A few comments: * Sidereal time is different from solar time; sidereal time measures the Earth's rotation relative to the stars, while solar time measures it r

A few comments: * Sidereal time is different from solar time; sidereal time measures the Earth's rotation relative to the stars, while solar time measures it relative to the Sun, which moves relative to the stars. Thus, UT1 and sidereal time have different rates. * Wikipedia's page on day length fluctuations (https://en.wikipedia.org/wiki/Day_length_fluctuations https://en.wikipedia.org/wiki/Day_length_fluctuations) has a discussion of the influences involved. It also has a handy graph, from which we can see that the average day length is around 2 milliseconds shorter than in the 1970s (hence why we had a leap second every year from 1973-1979, but the last leap second was over four years ago). * There's also UT2 and UT0. UT2 has a corrections for seasonal variations, while UT0 is derived from raw observations, without correcting for polar motion like UT1. These are rarely used.