| Summary: | this paper [Pavise 1982]. In order to simplify the present discussion we shall adopt the Gregorian calendar as our prototypical calendar. Calendar years are typically an integral number of days, yet there are not (currently) an integral number of days in any astronomically computed year. For example, for the Gregorian calendar year 1985, the anomalistic year was 365:259; 641 ephemeris days, the tropical year was in the northern hemisphere's Spring and fast in its Fall; and unpredictable variations, probably caused by differing rates of rotation between the core and the mantle. The gradual slowing of the rate of rotation adds about 1.5 milliseconds to the length of a day (in comparison to other clocks) during a century. The length of a day could fluctuate by 4 milliseconds over the course of a decade due to the unpredictable variations. Finally, seasonal variations can cause changes on the order of 1.2 milliseconds in the length of a day during a year [Howse 1980]. Another factor to consider in making precise sidereal time measurements is polar wander . Polar wander is a slight circular wobble of the Earth around the North/South pole on the order of 8 meters a year. Polar wander shifts an observer's meridian by a fraction each day (recall that the observer's meridian is used to determine when the sidereal and solar days start). The size of the shift depends on the observer's latitude. The family of universal times attempts to correct for these variations. UT0 is the mean solar time for the prime meridian computed by direct astronomical observations. The prime meridian is the 0
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