A sundial is an instrument that uses the sun to measure time. A sundial consists of an angled marker, called a gnomon, that casts a shadow on a plate, called a dial plane. On the dial plane there are marks indicating the hours of the day. During the day, as the sun moves across the sky, the shadow from the gnomon moves across the dial plane, indicating the hour. Sundials were used to measure time before clocks and watches were invented.