A sundial is a device that tells the time by using the position of the sun. It consists of a flat plate, called the dial, and a stick, known as the gnomon, that casts a shadow on the dial. As the sun moves across the sky, the shadow moves, indicating the time of day based on the markings on the dial. Sundials have been used for thousands of years and are one of the oldest timekeeping instruments. They rely on the sun's position, making them most effective on sunny days. Unlike modern clocks, sundials do not require batteries or electricity.