The leap second: its history and possible future

Abstract

This paper reviews the theoretical motivation for the leap second in the context of the historical evolution of time measurement. The periodic insertion of a leap second step into the scale of Coordinated Universal Time (UTC) necessitates frequent changes in complex timekeeping systems and is currently the subject of discussion in working groups of various international scienti c organizations. UTC is an atomic time scale that agrees in rate with International Atomic Time (TAI), but differs by an integral number of seconds, and is the basis of civil time. In contrast, Universal Time (UT1) is an astronomical time scale de ned by the Earth's rotation and is used in celestial navigation. UTC is presently maintained to within 0.9 s of UT1. As the needs of celestial navigation that depend on UT1 can now be met by satellite systems, such as the Global Positioning System (GPS), options for revising the de nition of UTC and the possible role of leap seconds in the future are considered.

The role of the ITU-R in time scale definition

“…International Atomic Time (TAI) was recommended by the International Astronomical Union (IAU) in 1967, the International Union of Radio Science (URSI) in 1969 and the International Radio Consultative Committee (CCIR) of the ITU in 1970. The 14th General Conference on Weights and Measures (CGPM) approved the establishment of TAI in 1971 as the coordinate time scale whose unit interval is the Second of the International System of Units (SI) as realized on the rotating geoid [6].…”

“…Coordination and technical exchange meetings were held to gather data on UTC utilization, to analyze usage, and to examine alternative approaches to reduce or eliminate any operational impact of UTC adjustments. Meetings were held in conjunction with international conferences dealing with time and frequency as well as special presentations to the Institute of Navigation and the Civil GPS Interface Committee [6,10,11]. Several bodies in the international community conducted surveys and information fact-finding independently from the SRG but the results did not provide any clear resolution [10].…”

The IAU Resolutions on Astronomical Reference Systems, Time Scales, and Earth Rotation Models (Draft 4)

“…Aside from monthly, annual, and decadal variations, the Earth's angular velocity of rotation is decreasing linearly, which means that the accumulated lag in UT1 increases quadratically; viewed over many centuries, the ∆T curve is roughly a parabola. The formulas for sidereal time, and length of the old ephemeris second to which the SI second was originally calibrated, are based on the average (assumed fixed) rate of Earth rotation of the mid-1800s (see [Nelson et al 2001]). All of our modern timekeping systems are ultimately based on what the Earth was doing a century and a half ago.…”

Error minimization of polynomial approximation of Delta T

“…The Universal time (UT), based on the rotation of earth around its own axis, Ephemeris time (ET), based on the revolution of the Earth in its orbit around the Sun and the Atomic time (AT) based on the quantum mechanics of atom. Each of these bases for measure of time has its history of evolution (Nelson et al 2001).…”

“…If the origin of this co-ordinate system is chosen at the centre of the Earth, we have Terrestrial Dynamic time (TDT) or if this refers to the centre of mass of the solar system we have Barycentric dynamical time (TDB) (Guinot & Seidelmann 1998). Later TDT was renamed as "Terrestrial time" (TT) (Nelson et al 2001), which is now considered to be uniform time scale, and is used as the time argument for the prediction of the astronomical events. Our clocks, locked on UT are gradually slowing down with respect to the uniform TT and hence using UT, astronomical events seem to occur earlier than predicted (Seidelmann 1985).…”