Sundials are a device that uses the sun to tell the time. A sundial is made up of a horizontal dial plate with numbers on it, and a vertical upright called a gnomon. As the sun moves, the shadow cast by the gnomon moves around the dial plate, and you can read the time off what numbers it points to. Some sundials use a shaft of light rather than a shadow, and the dial plate may be curved not flat. Some sundials have enough information on the dial plate to be able to tell the date as well as the time.