Definition
Universal Time is a time scale based on Earth's rotation angle. UT1 is the modern realisation: it is determined from observations of stars and quasars and may differ from Coordinated Universal Time (UTC) by up to 0.9 s because leap seconds keep UTC close to the Sun.
UT and ephemerides
Planetary positions in almanacs are often given in Terrestrial Time (TT) or Barycentric Dynamical Time (TDB). To convert to the time shown on a clock, astronomers apply ΔT = TT − UT. The difference grows irregularly as Earth's rotation slows.