Sundial's substyle distance is a term used to describe the distance between the gnomon and the hour lines on a sundial. There are many synonyms for this term, including shadow length, sun-shadow distance, shadow cast, shadow measurement, or simply sundial distance. Each of these terms describes the same phenomenon, which is the measurement of time based on the shadow cast by a sundial's gnomon. The distance between the gnomon and hour lines is essential for the accuracy of the sundial, and different cultures have used different methods for measuring this distance, such as the Italian, Babylonian, and Egyptian methods.