Time: From Earth Rotation to Atomic Physics

Time: From Earth Rotation to Atomic Physics , Dennis D. McCarthy and P. Kenneth Seidelmann Wiley-VCH, Weinheim, Germany, 2009. $145.00 (351 pp.). ISBN 978-3-527-40780-4

For nearly all of recorded history, and particularly the modern era, the interrelationship between timekeeping and astronomy has been a subject of interest. For example, the 18th-century “longitude problem” involved comparing the rotation angle of Earth to the time kept by mechanical devices that allowed precise navigation over long distances; the need to solve that problem led to a race between astronomers and mechanical clockmakers. As the astronomers discovered more details of Earth’s rotation, the clockmakers were inspired to build clocks whose rates were in sync with astronomical time.

An excellent look at the astronomical side of timekeeping is presented in Time: From Earth Rotation to Atomic Physics by Dennis McCarthy and Kenneth Seidelmann. The authors are recognized experts in Earth orientation, time scales, and timekeeping. More importantly, they write very well.

Time begins with a brief historical review of clocks and timekeeping, then moves into a discussion of how Earth rotation was used as the basis of time. The authors gently but rigorously explain the theory of Earth rotation and describe the progressive refinements of the physics used to define and model our planet’s motion. They also trace the historical progression of time scales, first moving from those based on Earth’s rotational rate to ephemeris time, which is based on Earth’s position in its yearly orbit. The reader learns how ephemeris time was briefly used to define the second and how it was soon abandoned when the early generation of the atomic clock was shown to be a more stable measurement reference.

Astronomical timekeeping is clearly the authors’ first love, and they devote roughly half the book to the topic. That half is well researched and was a delight to read, as it expertly, thoroughly, and lucidly describes the challenges involved in developing and using astronomical time scales. Much of the material, including an account of the origin of leap seconds (see the Quick Study by Brian Luzum, Physics Today, November 2006, page 78 ), is nearly impossible to find elsewhere in such detail. The reader is left with a clear impression that advances in timekeeping enabled new technologies that are ubiquitous in modern society, including the global positioning system, cellular phones, and computer systems and networks.

However, the other half of Time is less comprehensive and a bit disappointing. About a hundred or so pages unevenly cover a wide variety of topics; a few chapters consider the business of timekeeping by national laboratories and governments, and a short chapter is devoted to applications of timekeeping. Only 50 pages are devoted to microwave and optical atomic clocks. That section includes some information about the current definition of the second, which is based on a hyperfine transition of the cesium atom. But a reader who wants more than just a quick summary about atomic clocks would be better served by reading Fouad Major’s The Quantum Beat: Principles and Applications of Atomic Clocks (Springer, 2007) or Claude Audoin and Bernard Guinot’s The Measurement of Time: Time, Frequency, and the Atomic Clock (Cambridge University Press, 2001).

Time ends with two excellent chapters that hark back to the themes of Earth orientation and solar-system time, which is based on the Sun’s position in the sky. They tie together the book’s astronomical and mechanical themes and provide an interesting look at how the problem has inverted: Astronomical time was once used to measure the accuracy of mechanical clocks; today, dynamical systems like Earth and the solar system are better understood with atomic clocks.

Overall, Time is a welcome addition to the short stack of books that address the measurement of time, but one that is difficult to wholeheartedly recommend. For those whose main interest is astronomical timekeeping, the book is an indispensable introduction and reference. But it will be less satisfactory for those who are also interested in atomic timekeeping, measurement instrumentation and techniques, or state-of-the-art applications.