The culmination of many years of research, this book discusses ancient and medieval eclipse observations and their importance in studying Earth's past rotation. This is the first major book on this subject in twenty years. The author has specialized for many years in the interpretation of early astronomical records and their application to problems in modern astronomy. The book contains an in-depth discussion of numerous eclipse records from Babylon, China, Europe and the Arab lands. The author provides translations of almost every record studied. He shows that although tides play a dominant long-term role in producing variations in Earth's rate of rotation--causing a gradual increase in the length of the day--there are significant and variable nontidal changes in opposition to the main trend. This book is intended for geophysicists, astronomers (especially those with an interest in history), historians and orientalists.