An ancient sundial is a device used to tell time by the apparent position of the Sun in the sky. It typically consists of a flat plate (the dial) with markings for hours, and a gnomon (a thin rod or plate) that casts a shadow onto the dial. As the Sun moves across the sky during the day, the position and length of the gnomon's shadow change. People would read the time by observing where the shadow fell on the marked dial. It was effective during daylight hours but couldn't be used at night or on cloudy days.