Book Review: The Measurement of Time, Time: From Earth Rotation to Atomic Physics

Abstract

THE MEASUREMENT OF TIME Time: From Earth Rotation to Atomic Physics. Dennis D. McCarthy and P. Kenneth Seidelmann (Wiley-VCH Verlag, Weinheim, 2009). Pp. xxii + 351. euro99. ISBN 978-3-527-40780-4.Both Dennis McCarthy and Kenneth Seidelmann have spent many years in the study and measurement of time and are well qualified to produce a monograph with the above title.Until the late nineteenth century, the constant rate of rotation of the Earth was widely regarded as axiomatic. Throughout this period, timekeeping was governed by the presumed regular spin of the Earth. Although the constancy of the terrestrial spin was occasionally questioned in the eighteenth and nineteenth centuries, not until the early twentieth century was it widely challenged. Gradually it began to be recognised that the Earth's spin was influenced by numerous separate factors, both tidal and non-tidal. Although the largest changes in the length of the day only occur at the millisecond level, it soon became evident that other standards of reference, independent of the Earth's rotation, were needed. The concept of time and its measurement has thus become an increasingly complex subject, with the introduction of a variety of time-scales. Simultaneously, improved clock design has led to spectacular improvements in the accuracy with which time can be measured. In their monograph, McCarthy and Seidelmann discuss these diverse aspects in considerable detail.The twenty chapters in this book may be divided somewhat arbitarily into six main sections. In chaps. 1-3, time-keeping from Antiquity to the mid-twentieth century, defined by a uniformly rotating Earth, is the main theme. Points of special note include the measurement and definition of solar time, the adoption of what eventually became known as Universal Time (UT) - a global time standard based on the Greenwich meridian - and the development of ephemerides for the accurate computation of lunar and planetary positions. Chaps. 4-6 consider the variability of the Earth's rotation: not only in the terrestrial rate of spin but also in its orientation. In addition to UT, a new system - Ephemeris Time (ET) - was introduced in 1960. ET, based not on the terrestrial rotation but on the motion of the Sun, Moon and planets, was regarded as a theoretically uniform time-scale. A feature of special historical interest is the determination of the parameter ?? (= ET - UT). Using a wide variety of eclipse observations, the cumulative effect of small changes in the Earth's rate of spin, ?? has been determined with surprising accuracy as far back as 700 b.c.Relativistic effects and the introduction of further time scales are the themes of chaps. …