The Physical Basis of the Leap Second

International Atomic Time (TAI) is the internationally recognized timescale based on the second of the Système International d'Unités produced by the Bureau International des Poids et Mesures using data from timing laboratories around the world. TAI is an atomic timescale without steps. Coordinated Universal Time, the basis of civil time, is derived from TAI but is currently defined such that it is maintained within 0.9 s of Universal Time (UT1), the measure of time defined by the Earth's rotation angle, through the insertion of 1 s increments called leap seconds. The difference between UT1 and TAI that motivates the use of leap seconds is related to the tidal deceleration of the Earth's rotation. However, a recent paper by Deines and Williams claims that the divergence is caused by a relativistic time dilation effect. The purpose of this paper is to explain the physical basis of the leap second and to point out that leap seconds are unrelated to relativity.