The source provides an extensive overview of the sundial (日晷), detailing its historical importance as a crucial timekeeping invention before modern clocks became widespread. It thoroughly examines the sundial's fundamental components―the gnomon (晷針) and the dial plate (晷面)―and explains that its core principle relies on the apparent motion of the sun casting a moving shadow, with the gnomon needing to be aligned parallel to the Earth's axis (地軸). Furthermore, the text categorizes various structural types, such as horizontal, equatorial, and vertical sundials, and contrasts the unique technological advancements and materials (stone, metal, wood) used by ancient civilizations, including Greece, Rome, China, and India. Finally, the source describes the practical usage methods employed by ancient peoples, covering the necessary installation procedures, how to read the daily passage of time, and techniques for adjusting the device to account for seasonal variations.