This is an encyclopedia about time. ¿¿¿¿¿¿¿¿¿¿¿¿¿ This book takes the measurement of time as its central theme. It begins by introducing knowledge related to timekeeping, including how disciplines such as astronomy, metrology, Newtonian mechanics, and relativity were established through the understanding of time. It then explores monumental historical transformations and intriguing anecdotes tied to time measurement, such as the Age of Exploration and the Industrial Revolution, while also highlighting ancient China's achievements in astronomical observation and calendar systems. In its later sections, the book focuses on the current state of time measurement and its applications in modern science, as well as in domains critical to national development and people's livelihoods, helping readers grasp the significance of research in time measurement. Finally, it offers a forward-looking perspective on the future of time and its measurement. ¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿¿