Book excerpt: Of Clocks and Time takes readers on a five-stop journey through the physics and technology (and occasional bits of applications and history) of timekeeping. On the way, conceptual vistas and qualitative images abound, but since mathematics is spoken everywhere the book visits equations, quantitative relations, and rigorous definitions are offered as well. The expedition begins with a discussion of the rhythms produced by the daily and annual motion of sun, moon, planets, and stars. Centuries worth of observation and thinking culminate in Newton's penetrating theoretical insights since his notion of space and time are still influential today. During the following two legs of the trip, tools are being examined that allow us to measure hours and minutes and then, with ever growing precision, the tiniest fractions of a second. When the pace of travel approaches the ultimate speed limit, the speed of light, time and space exhibit strange and counter-intuitive traits. On this fourth stage of the journey, Einstein is the local tour guide whose special and general theories of relativity explain the behavior of clocks under these circumstances. Finally, the last part of the voyage reverses direction, moving ever deeper into the past to explore how we can tell the age of "things" - including that of the universe itself.