Book excerpt: Filling the need for a book that conveys the current technology as well as the underlying history and physical background, this book tells physicists and engineers how to measure time to the precision required for modern-day use. The authors draw on their longstanding research experience with timekeeping and high-precision measurement to cover the use of satellites in measuring earth movement variation and the influence of the moon, while also dwelling on such topics as timekeeping aboard satellites and time transfer. Indispensable for high-precision measurements of processes in astrophysics, and relevant for measurement, navigation and communication, this monograph can be equally used as a course book or as accompanying work at advanced undergraduate or graduate level.