Jo Ellen Barnett, Time's Pendulum

This is a well-written, well-argued book. It's divided into two parts. The first is a history of timekeeping—from sundials and water clocks (as the subtitle suggests) to the atomic clock. This part also begins with an interesting chapter on "The Planetary Basis of Our Day," which explains why an Earth day currently lasts about 24 hours and why it was shorter in the past. The second part of the book, less interesting to me, is about radiometric dating. The author refers to the various methods of dating using radioactive decay as "radioactive clocks," which is how the topic comes to be included in a book on timekeeping.