The times - They are a changin'?

Abstract

Coordinated Universal Time (UTC) is a hybrid timescale in widespread civil use, based partly on International Atomic Time (TAI) as maintained by the Bureau International des Poids et Mesures (BIPM), and also occasionally adjusted by intercalary (leap) seconds to keep it coordinated with Universal time (UT1). UT1 is an astronomical measure of Earth rotation; the unit of its rate has been closely matched with mean solar time. The rules governing UTC are specified in a broadcast standard maintained by the Radiocommunications bureau of the International Telecommunications Union (ITU-R), which have remained largely unchanged since 1972. In three decades, this standard has never allowed UTC to deviate from UT1 by more than 0.9 seconds. Technical confusion has led to interest in recent years to either relax, or abolish, the relationship between Earth rotation and civil-time broadcasts, thus inventing a new civil-time standard, fundamentally different than, and independent of, historic civil and legal standards based on Earth rotation. Since UTC, as currently defined, still satisfies the needs of many communities, this paper attempts to provide some rationale for and against the redefinition of UTC, and the status of leading proposals for change. Due to the recent adoption of the International Celestial Reference System (ICRS), UT1 has also been technically redefined. The ICRS has been recently introduced as an epoch-less, fixed reference system succeeding the former astronomical reference system based on the Fifth Fundamental Catalog (FK5). In addition, a new precession-nutation model and changes to the moving reference frame of date eliminate the use of the equinox. Details and references concerning these reference frames are given. Finally, recommended practices for astrodynamics are included for UTC time keeping. Copyright © 2004 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.