that recognized the role of general relativity in time keeping. Over the same period, the accuracy of time keeping and time transfer improved signifi cantly, and requirements for time have become even more demanding. The atomic second quickly achieved the status of being the most accurate and fundamental unit of measurement.

Although the Earth was no longer the basis for the most precise time keeping, the demands of new technologies made it even more critical to observe, analyze, and predict the actual variations of its rotation. The motions of its rotational axis, both in space and in the Earth itself, also required a parallel effort of observations, analysis and prediction. These activities pushed the improvement of celestial and terrestrial reference frames by orders of magnitude and encouraged new developments in the study of the dynamics of the Earth including the core, mantle, atmosphere, oceans, etc. and the forces acting on it due to the Sun, Moon, and planets. These studies have then gone on to spur the further development of even more accurate methods of observation.