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What does the Earth rotation coordinate measure?

The Earth rotation coordinate measures the angle through which the Earth has turned in a given period of time. This angle expressed as the difference between a time scale measured by the rotation of the Earth, UT1, and a uniform time scale, UTC, refers to the angular difference between the direction of the 0o meridian on the Earth and the direction to a point defined in space astronomically.
 
Historically some form of time based on the rotation of the Earth has always been the basis for civil time, the definition and the measurement procedures depending on available technology and precision requirements. In modern practice, UT1 is defined using a fiducial direction defined mathematically in the celestial reference system. This direction is referred to as the Mean Sun.
 
UTC, standing for Coordinated Universal Time, designates the atomic time scale which approximates the rotational time of the Earth. From the time of its inception, its rate and/or epoch have been adjusted to keep it near UT1. The current practice is to adjust UTC in epoch by integral seconds (leap seconds) to keep the difference between UT1 and UTC less than 0.9 seconds. UTC as defined by the International Radio Consultative Committee (CCIR) Recommendation 460-4, differs from TAI (Temps Atomique International) by an integral number of seconds. TAI is an atomic time scale determined by the Bureau International des Poids et Mesures (BIPM). Its unit is exactly one Système Internationale (SI) second at mean sea level.
 
Analyses of astronomical observations reveal different types of variations in the speed of rotation. The ancient observational data form the basis for estimates of the secular deceleration in the speed of rotation. The more recent information, having been obtained with higher accuracy and more regularity, has shown the changes in the acceleration causing irregular variations in the length of the day (LOD). These data have also been used to detect the periodic variations in the length of the day. Figure 2 (below) shows the difference between the rotationally determined length of day (corrected for the effects of known tidal variations, LODS) and 24 hours of UTC time, otherwise known as the excess length of day (in milliseconds per day).
 
 
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