In astronomical time scales, the difference between Terrestrial Time (TT) and Universal Time (UT). Currently this difference is nearly one minute and is increasing at the rate of about 2 s every three years. It arises because the Earth's rate of axial rotation is not precisely uniform, but shows certain irregularities and is gradually slowing down. Universal Time is tied to the rotation of the Earth through its link with sidereal time, while Terrestrial Time is defined by the orbital dynamics of bodies in the Solar System.
Subjects: Astronomy and Astrophysics.