The future(s) of UT1: a geophysical discussion

Abstract Once time became determined by quantum physics (International Atomic Time, TAI), rather than Earth orientation (universal Time, specifically UT1), the rules for the generally distributed time scale (Coordinated UT, UTC), have linked it to both. Since 1972 the rule for UTC has been that it has the TAI rate, with one-second discontinuities introduced as needed to keep UTC within 0.9 s of UT1. Whether in the 1960s or recently, discussions of rules for UTC have mostly focused on the steady change in the UT1 rate (Length of Day, LOD) caused by tidal friction slowing the Earth’s rotation. The main point of this paper is to develop a geophysically-based model for LOD changes that includes all known sources, and use it to produce Monte Carlo simulations of future UT1 variations over the next 75 years. These simulations show that, if UT1 is predicted over spans of decades using current values of LOD, the quadratic variation from tidal friction is much less important than irregular changes. They also show that the probability of a negative leap second before 2035 is about 0.25. The simulations can be used to evaluate a possible future rules for UTC, with two examples being shown.