David Helfand is a distinguished scientist, specially acclaimed in the community for his skills as an expositor. These talents - along with his intellectual range - are manifest in this highly original and culture-spanning book which gathers and recounts diverse ways whereby scientific analysis can enrich historical understanding. Few people could have written The Universal Timekeepers so well. It is fascinating, wide-ranging, and accessible; everyone should read it.–Lord Martin Rees, Astronomer Royal